Highly Efficient Spectral Hole-Burning in Oxygen-Evolving Photosystem II Preparations

Abstract

quinone (QA) either neutral or photoreduced to QA - via a low-temperature pre-illumination. Remarkably efficient hole-burning is observed within the chlorophyll Qy(0,0) absorption envelope in the wavelength range of 676695 nm. The hole-burning action spectrum of a sample poised in the S1(QA - ) state is dominated by a narrow feature (40 cm -1 ) at 684 nm, where hole depths of 30% are attainable. The photoproduct for spectral holes burnt in this region is distributed across the 50 cm -1 absorption feature centered at 683.5 nm, independent of the excitation wavelength within this band. Saturated hole-burning experiments indicate weak electron phonon coupling near 684 nm but stronger coupling for holes burnt near 690 nm. Selective excitation near 690 nm of samples in the S1(QA) state also results in efficient Q A - formation. Negligible hole-burning activity is observed at higher energies (<676 nm). Holewidths extrapolated to zero fluence and temperature are 2.0 ( 0.5 GHz near 685 nm for PSII samples in the S1(QA - ) state. Holewidths are twice as large and holeburning quantum efficiencies are up to an order of magnitude greater (approaching 1%) for samples in the S1(QA) state. We ascribe hole-burning near 684 nm to slow (40-210 ps) excitation transfer from a CP43 chlorophyll to the PSII reaction center, and we ascribe hole-burning at 690 nm to excitation transfer from a chlorophyll in CP47. The unusually high hole-burning efficiency that we observe is attributed to a mechanism that involves charge separation in the reaction center that follows excitation transfer from these “slow transfer” states in CP43 and CP47. A key result of this work is the observation that selective excitation in the range 685-695 nm leads to efficient charge separation, as indicated by QA - formation. This indicates the presence of (a relatively weak) P680 absorption in a native PSII, extending to low energy and underlying the CP47 chlorophyll trap absorption.