Picosecond spectroscopy of Cu(II) cytochrome c

We have observed a strong pH dependence in the relaxation rate of Cu(II) cytochrome c following excitation at 532 nm. At pH 8.0 the excited state relaxes with a lifetime of 10 +/- 5 ps while at pH extremes of 2.5 and 13.0 we find that the lifetime becomes longer than 1 ns. This change of more than two orders of magnitude in the lifetime may be due to the Cu coordination number, which is six at neutral pH but five at pH extremes.