The oxidation of water and the production of O-2 during photosynthesis, occur at a manganese-containing catalytic site within photosystem II. A number of questions remain regarding water substrate interactions and the nature of the other ligands to the catalytic site. Through the use of difference spectra, FTIR spectroscopy is being exploited in the mid-IR range to address these questions. Here, we investigate the isotope-induced effects in three regions of the mid-IR range S-2/S-1 difference spectrum of photosystem II (Noguchi, T.: Sugiura. M. Biochemistry 2000, 39, 10943 and Noguchi, T.; Inoue, Y.; Tang, X.-S. Biochemistry 1999, 38, 10187) by ab initio Hartree-Fock and density functional calculations, using the 6-31G(d). 6-3 1 + G(d,p), and 6-311 ++G-(d,p) bases. The first region contains the OH stretching modes of water. From a determination of the coupling between the two stretching modes and knowledge of die H2O/D2O isotope-induced shifts, the frequency of the symmetric stretching mode has been determined. It is shown that in the S-2 state, one OH is strongly H-bonded and the other much more weakly, whereas in the S, state the H-bonding for both OH,, is more similar. The second region involves frequencies of two ring modes near 1100 cm(-1). Calculations of N-deuteration and N-15 isotope-induced shifts both confirm that the histidine interaction with the catalytic site is represented by 5-methylimidazole. The third realon investigated spans the frequencies characteristic of the stretching vibrations of the carboxyl group. An apparent deuterium-induced frequency upshift of a band near 1600 cm(-1), and attributed to the carboxyl group, requires that the two CO bonds of the carboxyl group are similar.