Time-resolved absorption changes of the pheophytin Q(x) band in isolated photosystem II reaction centers at 7 K: Energy transfer and charge separation

The pheophytin a Q(x) spectral region of the isolated photosystem II reacti on center was investigated at 7 K using femtosecond transient absorption sp ectroscopy. At this temperature, uphill energy transfer, which greatly comp licates the interpretation of the kinetics at or near room temperature, sho uld be essentially shut off. Low-energy (similar to 100 nJ) pulses at 661 a nd 683 nm were used to excite the short-wavelength and long-wavelength side s of the composite Q(y), band, providing preferential excitation of the acc essory pigment pool and P680, respectively. The data analysis uses a backgr ound subtraction technique developed earlier (Greenfield et al. J. Phys. Ch em. B 1997, 101, 2251-2255) to remove the kinetic components of the data th at are due to the large time-dependent changes in the background that are p resent in this spectral legion. The instantaneous amplitude of the bleach o f the pheophytin a Q(x), band with 683 nm excitation is roughly two-thirds of its final amplitude, providing strong evidence of a multimer description of the reaction center core. The subsequent growth of the bleach shows bip hasic kinetics, similar to our earlier results at 278 K. The rate constant of the faster component is (5 ps)(-1) for 683 nm excitation (a factor of al most two faster than at 278 K), and represents the intrinsic rate constant for charge separation. The bleach growth with 661 nm excitation is also bip hasic; however, the faster component appears to be a composite of a (5 ps)( -1) component corresponding to charge separation following subpicosecond en ergy transfer to the long-wavelength pigments and a roughly (22 ps)(-1) com ponent corresponding to charge separation limited by slow energy transfer, The combined quantum yield for these two energy transfer processes is near unity. For both excitation wavelengths, there is also a roughly ( 100 ps)(- 1) component to the bleach growth. Exposure to high excitation energies (gr eater than or equal to 1 mu J) at 683 nm results in a substantial permanent loss of ground-state absorption at 680 nm. The transient behavior of these degraded samples is also examined and is consistent with the (5 ps)(-1) ra te constant for charge separation. Our results are compared to other low-te mperature transient absorption and hole burning studies, as well as to our 278 K results.