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

Calculations were done by applying the B3LYP/6-31++G(d) method on the zwitterionic l-diglycine and l-dialanine to study the solvent effects on their structures and vibrational features. Three models of solvation (implicit, explicit, and explicit in implicit) were used and the subsequent resulting values compared. Even though both dipeptides surrounded by 12 water molecules seem sufficient to stabilize their zwitterionic characters, notably to avoid the proton transfer between the backbone (N t H\(_{3}^{+}\), COO −) groups, the hybrid model of solvation (explicit in implicit noted 12W/Continuum) appears to be in better agreement with available IR and Raman experiments than explicit and implicit models. The harmonic vibrational modes derived from geometry optimization of l-diglycine and l-dialanine in 12W/Continuum, agree with the available IR and Raman experimental values within 1 % for l-diglycine and 2 % for l-dialanine, and they appear more accurate than those found using the explicit model (12W).