For seven representative tripeptide conformations, we carried out ab initio geometry optimizations and vibrational analyses. By using the Hessian matrix reconstruction method developed in the present paper, both the diagonal and off-diagonal coupling force constants of a given tripeptide were calculated and the relationship between the three-dimensional conformation and local amide I mode frequency was discussed. Isotope labeling effects on both the amide I local and normal modes were elucidated. It was found that the C=O bond length of a given peptide, which represents the structural distortion induced by neighboring peptides, is linearly proportional to the local amide I mode frequency. A theoretical model for predicting the local amide I mode frequencies is discussed and compared with the ab initio calculation results. (C) 2003 American Institute of Physics.