Microsolvation of cysteine: a density functional theory study.

Microsolvation of the neutral, zwitterion, and unconventional zwitterion (formed by the proton transfer from the thiol to the amine group) was performed using PBE1PBE/6-311+G(d,p) calculations. A large sampling of the configurations of the clusters involving one to six water molecules was created by analogy to glycine clusters and through analysis of hydrogen-bonding trends. Clusters of the neutral tautomer are lowest in energy with the inclusion up to five water molecules. With six water molecules the neutral and zwitterion are nearly isoenergetic. The unconventional zwitterion, while a stable structure when at least one water molecule is associated with it, remains energetically noncompetitive with the other two tautomers regardless of the degree of microsolvation.