Biophys J, October 1999, p. 2137-2144, Vol. 77, No. 4

A Difference Fourier Transform Infrared Study of Tyrosyl Radical Z· Decay in Photosystem II

Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, St. Paul, Minnesota 55108-1022

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 cm¹ to Z·; these studies were performed on highly resolved PSII preparations at pH 7.5, under conditions where Q_A and Q_B 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 and Z· decay. Q_B 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 cm¹ line. Quantitative comparison of the rates of Q_A and Z· decay with the decay of the 1478 cm¹ line supported the assignment of a 1478 cm¹ 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·.

Biophys J, October 1999, p. 2137-2144, Vol. 77, No. 4
© 1999 by the Biophysical Society   0006-3495/99/10/2137/08  $2.00