LOW-FREQUENCY VIBRATIONAL-MODES IN PROTEINS - CHANGES INDUCED BY POINT-MUTATIONS IN THE PROTEIN-COFACTOR MATRIX OF BACTERIAL REACTION CENTERS

As a step toward understanding their functional role, the low frequenc y vibrational motions (<300 cm(-1)) that are coupled to optical excita tion of the primary donor bacteriochlorophyll cofactors in the reactio n center from Rhodobacter sphaeroides were investigated. The pattern o f hydrogen-bonding interaction between these bacteriochlorophylls and the surrounding protein was altered in several ways by mutation of sin gle amino acids. The spectrum of low frequency vibrational modes ident ified by femtosecond coherence spectroscopy varied strongly between th e different reaction center complexes, including between different mut ants where the pattern of hydrogen bonds was the same. It is argued th at these variations are primarily due to changes in the nature of the individual modes, rather than to changes in the charge distribution in the electronic states involved in the optical excitation. Pronounced effects of point mutations on the low frequency vibrational modes acti ve in a protein-cofactor system have not been reported previously. The changes in frequency observed indicate a strong involvement of the pr otein in these nuclear motions and demonstrate that the protein matrix can increase or decrease the fluctuations of the cofactor along speci fic directions.