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

Photosystem II (PSII) contains a redox-active tyrosine, Z. Difference Fouri er transform infrared (FTIR) spectroscopy can be used to obtain structural information about this species, which is a neutral radical, Z ., in the pho tooxidized form. Previously, we have used isotopic labeling, inhibitors, an d site-directed mutagenesis to assign a vibrational line at 1478 cm(-1) to Z .; these studies were performed on highly resolved PSII preparations at p H 7.5, under conditions where Q((A)over bar) and Q((B)over bar) make no det ectable contribution to the vibrational spectrum (Kim, Ayala, Steenhuis, Go nzalez, Razeghifard, and Barry, 1998. Biochim. Biophys. Acta. 1366:330-354) . Here, time-resolved infrared data associated with the reduction of tyrosy l radical Z . were acquired from spinach core PSII preparations at pH 6.0. Electron paramagnetic resonance spectroscopy and fluorescence control exper iments 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 dec reases in the amplitude of a 1478 cm(-1) line. Quantitative comparison of t he 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 compon ents assignable to Z . and Z. To our knowledge, this is the first kinetic s tudy to use quantitative comparison of kinetic constants in order to assign spectral features to Z ..