Hydration structure and conformational dynamics of urocanic acid: a computer simulation study

Abstract

Urocanic acid, a potentially photocarcinogenic chromophore, has been studied in its trans and cis forms, and in its zwitterionic protomer in aqueous solution, based on molecular dynamics computer simulations. The hydration structure around the two conformers and the effect of the aqueous environment and protonation form on the conformational structure and dynamics have been investigated. It was found that explicit solvent effects and correct protonation form are crucial for a description of the dynamics and relative energies of the conformers, qualitatively changing the results from previous ab initio calculations corresponding to vacuum conditions. In particular, it was found that it is likely that a population of different rotamers is present under the experimental conditions of recent gas phase experiments. For the cis isomer, frequently the intramolecular hydrogen bond was broken, due to water molecules from solvent competing as hydrogen bond acceptors, and this led to a destabilizing effect, making this isomer more floppy than the trans form.