Femtosecond Spectroscopy of PSII Reaction Centers: New Results

Abstract

Time evolution of transient absorption changes excited by pumping Photosystem II (PSII) RCs at 683 nm (where P680 absorbs) or 665 nm (where RC antenna pigments absorb) and probing at 544 nm elicited a rapid increase in absorption within the time of the pump pulse, and, subsequently, three distinct kinetic components representing decreases in absorption: A fast component with {tau}s on the order of 1--3 ps, an intermediate one with {tau}s of 10--25 ps, and a slow one with {tau}s ranging from 50--100 ps. The amplitudes of the three bleach components versus pump energy are linear up to 250 nJ at both pump wavelengths. Between 250 nJ and 1 {mu}J the amplitudes continue to increase but at a lesser slope. Zero-crossing times increase with increasing pump energy from a minimum of about 10 ps at 683 nm excitation and 20 ps at 665 nm excitation. ZCTs provide a convenient way of comparing from laboratory to laboratory the actual pump intensity seen by a sample. Whereas the amplitudes of the three kinetic components increase with pump energy, there does not seem to be a consistent change in the relative percentage of the three components over the pump energy range investigated. However, the percentage of intermediate time component appears greater, and the lifetimes of all three components increase somewhat when pumping at 665 nm vs. 683 nm. To examine this point, we obtained transient absorption spectra of PSII RCs at 500 ps and 1 ps after low energy pump pulses at 665 nm and 683 nm. At 500 ps, transient absorption spectra in the 500-nm to 600-nm range are independent of pump wavelength. This indicates that energy transfer from antenna pigments, absorbing at 665 nm, to P680 is close to 100% efficient. On the other hand, at short time there is a substantial difference in the spectra generated by the two pump wavelengths.