In order to gain more information about the binding and interaction of quinones acting as electron acceptors in photosynthetic reaction centers, infrared spectra of ‘model” quinones and their reduced species (semiquinones, quinone dianions and quinols) were recorded in a spectroelectrochemical cell. The frequencies and the extinction coefficients of the quinone C = O and C = C stretching vibrations which are shifted upon reduction were obtained in different solvents. These spectra are used to estimate the contribution of quinone reduction to light-induced infrared difference spectra between the charge-separated and the relaxed state of photosynthetic reaction centers.
