Time Resolution of Electronic Transitions of Photosynthetic Reaction Centers in the Infrared

Abstract

Electronic transitions of the special pair excited state, P*, and of its positive ion, P+, have been identified by transient infrared spectroscopy in the 170&2000-~m-~ region. The P* transition is present immediately (ca. 300 fs) after light absorption and decays with time constant 3.4 ps. The P+ transition appears with time constant 3.4 ps. The transition dipoles of these transitions are both measured to have a squared projection of 0.63 onto the direction of the ground state to Qv-(870 nm) transition. This is interpreted to imply that both P* and P+ transitions have dipoles along the line joining the centroids of charge of the two bacteriochlorophylls (BChl) composing the dimer. The P* transition is assigned as an interexciton transition brought about by mixing of exciton and charge-separated states. The P+ transition is assigned as a transition between the symmetric and antisymmetric combination of the localized hole states of the dimer. The results are compared with theoretical calculations, static FTIR, and Stark effect measurements on the reaction center. While the results are in qualitative agreement with recent theoretical calculations, better agreement requires a larger admixture of charge resonance states in the QU-state than is found in most calculations.

Femtosecond coherent transient infrared spectroscopy of reaction centers from Rhodobacter sphaeroides.

“…Significant differences exist in both the amide I and cofactor regions of the spectra reported here and those of dry lipid films (37). These two preparations are known to differ functionally, with the most noticeable differences being the absence of a peak around 1640 cm-1, a much diminished feature around 1660 cm"-l", and the presence of a band around 1700 cm-' in the solution RC.…”

“…The initial, small positive component of the kinetics in Fig. 3 implies that the band being probed is broader than =50 cm-, which may correspond to an electronic transition of P* at even lower frequency than the electronic transition reported previously (37). It now appears likely that a weak electronic absorption spectrum could underlie the vibrational spectrum over the whole region 1560-1960 cm-'.…”

“…The bleach corresponds to the 9-keto C=O group of the neutral P (35,36). The positive component has the kinetic characteristics of an electronic absorption band ofthe special pair P* and P+ states (37). A sum of two exponentials are used for the positive component; the first rises instantaneously with the excitation and decays with a time constant of 2.5 ps (P* electronic absorption), and the second component rises with a time constant of 2.5 ps to a plateau.…”

Ultrafast Dynamics and Spectroscopy of Bacterial Photosynthetic Reaction Centers

“…Recently, rapid kinetics have been observed in anisotropy measurements applied to the P and B bands [10,21,31,39,51]. It is found that prior to ET, there exist a few steps in the P and B bands.…”

“…These bands have conventionally been assigned to the Q y electronic transitions of the P (870 nm), B (800 nm), and H (870 nm) components of RCs. By considering that the special pair P can be regarded as a dimer of two 2 s. h. lin et al ultrafast dynamics and spectroscopy 3 bacteriochlorophylls, the P band can be assigned to the lower excitonic band of P. By taking into account some difference in the protein environment in the L and M branches [2,3,[6][7][8][9][10][17][18][19][20][21]23,[30][31][32][33][34]41,42,46,47,52], the 800-nm band can be attributed to B L and B M [30][31][32][33][34][35][36][37][38][39][40][41][42]46,47]. The vibrational frequency of the special pair P and the bacteriochlorophyll monomer B have also been extracted from the analysis of the Raman profiles [39,40,42,44,51].…”