FEMTOSECOND COHERENT TRANSIENT INFRARED-SPECTROSCOPY OF REACTION CENTERS FROM RHODOBACTER-SPHAEROIDES

Protein and cofactor vibrational dynamics associated with photoexcitat ion and charge separation in the photosynthetic reaction center were i nvestigated with femtosecond (300-400 fs) time-resolved infrared (1560 -1960 cm(-1)) spectroscopy. The experiments are in the coherent transi ent limit where the quantum uncertainty principle governs the evolutio n of the protein vibrational changes. No significant protein relaxatio n accompanies charge separation, although the electric field resulting from charge separation modifies the polypeptide carbonyl spectra. The potential energy surfaces of the ''special pair'' P and the photoexci ted singlet state P and environmental perturbations on them are simil ar as judged from coherence transfer measurements. The vibrational dep hasing time of P modes in this region is 600 fs. A subpicosecond tran sient at 1665 cm(-1) was found to have the kinetics expected for a seq uential electron transfer process. Kinetic signatures of all other tra nsient intermediates, P, P, and P+, participating in the primary step s of photosynthesis were identified in the difference infrared spectra .