Hydration effects on the molecular structure and amide I mode frequency of a prototype peptide molecule, N-methylacetamide (NMA), when it is solvated by a few water molecules, were investigated by carrying out ab initio calculations for a number of NMA-water complexes. The harmonic frequency shift of the amide I mode in NMA-nD(2)O (n=1-5) complex was found to originate from the combination of the molecular cubic anharmonicity and displacement of the amide I coordinate when the NMA is hydrated. Using a multivariate least-square fitting method, the effective transition charges of six NMA sites were determined. A brief discussion on how this empirical model can be used to quantitatively describe solvatochromic frequency shift of the NMA amide I mode in solution is presented. (C) 2003 American Institute of Physics.