Theoretical study on the N-demethylation mechanism of theobromine catalyzed by P450 isoenzyme 1A2

• The N-demethylation mechanisms of theobromine catalyzed by CYP1A2 were explored using the unrestricted B3LYP theory.
• The rate-limiting N-methyl hydroxylation occurs in a HAT mechanism.
• The carbinolamines generated are prone to decomposition through the contiguous heteroatom-assisted proton transfer.
• The N-demethylation reaction proceeds through a SSM mechanism and HS is more favorable than LS.
• 3-N demethylation is kinetically more feasible than 7-N demethylation and 7-methylxanthine is the optimum metabolite of theobromine.

Theobromine, a widely consumed pharmacological active substance, can cause undesirable muscle stiffness, nausea and anorexia in high doses ingestion. The main N-demethylation metabolic mechanism of theobromine catalyzed by P450 isoenzyme 1A2 (CYP1A2) has been explored in this work using the unrestricted hybrid density functional method UB3LYP in conjunction with the LACVP(Fe)/6-31G (H, C, N, O, S, Cl) basis set. Single-point calculations including empirical dispersion corrections were carried out at the higher 6-311++G** basis set. Two N-demethylation pathways were characterized, i.e., 3-N and 7-N demethylations, which involve the initial N-methyl hydroxylation to form carbinolamines and the subsequent carbinolamines decomposition to yield monomethylxanthines and formaldehydes. Our results have shown that the rate-limiting N-methyl hydroxylation occurs via a hydrogen atom transfer (HAT) mechanism, which proceeds in a spin-selective mechanism (SSM) in the gas phase. The carbinolamines generated are prone to decomposition via the contiguous heteroatom-assisted proton-transfer. Strikingly, 3-N demethylation is more favorable than 7-N demethylation due to its lower free energy barrier and 7-methylxanthine therefore is the optimum product reported for the demethylation of theobromine catalyzed by CYP1A2, which are in good agreement with the experimental observation. This work has first revealed the detail N-demethylation mechanisms of theobromine at the theoretical level. It can offer more significant information for the metabolism of purine alkaloid.

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