Journal of Molecular Structure: THEOCHEM 901 (2009) 88–95

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Journal of Molecular Structure: THEOCHEM

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A computational investigation of the interactions between harmaneand the functional monomers commonly used in molecular imprinting

Agnieszka Kowalska *, Agnieszka Stobiecka, Stanisław WysockiInstitute of General Food Chemistry, Department of Biotechnology and Food Science, Technical University of Lodz, ul. Stefanowskiego 4/10, 90-924 Lodz, Poland

a r t i c l e i n f o a b s t r a c t

Article history:Received 2 October 2008Received in revised form 7 January 2009Accepted 8 January 2009Available online 18 January 2009

Keywords:Molecular modelingMolecular imprintingHydrogen-bonded interactionsb-Carbolines

0166-1280/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.theochem.2009.01.008

* Corresponding author.E-mail address: kawasak@tlen.pl (A. Kowalska).

In this study, the density functional (DFT) method with the hybrid B3LYP exchange-correlation functionalhas been applied to investigate the intermolecular interactions between harmane and the selected func-tional monomers: methacrylic acid (MAA) and 2-hydroxyethyl methacrylate (HEMA), most commonlyused in the preparation of molecularly imprinted polymers (MIPs). For this purpose possible conforma-tions of 1:n (n 6 4) harmane/functional monomer systems have been optimized with the use of theDFT(B3LYP)/6-31G(d,p) method. The most stable configurations of harmane/functional monomer 1:n(n 6 4) complexes have been selected. The positions and the geometrical parameters of hydrogen bond-ing sites in the optimized 1:n harmane/functional monomer systems have been determined. The bindingenergies DEbind of the optimized systems have been calculated taking into account the basis set superpo-sition error (BSSE) and the zero-point vibrational energies (DZPVE) corrections. Based on the conforma-tional analysis and the calculated binding energies of harmane/monomer molecular systems, we haveconcluded that the interactions between harmane and MAA are more specific and stronger in comparisonto the interactions between harmane and HEMA.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Harmane (1-methyl-9H-pyridinic[3,4-b]indole) is one of thenaturally occurring b-carboline alkaloids, which are compoundsof a great pharmaceutical importance [1–3]. b-Carbolines exhibitvariety of biological and pharmacological properties. The resultsof the extensive research indicated that b-carbolines are associatedin monoaminooxidase (MAO) inhibition as well as in blockage ofre-uptake sites and direct activation of MAO receptors [4,5].

MAO is a flavoenzyme located in the outer membranes of mito-chondria in the brain, liver, intestinal mucosa and other organs.MAO catalyses the oxidative deamination of xenobiotic andbiogenic amines including neurotransmitters as well as MPTP(N-methyl-4-phenyl-1,2,3,6-tetrahydropyrole) proneurotoxine.The primary role of MAO lies in the regulation of the levels ofneurotransmitters such as dopamine, serotonine, norephedrine,tryptamine and tyramine. In the brain MAO activates the dopami-nergic proneurotoxin MPTP to an active neurotoxin. In the gastroin-testinal track, the circulatory system and the liver, MAO may serve aprotective function as this enzyme regulates the level of exogenousdietary amines. However, the oxidation of amines by MAO results inthe production of potentially toxic H2O2, NH3 and aldehydes thatrepresent the risk factor for cell oxidative damage. Abnormal activ-ity of MAO is implicated in Parkinson’s, Alzheimer’s diseases as well

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as depression. Protection against Parkinsonism and neurodegenera-tion by MAO inhibitors may result from a reduced neurotoxin acti-vation as well as reduced production of H2O2, NH3 and aldehydespecies. Therefore MAO-inhibitors including harmane, which is awell-known inhibitor of one of the two MAO isozymes-MAO-A, havea great potency for being used as antidepressants and in the treat-ment and prevention of Parkinson disease [6].

b-Carbolines are synthesized in vivo in mammals and this bio-synthesis is enzymatic because it only proceeds in the presenceof mammalian tissue. b-Carbolines are also present in variousplants and foodstuffs (coffee) as well as in alcoholic beveragesand cigarette smoke [7–9].

Since b-carbolines occur in natural sources only in traceamounts (for example the concentration of harmane is estimatedto be 0.73 ± 0.34 lg/g in instant coffee brews, 0.4 ± 0.1 lg/g inground-brewed regular coffee [6]), their separation from naturalsources is a very difficult task, requiring the application of veryselective and specific methods. Therefore recently there has beena great demand for the synthesis of polymeric recognition materi-als, which are very selective and specific to the studied compounds.Such materials, mimicking the biological recognition systems, maybe synthesize applying molecular imprinting technique.

Molecularly imprinted polymers (MIP) are new highly selectivesynthetic receptors with molecular recognition sites designed for aparticular analyte. MIP technology has been developed as a methodfor preparation of synthetic receptors by polymerization of a self-assembled complexes, formed by functional monomers and a

Table 1The relative energies between the PM3 optimized conformations of 1:n harmane/monomer complexes.

Number of monomermolecules (n)

Name of thecomplex

Harmane/nMAA(kcal/mol)

Harmane/nHEMA(kcal/mol)

1 PM3-1A 1.266 4.325PM3-1B 2.175 2.330PM3-1C 2.693 0.000PM3-1D 0.000 4.934

2 PM3-2A 1.312 4.771PM3-2B 0.320 0.000PM3-2C 0.000 3.819PM3-2D 0.675 4.868

3 PM3-3A 0.000 3.798PM3-3B 3.569 2.277PM3-3C 3.743 0.847PM3-3D 2.810 0.000

4 PM3-4A 1.346 0.000PM3-4B 0.000 3.274PM3-4C 1.838 1.320PM3-4D 1.386 2.380

A. Kowalska et al. / Journal of Molecular Structure: THEOCHEM 901 (2009) 88–95 89

template in the pre-polymerization mixture. The selectivity andthe affinity of the recognition sites in the resulting MIP dependson the geometrical parameters of these complexes, which are inturn determined by the strength and the type of intermolecularinteractions involved in the complex formation in the pre-poly-merization phase [10–13]. Therefore, the knowledge of thestrength and the nature of the interactions involved in the tem-plate/monomer(s) complexes is crucial for the rational design ofa very selective MIP.

The success of molecular imprinting generally depends on thechoice of the optimal functional monomer used in the synthesisof MIP. Traditionally the choice of polymer composition is basedon the extensive, time-consuming experimental trials [14,15].Recently, thanks to rapid advances in both hardware and softwaretechnology, computational approach has been applied to molecularimprinting technology [16–22].

A large number of recently published theoretical studiesproved that computational simulations may be successfully ap-plied in the rational design, evaluation and prediction of affinityand selectivity of MIP. Piletsky group [23–29] have assembled alibrary of the most commonly used functional monomers, whichhas been subsequently screened on their possible interactionswith a given template molecule. The monomers capable of form-ing the strongest complexes with the template molecule were se-lected for the simulated annealing experiment. The results on thedevelopment of the MIPs for microcystin-LR [23], ochratoxine A[24], ephedrine [25], creatinine [26], cocaine [27], tylosine [28]and biotine [29] have proved that the proposed computationalselection of monomers for various templates is rapid and effectiveprocedure, which may be successfully used to determine the com-position of the pre-polymerization mixture for the synthesis ofhighly selective MIPs.

The most commonly applied theoretical models in the com-puter-aided studies of MIP are molecular mechanics (MM) models,which are simplified, therefore they do not provide in depth infor-mation with higher accuracy and reliability. This level of theorymay be not sufficient for the proper description of hydrogen bonds,which are most frequently involved in the intermolecular interac-tions in the pre-polymerization stage. Pavel [18] group performedthe analysis of various energy contributions (van der Waals, elec-trostatic, hydrogen-bonding contributions) to the total intermolec-ular interaction energy in theophylline/methacrylic acid system. Ithas turned out that, surprisingly and contrintuitative, the hydro-gen-bonding contributions in the system studied appeared to bevery small. Pavel group have performed more advanced Hartree–Fock HF/6-31G(d) calculations for theophylline/MAA system,which clearly indicated that H-bonding was involved in the theo-phyline/MAA system. They explained this inconsistency that themajor part of H-bonding in molecular mechanic simulations isimplicitly taken into account in the electrostatic van der Waalsinteractions rather than in the parametrization of H-bonding,which is not very sophisticated in the molecular mechanics calcu-lations. Due to the poor performance of molecular mechanicsmethods in the study of hydrogen-bonded interactions, mostresearchers have applied more sophisticated models (the semi-empirical Parametric Method 3 (PM3), HF/the second-orderMøller–Plesset method MP2 [16,17], DFT(B3LYP) [30,31]) in thedescription of the interacting molecular systems.

As far as we know there are few theoretical studies devoted tointermolecular interactions involving b-carbolines. In one of thesestudies [32] the HF/6-31G(d,p) method has been applied to studyproton transfer tautomerism of b-carbolines mediated by hydro-gen bonded interactions with acetic acid. So far, there has beenno report on using quantum chemical calculations to study theself-assembly process leading to the formation of harmane/func-tional monomer complexes in the pre-polymerization mixture.

Bearing in mind that only high level calculations are reliable inthe description of the electronic structure of hydrogen-bonded sys-tems, we have employed the DFT(B3LYP)/6-31G(d,p) method in thestudy of the specific interactions between harmane and the func-tional monomers (MAA, HEMA) most frequently used in molecularimprinting. Numerous studies have shown that this method maybe successfully applied in the prediction of geometries and stabil-ities of typical hydrogen-bonded systems [33–38].

Since the most common template/monomer molar ratio inmolecular imprinting is 1:4 [22], we have determined the geomet-rical parameters of hydrogen bonding sites in the most stable con-figurations of 1:n (n 6 4) harmane/MAA and 1:n (n 6 4) harmane/HEMA complexes. To quantitatively evaluate the strength of theintermolecular interactions, we have calculated the binding energyof the systems studied taking into account the BSSE and DZPVEcorrections. Based on the theoretically obtained binding energiesand conformational analysis of the most stable 1:4 harmane/monomer complexes, we will try to predict theoretically whichof the monomers studied is more suitable for harmane molecularimprinting.

2. Computational methods

The geometries of harmane, MAA, HEMA and four possible con-figurations of 1:n harmane/monomer molecular systems have beenfirstly optimized applying the relatively fast PM3 semi-empiricalmethod implemented in Gaussian 03 program. The relative totalelectronic energies between the PM3 optimized conformations of1:n harmane/monomer complexes are gathered in Table 1.

For searching the conformational space of the PM3 predicted lo-cal minima of the complexes studied, we used the combination ofquenched dynamics and simulated annealing techniques as recom-mended in [39]. The computation procedure used in this researchhas included the following main steps as presented in Fig. 1.

For each of the PM3 optimized 1:n harmane/monomer complexwe have performed molecular dynamics MD simulations at con-stant energy using the PM3 method implemented in HyperChem7.0 program. The convergence criteria were set to 0.000001 kcal/mol. The initial temperature of the simulation was set to 0 K (heat-ing time 0.1 ps), the simulation temperature was set to 600 K or298 K (simulation time 1 ps), the final temperature was set to 0 K(cooling time 0.1 ps). The temperature step and the time step wereset to 3 K and 0.0005 ps, respectively. After each of the simulationwe have saved the final geometry of each of the system studied.

Fig. 1. The main steps in the computational procedure used in this paper.

90 A. Kowalska et al. / Journal of Molecular Structure: THEOCHEM 901 (2009) 88–95

Then we have run geometry optimization calculations of the savedstructures and we have compared the calculated optimized ener-gies to find the lowest-energy conformations. As a result of theMD simulations and the subsequent geometry optimization calcu-lations, conformations of higher or nearly the same energy as theinitial configurations have been obtained.

The PM3 optimized conformations of the 1:n harmane–mono-mer complexes which had the minimum energy values, were fur-ther optimized using the DFT approach utilizing hybrid Beckethree-parameter exchange-correlation functional (B3LYP) and the6-31G(d,p) basis set implemented in Gaussian 3 program. We havealso performed frequency calculations of the systems at the samelevel of theory. The analysis of the DFT(B3LYP) calculated frequen-cies have been performed to verify whether the optimized struc-tures correspond to the local minima.

The ground state binding energies (DEbind) of the interactingsystems have been calculated using the super-molecule approach[40,41] as the electronic energy difference between the complexand the isolated molecules (harmane and monomers), then cor-rected for the basis set superposition error (BSSE) and the zeropoint vibrational energy (DZPVE).

DEbind = Ecomplex � (Eharmane + n�Emonomer) + BSSE + DZPVE.The basis set superposition error (BSSE) was estimated applying

the counterpoise method [42]. The zero-point vibrational energy(DZPVE) correction was estimated as the difference in zero-pointvibrational energies between the complex and the isolated mole-cules [43]:

DZPVE = ZPVEcomplex � (ZPVEharmane + n�ZPVEmonomer).The enthalpy of association (DHas) in the gas phase was calcu-

lated from the data listed in the output of Gaussian calculationwithout scaling their values.

DHas = Hcomplex � (Hharmane + n�Hmonomer) + BSSE.All calculations were performed using HyperChem 7.0 [44] and

Gaussian 03 [45] programs.

Fig.2. The DFT(B3LYP)/6-31G(d,p) optimized geometries of: (a) harmane, (b) MAAand (c) HEMA.

3. Results and discussion

3.1. Geometries of harmane, MAA and HEMA

The DFT(B3LYP)/6-31G(d,p) optimized configurations of har-mane and the two functional monomers studied are shown inFig. 2. The DFT(B3LYP)/6-31G(d,p) calculated charges on the atoms

of harmane and the functional monomers (MAA, HEMA) obtainedfrom Mulliken population analysis are shown in Table 2.

Fig.3. The DFT(B3LYP)/6-31G(d,p) optimized geometries of the most stable 1:nharmane/MAA systems.

Table 2Mulliken charges on the atoms of harmane, MAA and HEMA calculated with the use ofthe DFT(B3LYP)/6-31G(d,p) method.

Molecules No. Atom Atomic charges No. Atom Atomic charges

Harmane 1 C +0.048 13 C �0.0932 C +0.087 14 H +0.2583 C +0.311 15 H +0.0864 C �0.127 16 H +0.0835 N �0.731 17 H +0.0836 C +0.267 18 N �0.5007 C �0.147 19 C �0.3858 C �0.103 20 H +0.0909 C �0.114 21 H +0.08610 H +0.089 22 H +0.11311 C +0.074 23 H +0.14112 C +0.273 24 H +0.113

MAA 1 C �0.252 7 H +0.1342 H +0.106 8 H +0.1343 H +0.132 9 C +0.5544 C +0.090 10 O �0.4725 C �0.353 11 O �0.5006 H +0.104 12 H +0.323

HEMA 1 C �0.359 11 H +0.1252 C �0.254 12 H +0.1183 C +0.066 13 H +0.1034 O �0.440 14 H +0.0975 O �0.543 15 H +0.1086 C +0.038 16 H +0.1227 O �0.465 17 H +0.1528 C +0.551 18 H +0.1119 C +0.041 19 H +0.11410 H +0.315

A. Kowalska et al. / Journal of Molecular Structure: THEOCHEM 901 (2009) 88–95 91

Bearing in mind a bifunctional hydrogen-bonding character ofb-carboline ring, the pyridinic nitrogen atom may be a protonacceptor, while pyrrolic group may act as a proton donor. The func-tional monomer molecules studied also have proton donor–accep-tor properties, therefore they can form harmane/monomercomplexes of different hydrogen bonding sites. Moreover MAAmolecules may either interact with each other resulting in the for-mation of dimmers or with harmane molecule forming adducts ofdifferent stoichiometry.

From the DFT(B3LYP)/6-31G(d,p) calculated Mulliken charges(Table 2) and spatial considerations it can be deduced that mostprobably the proton donor in harmane/MAA and harmane/HEMA1:1 complexes may be hydroxylic group (H12 +0.323 in MAA,H10 +0.315 in HEMA); while the proton acceptor may be mostlikely pyridinic nitrogen atom (N18 �0.500). There is also a possi-bility of the hydrogen bonded interactions involving pyrrolic group(H14 in NH +0.258) and carboxylic oxygen of MAA (O10 �0.472) orHEMA (O4 �0.440). Therefore, harmane/monomer systems ofhigher than 1:1 stoichiometries may also be expected.

Considering the molar ratio 1=4 (template/monomer) generallyused in molecular imprinting, we docked the template and themonomers to find possible 1:4 harmane/monomer molecularsystems. These interacting systems were subsequently opti-mized applying the PM3/DFT(B3LYP)/6-31G(d,p) method andthe most stable 1:n (n 6 4) harmane/monomer complexes wereselected.

3.2. Geometries of 1:n harmane/MAA systems

The geometrical parameters of the most stable structures of 1:nharmane/MAA systems calculated using the DFT(B3LYP)/6-31G(d,p) method are shown in Fig. 3.

In the most stable of 1:1 harmane/MAA complex, H-1MAA, thehydrogen-bonding pair consists of MAA hydroxyl group and thepyridinic nitrogen of harmane. Additionally, the presence of the

weak interaction between MAA carboxylic oxygen and hydrogenatom of harmane ring enhances the interaction between pyridinic

Fig.4. The DFT(B3LYP)/6-31G(d,p) optimized geometries of the most stable 1:nharmane/HEMA systems.

92 A. Kowalska et al. / Journal of Molecular Structure: THEOCHEM 901 (2009) 88–95

nitrogen and hydroxylic group. H-1MAA complex is nearly planarand the N. . .HO distance is calculated to be about 2.7 ÅA

0

.The most stable among 1:2 harmane/MAA systems, H-2MAA,

consists of two acid molecules individually hydrogen bonded tothe pyridinic nitrogen and pyrrolic group, respectively. Inspectionof H-2MAA geometry indicates that there are two hydrogen-bond-ing interactions, i.e., between pyridinic nitrogen atom of harmaneand hydroxylic group of MAA and between the pyrrolic group andcarboxylic oxygen of the other MAA molecule, respectively. TheNH(pyrrolic). . .O distance was calculated to be about 2.9 Å(<NH. . .O = �150�). The strongest hydrogen bond in the most stableharmane/MAA 1:2 complex is the bond between pyridinic nitrogenand acidic hydrogen of MAA (N. . .HO = �2.7 ÅA

0

, <N. . .HO = �177�).This interaction is enhanced by the presence of the weak HB formedbetween pyridinic ring hydrogen atom and carboxylic oxygen atomof MAA (CH. . .O = �3.3 ÅA

0

, <CH. . .O = �134�). Based on the calcu-lated geometrical parameters of the hydrogen bonding sites, wemay deduce that the hydrogen-bonding interaction between har-mane pyridinic nitrogen and MAA hydroxylic group is stronger thanthat between harmane pyrrolic group and carboxylic oxygen atomof MAA.

In the most stable harmane/MAA 1:3 and 1:4 complexes, thelinkage of hydrogen bonds incorporating MAA molecules as wellas proton donor and acceptor sites in harmane is formed. The re-sults of our calculations indicate that nearly planar (178�) hydro-gen bond formed between harmane pyridinic nitrogen and MAAhydroxyl group in complex H-3MAA is stronger (the calculatedN. . .HO distance is 2.66 Å) in comparison to the HB interactionincorporating harmane pyrrolic group and carboxylic oxygen ofMAA (NH. . .O = 2.88 Å, <NH. . .0 = 169�). The strong hydrogen bondbetween pyridinic nitrogen and MAA hydroxylic group facilitates aproton transfer from the MAA molecule (which is in the contiguityof harmane pyridinic nitrogen atom) towards the pyridinic nitro-gen, which may lead to the protonation of harmane, resulting inthe formation of harmane cations. In the most stable harmane/MAA 1:4 complex, H-4MAA, the presence of the interaction be-tween carboxylic oxygen of the fourth MAA molecule and harmanepyridinic ring hydrogen atom results in the additional stabilizationof the structure. Inspection of H-4MAA geometry reveals the pres-ence of two strong hydrogen-bonding interactions, namelyN(pyridinic) . . .HO(MAA) and NH(pyrrolic). . .O@C(MAA). Similarlyto complex H-3MAA, the strongest hydrogen bonding interactionincorporates harmane pyridinic nitrogen atom and MAA hydroxylgroup. The hydrogen bond formed between harmane pyrrolicgroup and MAA carboxylic oxygen is planar (<NH. . .O = 180�) andthe NH. . .O distance is calculated to be 2.85 Å. The calculated dis-tance between N(pyridinic) and HO(MAA) is equal to 2.66 Å andthe calculated angle between atoms taking part in the hydrogenbond (<N. . .HO) is equal to 172.7�. Planarity of the above men-tioned hydrogen bond as well as the short distance between har-mane pyridinic nitrogen and MAA hydrogen atom favour theproton transfer towards harmane pyridinic nitrogen, resulting information of cationic form of harmane.

It should be noted that, in general, the geometrical parametersof two hydrogen bonding sites incorporating: pyridinic nitrogenand hydroxylic MAA group as well as pyrrolic harmane groupand carboxylic MAA oxygen have not been much changed whengoing from 1:2 to 1:4 harmane/MAA complexes. Therefore it maybe suggested that these interactions may be crucial in molecularrecognition of MIP specified for harmane.

3.3. Geometries of 1:n harmane/HEMA systems

The geometrical parameters of the most stable structures of 1:nharmane/HEMA systems calculated using the DFT(B3LYP)/6-31G(d,p) method are shown in Fig. 4.

Inspection of the geometry of the most stable among 1:1 har-mane/HEMA complexes, H-1HEMA reveals the presence of thehydrogen-bonded interaction involving pyridinic nitrogen andhydroxylic group of HEMA, which is stronger as compared to the

A. Kowalska et al. / Journal of Molecular Structure: THEOCHEM 901 (2009) 88–95 93

interaction incorporating harmane pyrrolic group and HEMA car-boxylic oxygen atom. The calculated hydrogen bond distance inH-1HEMA complex is 2.83 Å, whereas the angle between atomsparticipating in the hydrogen bond formation is <N. . .HO = 167.6�.Contrary to the most stable harmane/MAA 1:2 complex where twoMAAs are individually hydrogen bonded to pyridinic nitrogen andpyrrolic group, respectively, the most stable among harmane/HEMA 1:2 complexes is complex H-2HEMA with a triple-hydro-gen-bonded configuration. In this complex two HEMA moleculesforming hydrogen bonds with two HB sites in harmane are linkedto each other by an additional hydrogen bond. The calculated dis-tance of pyridinic nitrogen–hydroxylic group is 2.84 Å (<N. . .HO =166.2�). The hydrogen bond distance involving pyrrolic groupand HEMA carboxylic oxygen is calculated to be 2.93 Å (<NH. . .O =163.7�). In the most stable 1:3 and 1:4 harmane/HEMA complexes,H-3HEMA and H-4HEMA, respectively, the linkage of hydrogenbonding incorporating HEMA molecules and two hydrogen bond-ing sites provided by b-carboline, results in the enhancement ofthe system stability. The calculated distance between HEMA mole-cules is about 2.8 Å. The analysis of the geometrical parameters ofhydrogen bonding sites in 1:3 and 1:4 harmane/HEMA complexessuggest that the interaction involving pyridinic nitrogen and HEMAhydroxylic group is stronger (N. . .HO = �2.8 Å, <N. . .HO = �168�)comparing to the interaction involving pyrrolic group and HEMAcarboxylic oxygen (NH. . .O = 3 Å, <NH. . .O = �159�).

3.4. The nature of harmane/MAA and harmane/HEMA intermolecularinteractions

It is well known that the proof for the existence of hydrogenbonds in the interacting systems is that vibrations involving theproton-donating groups (NH, OH) show shifts in their absorptionfrequencies towards lower energy, whereas the intensities of cor-responding vibrations are enhanced largely. The magnitude ofthe above-mentioned red shifts and changes of vibrational intensi-ties of proton-donating groups depends on the strength of thehydrogen bonded interactions [46].

In order to get insight into the nature of the intermolecularinteractions involving harmane and the functional monomersstudied (MAA, HEMA), we have calculated the frequencies andintensities of pyrrolic NH and hydroxylic OH stretching vibrationsin the most stable harmane/MAA and harmane/HEMA 1:n systems.These values have been compared with the DFT(B3LYP)/6-31G(d,p)calculated spectroscopic parameters of OH and NH stretchingvibrations in the isolated functional monomer and harmane mole-cule, respectively (Table 3).

From Table 3 it may be noticed that hydrogen bonding interac-tions between harmane pyridinic nitrogen and hydroxylic group ofMAA or HEMA are accompanied by the red shift of the calculatedOH stretching vibration frequencies. Moreover the intensities of

Table 3The DFT(B3LYP)/6-31G(d,p) calculated spectroscopic parameters of NH and OH stretching

mNH(cm�1) INH DmNH(cm-1) INHC/INH

Harman 3690.6 57.4 – –MAA – – – –HEMA – – – –H-1MAA 3689.9 60.5 �1.1 1.1H-2MAA 3546.0 419.2 �145.0 7.3H-3MAA 3435.7 1247.9 �255.3 21.7H-4MAA 3448.0 1278.1 �243.0 22.2

H-1HEMA 3689.0 62.6 �1.6 1.1H-2HEMA 3458.0 862.0 �232.6 15.0H-3HEMA 3535.4 514.1 �155.3 9.0H-4HEMA 3550.3 459.0 �140.3 8.0

the above-mentioned OH stretching vibrations in the most stableharmane/MAA and harmane/HEMA systems are enhanced largely.Additionally, in the harmane/MAA and harmane/HEMA molecularsystems of stoichiometry greater than 1:1, the decrease of the cal-culated frequency of pyrrolic NH stretching vibration together withthe enhancement of the calculated intensity of NH stretchingvibration are also noticed, but these changes are less evident com-paring to the changes in spectroscopic parameters involving OHgroup. This proves the existence of hydrogen bonds in the har-mane/MAA and harmane/HEMA associates and indicates that theinteraction between functional monomer hydroxylic group andharmane pyridinic nitrogen atom is stronger than the interactioninvolving harmane pyrrolic group and carboxylic oxygen of thefunctional monomers studied.

3.5. Stabilities of 1:n harmane/MAA and 1:n harmane/HEMA molecularsystems

As the most stable configurations of harmane/MAA and har-mane/HEMA 1:n systems have been determined, we can now focusour attention on the prediction which of functional monomersstudied interacts more strongly and more specific with harmane.As mentioned earlier, the more stable complexes are formed inthe pre-polymerization mixture, the more selective the resultingMIP will be. As a measure of the complex stability, the interaction(binding) energy calculated in the super-molecule approach hasbeen commonly used. As has been already proven, the larger (themost negative value) interaction energy of the molecular system,the stronger hydrogen-bonding interactions and the more stableresulting complex [12–17,21,22,31].

On the other hand, the extent of template complexation at equi-librium is governed by the change in Gibbs free energy for the for-mation of a template-functional monomer complex. However,analyzing the calculated frequencies of the studied harmane/MAA or harmane/HEMA systems, it may be noticed that the molec-ular systems studied are characterized by one or more verylow-frequency vibrations, for which applied harmonic oscillatorapproximation is expected to give very poor description. The poorperformance of the harmonic oscillator approximation results inerrors in low frequency vibrations, which may lead to very largeerrors in entropies, which in turn mean large errors in free energy.Therefore we expect that the calculated DG of the associationis not reliable to predict stabilities of the systems studied andinstead DG, we have restricted ourselves to calculate enthalpy ofassociation [47].

Not only the strength of intermolecular interactions betweenimprinted analyte and functional monomers but also the numberof binding (activity) sites in the pre-polymerization mixture is animportant criteria for the selection of the proper monomer usedin the preparation of a very selective MIP. The number of the

vibrations and its changes due to the complex formation.

M mOH(cm�1) IOH DmOH(cm�1) IOHC/IOH

M

– – – –3766.6 62.7 – –3837.7 26.1 – –2901.1 4104.2 �865.5 65.52876.0 4249.9 �890.6 67.82360.0 5137.9 �1406.6 82.02608.8 2852.7 �1157.8 45.5

3424.4 1752.3 �413.3 67.23387.8 1194.3 �449.9 45.83373.3 1378.4 �464.4 52.83344.2 1483.7 �493.5 56.9

Table 4The DFT(B3LYP)/6-31G(d,p) calculated binding energy (DEbind [kcal/mol]), enthalpy ofassociation (DHas [kcal/mol] and the number of the activity sites created by harmaneinteracting with monomers studied (MAA, HEMA) in the most stable complexes.

Molecular system DEbind

(kcal/mol)DHas

(kcal/mol)Number ofactivity sites

Harmane/MAA H-1MAA �9.747 �9.276 2H-2MAA �14.363 �13.344 3H-3MAA �28.941 �27.867 2H-4MAA �37.703 �36.403 4

Harmane/HEMA H-1HEMA �6.505 �6.215 1H-2HEMA �17.604 �17.390 2H-3HEMA �23.096 �22.515 2H-4HEMA �25.670 �24.543 3

94 A. Kowalska et al. / Journal of Molecular Structure: THEOCHEM 901 (2009) 88–95

above-mentioned activity sites is equivalent to the number ofhydrogen bonds formed between the imprinted analyte and thefunctional monomers [22].

The DFT(B3LYP)/6-31G(d,p) calculated binding energy (DEbind

[kcal/mol]), enthalpy of association (DHas [kcal/mol]) and thenumber of activity sites created by template interacting withmonomers studied (MAA, HEMA) in the most stable complexesare gathered in Table 4.

Comparing the calculated binding energy values for 1:1 har-mane/MAA and 1:1 harmane/HEMA complexes, it should be notedthat the largest binding energy has been obtained for H-1MAAcomplex. Therefore, the results of our calculations suggest thatthe interaction between harmane pyridinic nitrogen atom andOH group of MAA is stronger than that involving pyridinic nitrogenand HEMA hydroxylic group. Additionally H-1MAA complex is sta-bilized by the presence of the weak interaction between harmanering CH group and carboxylic oxygen of MAA. Therefore the moststable 1:1 harmane/MAA complex provides two binding sites,while H-1HEMA complex-only one.

As far as 1:2 complexes are considered, now the situation is re-versed: the most stable is the complex formed between harmaneand two HEMA molecules. Giving the closer look to the geometriesof H-2MAA and H-2HEMA, this result is not surprising: complexH-2HEMA is more stable because this complex is dominated by atriple-hydrogen-bonded configuration, in which two HEMA mole-cules forming hydrogen bonds with two HB sites: pyridinic nitro-gen and pyrrolic group, respectively, are linked to each other byan additional hydrogen bond. It is proposed [32] that the protondonor and acceptor sites are mutually induced through a conju-gated triple hydrogen binding (CTHB) effect, resulting in a redistri-bution of electronic density from the pyrrolic group to the pyridinering of harmane. The linkage of hydrogen bonds with the combina-tion of CTHB effect stabilizes H-2HEMA complex, resulting in largerstabilization as compared to H-2MAA complex, in which each oftwo MAA molecules interacts individually with one of two hydro-gen bonding sites in harmane (pyrrolic group and pyridinic nitro-gen atom, respectively). The far separation between twohydrogen bonding sites in harmane molecule eliminates the possi-bility of forming 1:2 harmane/MAA complex of a triple-hydrogen-bonded configuration. Due to the smaller size of MAA moleculecomparing to HEMA molecule, at least three acid molecules areneeded for CTHB effect to take place in harmane/MAA system.The CTHB effect in H-3MAA contributes to the significant enhance-ment of the complex stability; the calculated binding energy forH-3MAA complex is higher than for H-3HEMA system.

Considering 1:4 complexes, the largest binding energy has beenobtained for 1:4 harmane/MAA complex (-37.703 kcal/mol), inwhich four activity sites may be found. For the most stable 1:4 har-mane/HEMA complex, H-4HEMA, the calculated binding energy ismuch lower (�25.670 kcal/mol) and there are also less activitysites.

Based on the above considerations, we may conclude that theinteractions between harmane and MAA are more strong and spe-cific comparing to the interactions between harmane and HEMA.

4. Conclusions

The knowledge of the intermolecular interactions in pre-poly-merization mixture is crucial for the rational design of the molec-ularly imprinted polymer specified for the studied b-carboline. Thequantity and quality of the recognition sites in the resulting MIP isa direct function of the mechanism and extent of the monomer-template interactions present in the pre-polymerization mixture.Therefore the strength and the nature of the intermolecular inter-actions involved in the pre-polymerization phase itself determinethe selectivity and affinity of the polymer product. A large numberof binding sites as well as a large binding energy between the tem-plate and the functional monomer results in a MIP of better affinityand selectivity.

In this study, we have applied the DFT(B3LYP)/6-31G(d,p)method to examine the intermolecular interactions between har-mane and the selected functional monomers (MAA, HEMA) com-monly used in molecular imprinting. Our purpose was todetermine which of the monomer studied interacts more stronglywith harmane. To achieve this goal possible configurations of 1:nharmane/functional monomer have been optimized with the useof the DFT(B3LYP)/6-31G(d,p) method and the most stable 1:n(n 64) harmane/monomer systems have been selected. The 1:4harmane/monomer molar ratio was considered in this study, be-cause this molar ratio is generally used in molecular imprinting.The geometrical parameters involving hydrogen bonding sites ofthe most stable interacting systems have been determined. Thebinding energies of the most stable associates have been calculatedtaking into account the BSSE and DZPVE corrections. The analysisof the changes in the calculated spectroscopic parameters of NHand OH stretching vibrations upon complexes formation revealedthe presence of the specific hydrogen bonded interactions in bothharmane/MAA and harmane/HEMA systems.

Finally, based on the conformational analysis as well as the cal-culated values of binding energy and enthalpy of association forthe most stable 1:4 harmane/monomer complexes, we have con-cluded that the interactions between harmane and MAA are stron-ger and provide more activity sites as compared to the interactionsbetween harmane and HEMA. Hence, the results of our calculationspredict that a MIP specified for harmane generated with MAA ismore likely to have greater rebinding and selectivity than a MIPmanufactured with HEMA. We hope that the results of our calcula-tions will be helpful for the rational design of MIP specified forharmane.

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