N2-saturated solutions of 6MP were prepared in PBS pH 7.4 and 5.5 with phosphate concentration of 16 mM and transient absorption data were collected in 2 mm optical quartz cells. Solutions were continuously stirred with a Teflon-coated magnetic stir bar. The sample was replaced with a fresh solution to maintain less than five percent decrease in absorbance through the entire length of the experiment, as monitored from the steady-state absorption spectra. Forty kinetic traces were extracted from each data set for target analysis using the Igor Pro 6.3.7.2 software (Wavemetrics, Inc.). The pH 5.5 sample was fitted to a single-exponential function, while pH 7.4 sample was fitted to a two-exponential function. Both fit functions were convoluted with an instrument response function of 400 ps. The uncertainties in the lifetimes are reported as twice the standard deviation (2) obtained from the target analysis using the Igor Pro software.