Keto‐enol tautomerism of the 4,5‐dimethyl‐2‐(2′‐hydroxyphenyl)imidazole in water solution: Modeling equilibrium between neutral forms and accurate assignment of the absorption bands

Abstract

Tautomerizarion between keto, cis‐ and trans‐enol forms of the 4,5‐dimethyl‐2‐(2′‐hydroxyphenyl)imidazole (DMHI) is studied using molecular dynamics simulations with the quantum mechanics/molecular mechanics (QM/MM) potentials. We applied the live solvent selection approach that presumes division of water molecules to either QM or MM subsystems on the fly depending on their distances from the chromophore. This allowed us to treat a water shell around the chromophore at the QM level and to describe properly all chromophore‐water interactions. According to the calculated Gibbs energy profiles the most stable is a keto form and a cis‐enol form is ~1 kcal/mol higher. The trans‐enol form is much higher in energy and should not be observed in experiment. Vertical S0,min − S1 excitation energies of all neutral tautomers were calculated at the XMCQDPT2 level: both enol forms have similar values ~4 eV and keto form is red‐shifted, with the excitation energy of 3.3 eV. Thus, we conclude that two absorption bands in experimental absorption spectrum of DMHI in water solution are due to the presence of cis‐enol and keto forms of the chromophore.

Imidazole-amino acids. Conformational switch under tautomer and pH change

“…The presence or absence of the NH group results in the different conformational properties and the formation of different hydrogen bonds pattern (Hamissa et al 2022 ). The environment or type and position of substituents influence the equilibrium of tautomers and their properties (Kusakiewicz-Dawid et al 2019 ; Podolyan et al 2003 ; Kapusta et al 2021 ; Alkorta and Elguero 2019 ). Despite the potential utility of imidazole-amino acids their conformational properties were not so far fully recognised.…”