SCALED QUANTUM-MECHANICAL FORCE-FIELD FOR GLYCINE IN BASIC SOLUTION

We obtain scale factors for three glycinate-nH2O ab initio force field s, using the 4-31G basis set, that can be used in building a scaled qu antum mechanical force field for alanine and, subsequently, for peptid es in aqueous solutions. Force constants from the fully optimized glyc inate-nH2O supermolecules were scaled by using experimentally determin ed vibrational frequencies of glycine in water at pH 13. Similar calcu lations were performed for methylamine and acetate. Scale factors for the stretching modes of acetate are within 2% of the related scale fac tors for glycinate. The scale factor for the NH2 scissor mode in methy lamine is also in agreement with that of glycinate. Changes in the sca le factors as a function of the number of hydrating water molecules we re also similar between glycinate and acetate. Amine groups showed rel atively small changes. Scale factors for glycinate with no hydrating m olecules were extrapolated from the supermolecule results, since the o ptimized structure of isolated glycinate obtained with the 4-31G basis set yielded one imaginary frequency. Good agreements between calculat ed and experimental frequencies for glycinate, acetate, and methyl ami ne were obtained for each set of scale factors. Scaling appears to com pensate for the systematic effects of hydration on force constants, ma king it possible to obtain reliable frequency predictions for amino ac ids in water without resorting to expensive supermolecule calculations .