o-Quinone amines, which are relevant to various biological processes, can undergo spontaneous intramolecular cyclization (ring closure reaction by amino-terminated hydrocarbon side chain) that deactivates them toward another possible reactions, that is, thiol binding. Density functional theory-based calculation is employed for obtaining the potential energy curves along the CN bond formation in the intramolecular cyclization of various o-quinone amines, viz., dopaminequinone, dopaquinone, N-methyl-dopaminequinone, N-formyl-dopaminequinone, and the corresponding methylene-inserted analogues. The activation barrier is decreased by introduction of α-carboxylate and N-methyl group whereas increased by introduction of N-formyl group. A negative correlation between the activation barriers and the level of highest occupied molecular orbital is pointed out. Furthermore, the methylene-inserted analogues show decreased activation barriers. This is explained by reduction of steric repulsion in the transition state.
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