Time-resolved FTIR difference spectroscopy for the study of quinones in the A binding site in photosystem I: Identification of neutral state quinone bands.

Infrared absorption bands associated with the neutral state of quinones in the A binding site in photosystem I (PSI) have been difficult to identify in the past. This problem is addressed here, where time-resolved step-scan FTIR difference spectroscopy at 77 K has been used to study PSI with six different quinones incorporated into the A binding site. (P700A - P700A) and (A - A) FTIR difference spectra (DS) were obtained for PSI with the different quinones incorporated, and several double-difference spectra (DDS) were constructed from the DS. From analysis of the DS and DDS, in combination with density functional theory based vibrational frequency calculations of the quinones, the neutral state bands of the incorporated quinones are identified and assigned. For neutral PhQ in the A binding site, infrared absorption bands were identified near 1665 and 1635 cm, that are due to the CO and CO stretching vibrations of the incorporated PhQ, respectively. These assignments indicate a 30 cm separation between the CO and CO modes, considerably less than the ~80 cm found for similar modes of PhQ. The CO mode downshifts due to hydrogen bonding, so the suggestion is that hydrogen bonding is weaker for the neutral state compared to the anion state, indicating radical-induced proton dynamics associated with the quinone in the A binding site in PSI.