A theoretical study on reaction pathways to carbanions

Area of Science:

• Computational Chemistry
• Biochemistry
• Enzyme Mechanisms

Background:

• Carbanion formation is crucial in many enzymatic reactions.
• Semicarbazide-sensitive amine oxidases (SSAO) catalyze reactions involving amine substrates.
• Understanding the initial proton transfer step is key to elucidating SSAO mechanisms.

Purpose of the Study:

• To investigate the conditions favoring carbanion formation by proton transfer to methylamine.
• To computationally model the reaction mechanism relevant to semicarbazide-sensitive amine oxidases.
• To assess the impact of various substituents on the proton transfer barrier.

Main Methods:

• Density Functional Theory (DFT) calculations at SCF/3-21G and 6-31G* levels.
• Investigation of potential energy surfaces for methylamine approaching a substituted methylene group.
• Inclusion of MP2 correlation correction for enhanced accuracy.
• Modeling of substrate-cofactor Schiff base complexes.

Main Results:

• Proton transfer barriers were calculated for different substituents (NO2, C6H5, NH2, NH-CH=CH-CHO).
• Electron-withdrawing groups like nitro and phenyl substantially increased the proton transfer barrier.
• A model Schiff base complex yielded a barrier comparable to phenyl-substituted systems.
• Substitutions on the pyridoxal ring and cofactor stability were preliminarily explored.

Conclusions:

• The nature of substituents on the methylene group significantly influences carbanion formation energetics.
• High barriers suggest that direct proton transfer to methylamine might not be the primary mechanism in all SSAO-catalyzed reactions.
• Computational modeling provides valuable insights into enzyme reaction pathways and cofactor interactions.