Adsorption of Glycine Amino Acid in Zeolite: An Embedded QM/MM Study

Adsorption of glycine on the acid site of H-ZSM-5 has been studied at an embedded ONIOM (MP2/6-31G(d,p): UFF) level of theory. Four possible modes of adsorptions are considered. The zwitterionic adsorption is the most stable conformation with the highest adsorption energy of 47.22 kcal/mol. The glycine zwitterion is protonated and stabilized by the anionic zeolite framework. The second most stable complex of adsorbed glycine involves the interactions of the protonated amino group and the anionic zeolite surface with the adsorption energy of -33.41 kcal/mol. The next stable glycine/zeolite adsorbs via the carbonyl oxygen of glycine with the adsorption energy of 26.93 kcal/mol. The adsorption, via the glycine OH has the weakest interaction of -21.91 kcal/mol. The influence of the extended zeolite lattice ranges from -5.55 to -8.14 kcal/mol, depending on the modes of interactions. This finding suggests the important role of the zeolite framework as “solid solvent molecules” to stabilize the zwitterionic form of glycine which is normally present in aqueous solution.