Analysis of the relative stability of cis-urocanic acid in condensed phase.: The use of Langevin dipoles

A series of ab initio calculations employing a modified Langevin dipoles method to model neutral, cationic, and anionic cis-urocanic acid in human skin is presented. A comparison between the stability of the conformers in gas phase and in a condensed phase is performed. In particular, the energy barrier and transition state of the isomerization reaction of the anionic forms of cis-urocanic acid have been characterized. The modifications of the Langevin dipoles method, a procedure to obtain the required model parameters, and model verifications are presented. The latter include computing the water solvation energy and the free energy of the water dissociation.