Effects of Microsolvating Water on the Stability of Zwitterionic vs. Canonical Diglycine

Abstract

We present calculations for diglycine -(H 2 O) n (n = 0-3) to examine the effects of microsolvating water on the relative stability of the zwitterionic vs. canonical forms of the dipeptide. We calculate the structures, energies and Gibbs free energies of the conformers at wB97XD/6-311++G** and MP2/aug-cc-pvdz levels of theory level of theory. We predict that microsolvation by up to three water molecules does not give thermodynamic stability of the zwitterion relative to the canonical forms. Our calculations also suggest that zwitterionic diglycine -(H 2 O) 3 is not stable kinetically in low temperature gas phase environment.

Solvent effects on the structures and vibrational features of zwitterionic dipeptides: L-diglycine and L-dialanine

“…Ramegowda [7,25] has studied the structures and the relative stability of dialanine and diglycine in gas and water, using the effective fragment potential method associated to the RHF. Kim's group [3,6,26] has also presented a quantum chemical study of the structures of diglycine -(H 2 O) 3 and dialanine -(H 2 O) 3 systems to elucidate the relative stability of canonical vs. zwitterionic forms. They have predicted that microsolvent by up to three water molecules does not yield stable zwitterionic diglycine as compared with those of amino acids because the functional groups N t H + 3 and COO − terminals in dipeptides are separated by too large of distance than in amino acids.…”

“…The first is present in hight concentrations in muscle and brain tissues, and the second is found in the skeletal muscle and brain of the mammals. L-diglycine (L-Gly-Gly) [3][4][5][6] and L-dialanine(L-Ala-Ala) [6][7][8] are the simplest dipeptides formed respectively with L-glycine [9] and L-alanine [10] amino acids.…”