A difference fourier transform infrared study of tyrosyl radical Z• decay in photosystem II

Photosystem II (PSII) contains a redox-active tyrosine, Z. Difference Fourier transform infrared (FTIR) spectroscopy can be used to obtain structural information about this species, which is a neutral radical, Z ., in the photooxidized form. Previously, we have used isotopic labeling, inhibitors, and site-directed mutagenesis to assign a vibrational line at 1478 cm(-1) to Z .; these studies were performed on highly resolved PSII preparations at pH 7.5, under conditions where Q((A)over bar) and Q((B)over bar) make no detectable contribution to the vibrational spectrum (Kim, Ayala, Steenhuis, Gonzalez, Razeghifard, and Barry, 1998. Biochim. Biophys. Acta. 1366:330-354). Here, time-resolved infrared data associated with the reduction of tyrosyl radical Z . were acquired from spinach core PSII preparations at pH 6.0. Electron paramagnetic resonance spectroscopy and fluorescence control experiments were employed to measure the rate of Q((A)over bar) and Z . decay. Q((B)over bar) did not recombine with Z . under these conditions. Difference FTIR spectra, acquired over this time regime, exhibited time-dependent decreases in the amplitude of a 1478 cm(-1) line. Quantitative comparison of the rates of Q((A)over bar) and Z . decay with the decay of the 1478 cm(-1) line supported the assignment of a 1478 cm(-1) component to Z .. Comparison with difference FTIR spectra obtained from PSII samples, in which tyrosine is labeled, supported this conclusion and identified other spectral components assignable to Z . and Z. To our knowledge, this is the first kinetic study to use quantitative comparison of kinetic constants in order to assign spectral features to Z ..