Fluorescence lifetimes were estimated using the Strickler-Berg (SB) equation ( ) as previously described. Briefly, the steady-state absorption spectra measured in 16 mM phosphate buffer, pH 7.4 were plotted versus wavenumber. The area under the lower energy absorption band was integrated from 39290-31013 cm -1 for 5mCyd and 39908-32265 cm -1 for Cyd.