Path-integral molecular dynamics simulations of glycine-(HO) ( = 1-7) clusters on semi-empirical PM6 potential energy surfaces

Yoshikawa, Takehiro*; Motegi, Haruki*; Kakizaki, Akira*; Takayanagi, Toshiyuki*; Shiga, Motoyuki   

Path-integral molecular dynamics simulations for the hydrogen-bonded glycine-(HO) ( = 1-7) clusters have been carried out using an on-the-fly direct dynamics technique at the semiempiricalPM6 level of theory. In the case of smaller clusters with = 1-3, the simulations show that the clusterstructure takes exclusively the hydrogen-bonded complex between a canonical neutral glycine and a water cluster moiety. In contrast, it was found that proton-exchange processes effectively occur between the COOH carboxylic group and water cluster moiety for = 4-6 clusters although the overall structures are the complex between a neutral glycine and water clusters. In the case of the = 7 cluster, glycine preferentially takes a zwitterionic form having NH and COO functional groups.