Changes in Low-Frequency Vibrational Modes Induced by Point-Mutations in the Protein-Cofactor Matrix of Bacterial Reaction Centers

The formation of the excited state P* of the primary donor of bacterial RCs sets in motion low-frequency (mostly <250 cm-1) vibrational motions [1]. These motions are coherent and persist on the time scale of primary electron transfer. The molecular ‘identity’ of these motions is essentially unknown. Their low frequency suggests that they are delocalized over the protein environment to a certain extent. In the present work, we have examined the effects of modification of the bonding between the cofactors and protein, by studying mutants designed to add or remove hydrogen bonds between the protein and the Bchls of P [2]. In WT, one hydrogen bond between the 2-acetyl carbonyl group of PL and His L168 is present. Using low-temperature femtosecond spectroscopy, we compared the frequency spectrum of membrane-bound RCs from WT with mutants in which this hydrogen bond had been deleted by replacement of HL168 by F, L or D, and where a second hydrogen bond has been introduced at the 2-acetyl carbonyl group of PM (FM197H) and at the 9-keto carbonyl group of Pm (LL131H).