The structure and the electronic properties of P680 and its radical cation in photosystem II (PSII) were studied by means of Fourier transform infrared spectroscopy (FTIR). Light-induced P680(+)/P680 FTIR difference spectra in the mid- and near-IR regions were measured using PSII membranes from spinach, core complexes from Thermosynechococcus elongatus, and reaction center (RC) complexes (D1-D2-Cytb559) from spinach. The spectral features of the former two preparations were very similar, indicating that the structures of P680 and its radical cation are virtually identical between membranes and cores and between plants and cyanobacteria. In sharp contrast, the spectrum of the RC complexes exhibited significantly different features. A positive doublet at similar to 1724 and similar to 1710 cm(-1) due to the 13(1)-keto CO stretches of P680(+) in the membrane and core preparations were changed to a prominent single peak at 1712 cm(-1) in the RC complexes. This observation was interpreted to indicate that a positive charge on P680(+) was extensively delocalized over the chlorophyll dimer in RC, whereas it was mostly localized on one chlorophyll molecule (70-80%) in intact P680. The significant change in the electronic structure of P680(+) in RC was supported by a dramatic change in the characteristics of a broad intervalence band in the near-IR region and relatively large shifts of chlorin ring bands. It is proposed that the extensive charge delocalization in P680(+) mainly causes the decrease in the redox potential of P680(+)/P680 in isolated RC complexes. This potential decrease explains the well-known phenomenon that Y(Z) is not oxidized by P680(+) in RC complexes.