two new iridoids from selected penstemon species – antimicrobial...

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Two new iridoids from selectedPenstemon species – antimicrobialactivitySybilla M. Zajdel a , Konstantia Graikou b , Georgios Sotiroudis c ,Kazimierz Głowniak a & Ioanna Chinou b

a Department of Pharmacognosy with Medicinal Plant Unit ,Faculty of Pharmacy, Medical University of Lublin , Lublin , Polandb Department of Pharmacognosy and Chemistry of NaturalProducts , School of Pharmacy, University of Athens , Athens ,Greecec National Hellenic Research Foundation, Institute of BiologicalResearch and Biotechnology , Athens , GreecePublished online: 05 Sep 2013.

To cite this article: Sybilla M. Zajdel , Konstantia Graikou , Georgios Sotiroudis , KazimierzGłowniak & Ioanna Chinou , Natural Product Research (2013): Two new iridoids from selectedPenstemon species – antimicrobial activity, Natural Product Research: Formerly Natural ProductLetters, DOI: 10.1080/14786419.2013.825913

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Two new iridoids from selected Penstemon species – antimicrobial activity

Sybilla M. Zajdela, Konstantia Graikoub, Georgios Sotiroudisc, Kazimierz Głowniaka and

Ioanna Chinoub*

aDepartment of Pharmacognosy with Medicinal Plant Unit, Faculty of Pharmacy, Medical Universityof Lublin, Lublin, Poland; bDepartment of Pharmacognosy and Chemistry of Natural Products,School of Pharmacy, University of Athens, Athens, Greece; cNational Hellenic Research Foundation,Institute of Biological Research and Biotechnology, Athens, Greece

(Received 28 January 2013; final version received 7 June 2013)

Eighteen secondary metabolites, belonging to three different chemical groups, wereisolated from the methanolic extracts of the aerial parts of selected penstemon plants[Penstemon fruticosus (Pursh) Greene var. fruticosus, Penstemon palmeri Gray andPenstemon venustus Doug. ex Lindl.], and their structures were elucidated on the basisof spectral evidence. Six iridoid glucosides (1–6), three phenylpropanoid glucosides(13–15) and two acetophenone derivatives (16,17), obtained from P. fruticosus, fiveiridoids (2, 7–10), one phenylpropanoid glucoside (15) and two acetophenones(16, 18), isolated from P. palmeri while three iridoids (2, 11, 12) and threephenylpropanoids (13–15) were identified in P. venustus. Two of the iridoid glucosides(4, 5) from P. fruticosus are new natural products named accordingly as cis- and trans-forms of 10-O-p-methoxycinnamoylaucubin. All isolated compounds, as well as crudemethanolic extracts, were evaluated for their antimicrobial activities against six Gram-positive and Gram-negative bacteria and three human pathogenic fungi.

Keywords: Penstemon; 10-O-p-methoxycinnamoylaucubin; iridoid glucosides;phenylpropanoid glucosides; acetophenone derivatives; antimicrobial activity

1. Introduction

In continuation of our phytochemical studies on Penstemon species (Zajdel et al. 2012),

we present results of the isolation and structure determination of compounds obtained from

three Penstemon species grown and collected in their natural habitat (USA), namely

Penstemon fruticosus (Pursh) Greene var. fruticosus, Penstemon palmeri Gray and Penstemon

venustus Doug. ex Lindl. (Plantaginaceae). The antimicrobial profiles of the extracts and the

isolated compounds were further studied against Gram-positive, Gram-negative bacteria and

pathogenic fungi.

Penstemons, native plants of the North America Continent, belonging to the genus

Penstemon, were widely used by Native Americans as medicinal plants (Moerman 1998).

Previous phytochemical research on penstemons showed dominant presence of various types of

iridoids and their glucosides (Stermitz et al. 1994; Franzyk et al. 1998), phenylpropanoid

glucosides (Foderaro & Stermitz 1992; Ismail et al. 1995) as well as acetophenone derivatives

(Junior 1986; Foderaro & Stermitz 1992; Stermitz et al. 1994).

To our best knowledge, this research on chemical composition of the aerial parts (herb) of

P. fruticosus var. fruticosus, P. palmeri and P. venustus is undertaken for the first time, as

previously only chemical composition of the roots of P. venustus (Roth et al. 1995) and a single

q 2013 Taylor & Francis

*Corresponding author. Email: [email protected]

Natural Product Research, 2013

http://dx.doi.org/10.1080/14786419.2013.825913

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http://dx.doi.org/10.1080/14786419.2013.825913

report dedicated to the leaves from P. fruticosus ssp. fruticosus (Ofterdinger-Daegel & Junior

1993) were referred.

2. Results and discussion

2.1. Identification of the isolated compounds

The aerial parts of three investigated Penstemon species led to the isolation and identification of

18 secondary metabolites. In the methanolic extract of P. fruticosus, six iridoid glucosides were

identified, namely aucubin (1) (Foderaro & Stermitz 1992), geniposidic acid (2) (Guvenalp et al.

2006), 8-epiloganin (3) (Bianco & Passacantilli 1981), 10-O-foliamenthoylaucubin (6) (Justice

et al. 1992) as well as a mixture of two new iridoid glucosides 10-O-cis-p-methoxycinnamoy-

laucubin (4) and 10-O-trans-p-methoxycinnamoylaucubin (5) (Figure 1).

O

OGlcHO

OHO

(2)

O

H3C OGlc

OH3CO

HO

H

H

(3)

O

H3C OGlc

OHO

(7)

O

HO

ROH

H

OGlc

O

OCH3R = H (1)R = cis-p-methoxycinnamoyl (4)R = trans-p-methoxycinnamoyl (5)R = foliamenthoyl (6)

O

H3C

OHO

OO

HO

HOOH OR O

OHR = trans-coumaroyl (8)R = cis-coumaroyl (9)R = foliamenthoyl (10)

OOH

O

O CH3

CH3O

HOH

H

O

OHOH

OGlc

R:

OR

R: Glc (11)

(12)

OO

OH

OHOO

O

HO

OH

ORO

OHHOHOH3C

RO

R:CH3 7"'-8"' = trans (13)R:CH3 7"'-8"' = cis (14)R:H 7"'-8"' = trans (15)

7'"

8'"

OH3C

OR1

R1:Glc, R2:H (16)R1:Glc, R2:OCH3 (17)R1, R2:H (18)

R2

Figure 1. Structure of compounds 1–18.

2 S.M. Zajdel et al.

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The structure of these two compounds was similar to that of p-hydroxycinnamoyl aucubin

(Kouno et al. 1988) by comparison of spectral data. The 1H NMR and the 13C NMR spectra

revealed very good agreement with the respective data recorded for hydroxyl derivative

regarding the iridoid skeleton and the glucoside. It indicated the presence of cinnamoyl group

esterified with the 10-CH2O group of the iridoid skeleton and glycosidation at C-1 (98.3 ppm)

(according to the HMBC experiments), but it also indicated the presence of a methoxy group

(56.3 ppm) at para-position of the cinnamoyl skeleton. It is also comparable with the structure of

6-O-( p-methoxycinnamoyl)-aucubin (Eribekyan et al. 1989).

Moreover, three phenylpropanoid glucosides: martynoside (13) (Teborg & Junior 1989) in a

mixture with cis-martynoside (14) (Skrzypek et al. 1999) and verbascoside (15) (Gvazava &

Kikoladze 2007), and two acetophenone derivatives, namely picein (16) (Delvas et al. 2011) andandrosin (17) (De Rosa et al. 1996) were identified in this plant.

In the methanolic extract of P. palmeri, the presence of five iridoid glucosides was stated:

plantarenaloside (7) (Damtoft et al. 1981), 20-O-Coumaroyl-plantarenaloside (8) and (9) (cis-

and trans-isomer) (Stermitz et al. 1994), 20-O-foliamenthoyl-plantarenaloside (10) (Stermitz

et al. 1994) and geniposidic acid (2). In addition, p-hydroxyacetophenone (18) (Kwon & Lee

1995) and picein (16), together with verbascoside (15), were identified in this plant.

In a methanolic extract of P. venustus the following three iridoid glucosides were identified:

geniposidic acid (2), penstemide (11) (Gering et al. 1986) and serrulatoside (12) (Junior 1984),as well as three phenylpropanoid glucosides, verbascoside (15) and martynoside (13) in a

mixture with cis-martynoside (14).

Among the isolated compounds, except of the two new iridoid glucosides, it is also

noteworthy that 10-O-foliamenthoylaucubin is found for the first time in Plantaginaceae family,

as it has been previously isolated only once from Cordylanthus species (Scrophulariaceae,

re-classified to Orobanchaceae) (Justice et al. 1992). It is also remarkable that Penstemon genus

previously belonged to Scrophulariaceae family and recently re-classified to Plantaginaceae.

Moreover, once they were previously referred in the genus: 20-foliamenthoyl-plantarenaloside in

Penstemon barrettiae (Stermitz et al. 1994) and penstemide in Penstemon gentianoides

(Domınguez et al. 2011). 20-O-coumaroyl-plantarenaloside was previously found in the genus

(as a mixture of cis- and trans-isomers) (Stermitz et al. 1994; Rodriguez-Loaiza et al. 2003) and

serrulatoside in Penstemon serrulatus (Junior 1984; Bazylak et al. 1996) and in Penstemon

richardsonii (Gering-Ward &Wichtl 1988). The most commonly identified iridoid glucosides in

penstemons are plantarenaloside (Lira-Rocha et al. 1987; Arslanian & Stermitz 1991; Stermitz

et al. 1994; Ismail et al. 1995; Zhou et al. 1998; Domınguez et al. 2007, 2010, 2011), aucubin

(Abdel-Kader & Stermitz 1993; Konig & Karlheinz 1995; Bazylak et al. 1996), 8-epiloganin

(Foderaro & Stermitz 1992; Stermitz et al. 1993, 1994; Vesper & Seifert 1994) and geniposidic

acid (Arslanian & Stermitz 1991; Stermitz et al. 1994).

Among the isolated phenylpropanoid glucosides, several were previously reported in certain

Penstemon species such as verbascoside (Arslanian & Stermitz 1991; Stermitz et al. 1994;

Domınguez et al. 2011; Zajdel et al. 2012), picein (Junior 1986; Stermitz et al. 1994) and

martynoside (Teborg & Junior 1989; Domınguez et al. 2011; Zajdel et al. 2012), whereas

cis-martynoside was previously reported as a mixture with martynoside from P. serrulatus

(Skrzypek et al. 1999) during cell culture. The presence of the acetophenone derivative

p-hydroxyacetophenone is reported for the first time in Penstemon genus, whereas androsin has

been reported once before in Penstemon pinifolius (Junior 1986).

2.2. Antimicrobial assay

Even though Penstemon species are still not well studied phytochemically, they have been used

traditionally, as it was referred to possess several biological properties, including anti-oxidant

Natural Product Research 3

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and anti-inflammatory properties. Extracts and pure compounds of P. gentianoides and

Penstemon campanulatus (Domınguez et al. 2010, 2011), as well as Penstemon roseus (Garcıa-

Rodrıguez et al. 2011), Penstemon barbatus and P. campanulatus (Moreno-Escobar et al. 2011),

have revealed anti-oxidant and cytotoxic activities. Moreover, some very interesting results have

been published recently from several compounds (phenylethanoid glycosides as well as iridoid

glycosides) isolated from Penstemon centranthifolius, showing significant inhibition of the

formation of bacterial biofilms by Escherichia coli UTI89 (Ye et al. 2010).

The methanolic extracts of the studied penstemons together with the pure isolated

compounds have been tested for their antimicrobial activities against two Gram-positive

bacteria, four Gram-negative bacteria and three human pathogenic fungi. The antibacterial

studies showed that the crude methanolic extracts of P. palmeri and P. fruticosus among the

assayed samples exhibited the strongest antibacterial activity (minimal inhibition concentration

(MIC) values 0.98–1.28mg/mL) and a weak antifungal activity. The isolated acetophenones

(16–18) appeared as the most active among the pure assayed constituents, against all tested

microorganisms (bacteria and fungi with MIC values 0.60–1.95mg/mL). It is noteworthy that

iridoids showed stronger activities against Gram-positive bacteria, moderate activities against

Gram-negative, whereas they were almost inactive against the tested fungi, which is in

accordance with previous antimicrobial studies of iridoids and iridoid glucosides (Graikou et al.

2002; Christopoulou et al. 2008; Zajdel et al. 2012) (Table 1).

3. Experimental

3.1. General1H NMR (400MHz, CDCl3 or CD3OD) and

13C NMR (50MHz, CDCl3 or CD3OD) spectra were

recorded on Bruker DRX400 (Bruker BioSpin, Rheinstetten, Germany) and Bruker AC200

(Bruker, Germany) spectrometers, respectively. Tetramethylsilane was used as an internal

standard, d in ppm, J in Hz. 2D NMR spectra were recorded using standard Bruker

microprograms.

3.2. Plant material

Aerial parts (inflorescence included) of P. fruticosus (Pursh) Greene var. fruticosus, P. palmeri

Gray and P. venustus Doug. ex Lindl. were collected and identified in September 2009 by

Mr James Swayne from American Penstemon Society and Washington Native Plant Society,

in private plant collection in Walla Walla, Washington, USA. Voucher specimens of the

investigated plants (P. fruticosus: PFP150909JSWA-1, P. palmeri: PP150909JSWA-2,

P. venustus: PV150909JSWA-4) have been deposited in the Chair and Department of

Pharmacognosy, Medical University of Lublin, Poland.

3.3. Extraction and isolation

Air-dried, powdered plant material (P. fruticosus var. fruticosus – 200.0 g, P. palmeri – 43.95 g,

P. venustus – 70.03 g) was extracted by 24-h maceration at room temperature with MeOH

(P. fruticosus var. fruticosus – 1000mL, P. palmeri – 300mL, P. venustus – 500mL) followed

by ultrasound extraction for 30min at 408C. The procedure was repeated three times. The

obtained methanolic extracts of each plant were combined and evaporated to dryness, giving dry

residues named as PfM (55.04 g), PpM (14.91 g) and PvM (12.98 g).

The extract PfM (55.04 g) was separated by silica gel vacuum liquid chromatography (VLC)

eluted by organic solvents with increasing polarities: cyclohexane:CH2Cl2 (100:0 ! 0:100),

CH2Cl2:ethyl acetate (100:0 ! 0:100) and ethyl acetate:MeOH (100:0 ! 0:100), giving 14


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Table

1.Resultsoftheantimicrobialactivityoftheisolatedcompoundsandthemethanolicextractsoftheinvestigated

plants(M

ICvalues

inmg/m

L).

Testedsamples

S.aureus

S.epidermidis

P.aeruginosa

E.cloacae

K.pneumoniae

E.coli

C.albicans

C.tropicalis

C.glabrata

P.venustus

1.34

1.39

1.25

1.48

1.50

1.47

2.93

2.45

2.54

P.palmeri

0.99

1.05

1.10

1.14

1.23

1.28

2.30

2.24

2.20

P.fruticosus

0.98

1.00

1.04

1.09

1.18

1.23

2.32

2.26

2.15

Aucubin

(1)

1.12

1.10

1.44

1.72

1.80

1.92

2.98

2.80

2.77

Geniposidic

acid

(2)

0.94

0.85

1.10

1.22

1.53

1.97

2.59

2.28

2.10

Loganin

(3)

0.92

0.98

1.72

1.65

1.89

1.53

3.12

2.74

2.70

10-O

-p-M

ethoxycinnam

oyl-

aucubin

(41

5)

1.12

1.10

1.36

1.90

1.94

1.82

2.94

2.76

2.70

10-O

-Foliam

enthoylaucubin

(6)

0.99

1.12

1.45

1.84

1.89

1.75

2.69

2.65

2.62

Plantarenaloside(7)

1.13

1.36

1.86

1.44

1.90

2.15

.10

.10

.10

20 -O

-trans-Coumaroyl-plantare-

naloside(8)

0.98

1.04

1.47

1.26

1.62

2.06

.5

.5

.5

20 -O

-cis-Coumaroyl-plantarena-

loside(9)

1.12

1.33

1.72

1.46

1.97

2.24

.5

.5

.5

20 -O

-Foliam

enthoyl-plantarena-

loside(10)

1.15

1.21

1.72

1.35

1.76

2.12

.5

.5

.5

Penstem

ide(11)

1.18

1.15

1.58

1.32

1.79

2.00

.5

.5

.5

Serrulatoside(12)

1.10

1.18

1.70

1.38

1.82

1.97

.5

.5

.5

Martynoside(13þ

14)

0.98

1.05

1.24

1.32

1.78

1.90

2.90

2.48

2.34

Verbascoside(15)

1.04

1.00

1.73

1.78

1.96

1.88

2.95

2.76

2.64

Picein(16)

0.65

0.70

1.00

0.98

1.10

1.14

1.94

1.75

1.77

Androsin(17)

0.60

0.68

0.97

0.85

1.00

1.08

1.85

1.70

1.65

p-H

ydroxy-acetophenone(18)

0.90

0.86

1.00

0.93

0.98

1.24

1.95

1.87

1.80

Netilmicin

3.8

£1023

3.95£1023

8.5

£1023

7.8

£1023

7.9

£1023

10.2

£1023

NT

NT

NT

AmphotericinB

NT

NT

NT

NT

NT

NT

0.80£1023

0.4

£1023

0.45£1023

Note:NT,nottested.


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fractions. Further separations yielded the iridoid glucosides aucubin (1) (16.7mg), geniposidic

acid (2) (2.9mg), 8-epiloganin (3) (11.7mg), 10-O-cis-p-methoxycinnamoylaucubin (4) in a

mixture with 10-O-trans-p-methoxycinnamoylaucubin (5) (2.0mg) and 10-O-foliamenthoylau-

cubin (6) (8.5mg), a mixture of martynoside and cis-martynoside (13, 14) (7.1mg), verbascoside

(15) (21.1mg), picein (16) (2.3mg) and androsin (17) (2.5mg).

A part of extract PpM (4.53 g) was separated by silica gel VLC eluted by organic solvents

with increasing polarities: cyclohexane:CH2Cl2 (100:0 ! 0:100) and CH2Cl2:MeOH

(95:5 ! 0:100), giving 18 fractions. Further separations yielded plantarenaloside (7), 20-O-trans-coumaroyl-plantarenaloside (8) (3.0 mg), 20-O-cis-coumaroyl-plantarenaloside (9)

(17.9mg), 20-O-foliamenthoyl-plantarenaloside (10) (9.2mg), verbascoside (15) (5.6mg) picein

(16) (4.2mg) and p-hydroxyacetophenone (18) (8.3mg). Reversed-phase preparative thin layer

chromatography fractionation (Merck KGaA, Darmstadt, Germany); Silica Gel RP-18 F254sglass plates) in MeOH:H2O (1:1) yielded geniposidic acid (2) (2.8mg).

A part of extract PvM (6.04 g) was separated by silica gel VLC eluted by organic solvents

with increasing polarities: CH2Cl2:MeOH (100:0 ! 0:100), giving 15 fractions. Further

separation by classic column chromatography yielded penstemide (11) (8.8mg), serrulatoside

(12) (12.7mg), a mixture of martynoside (13) and cis-martynoside (14) (2.5mg) and

verbascoside (15) (2.1mg). Medium pressure liquid chromatography (RP-18) eluted by solvents

with decreasing polarities: H2O:MeOH (100:0 ! 0:100) pressed by two Series 1 pumps with

flow speed 8mL/min yielded geniposidic acid (2) (10.2mg).

10-O-cis-p-Methoxycinnamoylaucubin (4) in a mixture with 10-O-trans-p-methoxycinna-

moylaucubin (5): white amorphous powder.

10-O-cis-p-Methoxycinnamoylaucubin: 1H NMR (400MHz, CD3OD); d 7.71 (2H, d,

J ¼ 8.9Hz, H-200 and H-600), 6.97 (1H, d, J ¼ 12.5 Hz, H-b), 6.92 (2H, d, J ¼ 8.9Hz, H-300 andH-500), 6.36 (1H, dd, J ¼ 6.3 and 2.0Hz, H-3), 5.90 (1H, d, J ¼ 12.5 Hz, H-a), 5.83 (1H, br s,

H-7), 5.10 (1H, dd, J ¼ 6.1 and 4.0Hz, H-4), 4.95 (1H, d, J ¼ 7.6Hz, H-1), 4.92 (1H, d,

J ¼ 15.4 Hz, H-10a), 4.76 (1H, obscured by solvent peak, H-10b), 4.72 (1H, d, J ¼ 7.6Hz,

H-10), 4.46 (1H, br s, H-6), 3.87 (1H, dd, J ¼ 11.7 and 1.6Hz, H-60a), 3.85 (3H, s,ZOCH3), 3.64

(1H, dd, J ¼ 11.7 and 5.3Hz, H-60b), 2.96 (1H, t, J ¼ 7.2Hz, H-9), 2.70 (1H, m, H-5); 13C NMR

(50MHz, CD3OD); d 169.0 (ZCO), 162.9 (C-400), 145.4 (C-8), 143.2 (C-b), 142.2 (C-3), 133.4

(C-200 and C-600), 133.0 (C-7), 128.7 (C-100), 117.7 (C-a), 114.9 (C-300 and C-500), 106.0 (C-4),

100.6 (C-10), 98.3 (C-1), 83.3 (C-6), 63.8 (C-10), 63.1 (C-60), 56.3 (ZOCH3), 46.6 (C-5

and C-9).

10-O-trans-p-Methoxycinnamoylaucubin: 1H NMR (400MHz, CD3OD); d 7.71 (1H, d,

J ¼ 16.0 Hz, H-b), 7.56 (2H, d, J ¼ 8.9Hz, H-200 and H-600), 6.98 (2H, d, J ¼ 8.9Hz, H-300 andH-500), 6.45 (1H, d, J ¼ 16.0Hz, H-a), 6.36 (1H, dd, J ¼ 6.3 and 2.0Hz, H-3), 5.83 (1H, br s,

H-7), 5.10 (1H, dd, J ¼ 6.1 and 4.0Hz, H-4), 4.95 (1H, d, J ¼ 7.6Hz, H-1), 4.92 (1H, d,

J ¼ 15.4 Hz, H-10), 4.76 (1H, obscured by solvent peak, H-10b), 4.72 (1H, d, J ¼ 7.6Hz, H-10),4.46 (1H, br s, H-6), 3.87 (1H, dd, J ¼ 11.7 and 1.6Hz, H-60a), 3.85 (3H, s,ZOCH3), 3.64 (1H,

dd, J ¼ 11.7 and 5.3Hz, H-60b), 2.96 (1H, t, J ¼ 7.2Hz, H-9), 2.70 (1H, m, H-5); 13C NMR

(50MHz, CD3OD); d 169.0 (ZCO), 163.9 (C-400), 145.4 (C-8), 147.0 (C-b), 142.2 (C-3), 133.0

(C-7), 131.4 (C-200 and C-600), 128.7 (C-100), 116.3 (C-a), 115.9 (C-300 and C-500), 106.0 (C-4),

100.6 (C-10), 98.3 (C-1), 83.3 (C-6), 63.8 (C-10), 63.1 (C-60), 56.3 (-OCH3), 46.6 (C-5 and C-9).

3.4. Microorganism cultures

A panel of microorganisms, including two Gram-positive bacteria: Staphylococcus aureus

(ATCC 25923) and S. Staphylococcus epidermidis (ATCC 12228); four Gram-negative bacteria:

E. coli (ATCC 25922), Enterobacter cloacae (ATCC 13047), Klebsiella pneumoniae (ATCC

13883) and Pseudomonas aeruginosa (ATCC 227853) and three pathogenic fungi: Candida


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albicans (ATCC 10231), Candida tropicalis (ATCC 13801) and Candida glabrata (ATCC

28838) were used. Standard antibiotics netilmicin and amphotericin B were used in order to

control the tested bacteria and fungi.

3.5. Determination of antibacterial activity

Antimicrobial activity of the crude methanolic extracts obtained from both species and the

isolated compounds was determined using the agar dilution technique (Popova et al. 2009). For

all assays, stock solutions of the tested samples were prepared at 1mg/mL. Serial dilutions of the

stock solutions in broth medium (100mL of Muller–Hinton broth or on Sabouraud broth for the

fungi) were prepared in a microtitre plate (96 wells). Then 1mL of the microbial suspension (the

inoculum, in sterile distilled water) was added to each well. For each strain, the growth

conditions and the sterility of the medium were checked, and the plates were incubated as

referred above. MICs were determined as the lowest concentrations preventing visible growth.

Standard antibiotic netilmicin (at concentrations 4–88mg/mL) was used in order to control the

sensitivity of the tested bacteria, whereas amphotericin B (at concentrations 0.4–1mg/mL) was

used as control against the tested fungi (Sanofi Diagnostics Pasteur, Marnes-La Coquettes,

France, at concentrations of 30, 15 and 10mg/mL). For each experiment, any pure solvent used

was also applied as blind control. The experiments were repeated three times and the results

were expressed as average values (Table 1).

4. Conclusions

From three selected penstemon plants: P. fruticosus var. fruticosus, P. palmeri and P. venustus,

18 secondary metabolites were isolated and structurally determined, belonging to iridoids

(1–12), phenylpropanoids (13–15) and acetophenone derivatives (16–18). Two of the iridoid

glucosides (4, 5) isolated from P. fruticosus are new natural products named as cis- and trans-

forms of 10-O-p-methoxycinnamoylaucubin. All isolated compounds, as well as crude

methanolic extracts, were evaluated for their antimicrobial activities against six Gram-positive

and Gram-negative bacteria, and three human pathogenic fungi. The crude extracts of P. palmeri

and P. fruticosus exhibited the strongest antibacterial activity, and among the tested secondary

metabolites, acetophenones showed the stronger antimicrobial profile.

Supplementary material

Supplementary material relating to this article is available online, alongside Figures S1–S5,

showing the NMR spectra of the two new compounds.

Acknowledgements

The authors would like to thank Mr James Swayne from American Penstemon Society and WashingtonNative Plant Society for collection, identification and provision of investigated plant material.

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two new iridoids from selected penstemon species – antimicrobial...

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