Low-temperature x-ray absorption spectroscopy of plastocyanin: evidence for copper-site photoreduction at cryogenic temperatures

X-ray absorption spectra for copper(II) plastocyanin have been measured at low temperature. These data confirm our previous observation (based on room-temperature measurements) that the methionine sulfur in plastocyanin makes no significant contribution to the Cu EXAFS and that the Cu-S(Met) interaction must therefore be extremely weak. Both preliminary data, having a very poor signal/noise ratio and subsequent measurements with much higher quality data are reported. Analysis of the preliminary data provides a cautionary example of the dangers of deriving structural results from noisy EXAFS data, at least in cases where the scatter is weakly contributing to the signal. Analysis of the near-edge structure gives direct evidence for x-ray-induced photoreduction of Cu(II) when extremely intense x-rays from synchrotron sources are used. The rate of photoreduction decreases by approximately a factor of 2 between 190 and 4 K, but it is still significant even at the lower temperatures studied. 38 refs., 7 figs., 3 tabs.