tanko research group · 2020-03-16 · parkinson’s disease/parkinsonism clinical syndrome...
TRANSCRIPT
The two currently accepted mechanisms for MAO-catalyzed oxidations
TANKO RESEARCH GROUPWhat do Designer Drugs, Parkinson’s Disease, and the Persistent Radical Effect have in Common?Nathan Price (presenting)
Parkinson’s Disease/Parkinsonism
Clinical syndrome characterized by tremor, bradykinesia, rigidity, and postural instability
• Lewy bodies (abnormal aggregates of protein that develop inside nerve cells)
• Destruction of dopaminergic neurons in the substantia nigra
• Generally a disease that affects the elderly
N
O
CH3
CH2CH3
O Δ
NCH3
- HOAc
1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine
(MPTP)
• MPTP is lipophilic and can cross blood-brain barrier• Undergoes oxidation catalyzed by monoamine oxidase B to produce a potent
neurotoxin• Kills primarily dopamine-producing neurons in a part of the brain called the pars
compacta of the substantia nigra• MPTP used to simulate Parkinson’s disease to better understand etiology, develop
treatment strategies, etc.
What is a designer drug? A structural or functional analog of a controlled substance that has been designed to mimic the pharmacological effects of the original drug, while avoiding classification as illegal and/or detection in standard drug tests
N
Ph
CH3
Fl Fl
N
Ph
CH3
- H
N
Ph
CH3H
HH
N
Ph Fl Fl
N
Ph
N
Ph
HH
H
- H
N
Ph
HH
fast
SET Hypothesis for MAO oxidation of amines (Current Status)
• Paramagnetic intermediates derived from the flavin moiety have never been detected
• Chemical model studies have shown that rearranged products (and flavin-adducts) can arise via a polar, nucleophilic pathway
• Numerous experimental & computational studies strongly support other mechanisms
Polar mechanism:
Hydride transfer mechanism:
N
N
N
N O
OR
R
N
N
N
N O
OR
R
H2NCHPh
H
NH
N
N
HN O
OR
R
+ PhCH=NHPhCH2NH2
NH
N
N
N O
OR
R
+ PhCH=NH2
NH
N
N
HN O
OR
R
+ PhCH=NH
N
N
N
N O
OR
R
CH
PhH
NH2
A glaring omission from recent studies – none have considered the reaction of MAO with tertiary amines
Tertiary amines are not MAO substrates, with the exception of MPTP and itsstructural analogs are the only reported tertiary amines with MAO-B substrateproperties
This work: To ascertain the mechanism of reaction of chemical model compounds(biomimetics) with an MPTP derivative
MMTP: 1-methyl-4-(1-methyl-1H-pyrrol-2-yl)-1,2,3,6-tetrahydropyridine
N CH3NCH3
MMTP
N
N
N
N
O
O
N
N
N
N
O
O
CH3CH2
ClO4
3MLF Fl+
Reaction of MMTP with Fl+ gave rise to two principal products, in nearly quantitative yield: 1) the expected oxidation product MMP+ and 2) MMTPH+, the protonated form of the starting material MMTP.
Two small molecules, molecular oxygen (O2) and water (H2O) play critical roles in this chemistry
Water (in CH3CN) introduces a ”sideshow” equilibrium. This reaction needs to be studied under rigorously dry conditions and in a glovebox.
N CH3 N CH3
MMTP MMP+
NCH3
HN N NCH3CH3CH3
Fl +
MMTPH+
N
N
N
N
O
O
CH3CH2
Fl+ + MMTP + H2O
HOMMTPH+
N
C6H6N
H CH3
+
Headline News: It works!
5.56.06.57.07.58.08.59.0f1(ppm)
1
2
3
4
5
6
15min
30min
60min
90min
120min
180min
N CH3 N CH3
MMTP MMP+
NCH3
HN N NCH3CH3CH3
Fl +
MMTPH+
Reaction conducted in the presence of O2, monitored by 1HNMR (d 5.5 – 9.0)
MMTPH+
MMP+
N
N
6.06.57.07.58.08.5f1(ppm)
1
2
3
4
12days
17days
23days
afteradditionofair
Reaction conducted under anaerobic conditions: “Silent” NMR – only MMTPH+ is detected
The final two pieces of the puzzle: The UV/Vis spectrum of the reaction of MMTP and Fl+ (anaerobic conditions) ...and a heartbeat! The EPR spectrum of the reaction of MMTP and Fl+ (anaerobic conditions)
0. 0
0. 5
1. 0
1. 5
2. 0
300 340 380 420 460 500 540 580 620 660 700 740 780
Abs
orba
nce
Wavelength (nm)
B
N
N
N
N
O
O
CH3CH2N
N
N
N
CH3CH2 O
O
Fl+ Fl
e-
- 600
- 400
- 200
0
200
400
600
• SET is back on the menu as a viable mechanism for the MAO catalyzed oxidation of tertiary amines such as MPTP. Failure to detect this pathway in previous studies is because other mechanisms operate for primary and secondary amines.
• The ability to form a stabilized radical such as MMTP•, which drives the SET process, is unique to allylic tertiary amines such as MPTP and derivatives.
Conclusions and implications
Fl +
+ Fl
N
Ar
HH
N
Ar
H
N
Ar
HN
Ar
H
O2
O2
N
Ar
+ H+
O2H2O2
N
Ar
H
+ FlO2 H2O22 H+ +
• Some mechanism-based MAO-B inhibitors such as the propargylic aminespargyline and selegiline (deprenyl) possess similar functionality, and thus, mayalso operate via a single electron transfer process.
• While the role of O2 in completing the catalytic cycle for MAO is known, apossible role of O2 in the early steps of the mechanism has not been considered.Future studies of MAO catalysis under anaerobic conditions are needed.
N
pargyline
N
deprenyl
Mechanistic studies with 5-ethyl-3-methyllumiflavinium (Fl+) perchlorate, a biomimetic model for flavoenzyme monoamine oxidase B (MAO-B) catalysis, and the tertiary, allyl amine 1-methyl-4-(1-methyl-1H-pyrrol-2-yl)-1,2,3,6-tetrahydropyridine (MMTP) reveal that proton coupled electron transfer (PCET) may be an important pathway for MAO catalysis. The first step involves single electron transfer (SET) leading to the free radicals Fl• and MMTP•, the latter produced by deprotonation of the initially formed and highly acidic MMTP•+. Molecular oxygen (O2) is found to play a hitherto unrecognized role in the early steps of the oxidation. MMTP and several structurally similar tertiary amines are the only tertiary amines oxidized by MAO, and their structural/electronic properties provide the key to understanding this behavior. A general hypothesis about the role of SET in MAO catalysis, and the recognition that PCET occurs with appropriately substituted substrates is presented.
+
+
MMTPH+