The interest in the inner proton tautomerism of porphyrin systems in both the ground and excited states has experienced a rekindled interest in recent years due to the possibility of applying selective photoactivated tautomer switching via hole burning. This has the potential of serving as a modality for data storage that would far exceed current optical storage systems by several orders of magnitude. Moreover, in biological systems,. the transfer of protons in enzymatic reactions and for the establishment of transmembranous """"""""protonmotive force"""""""" remains an area of active endeavor. Previous ESR experiments suggest that that the protons are moving in the condensed phase at cryogenic temperatures. We are now examining this hypothesis and we are specifically examining the role of the protein in modulating the proton transfer reaction. Since the tautomerization reaction will change the fluorescence anisotropy, time resolved fluorescence measurements allow us to measure the proton transfer rates that occur in the singlet state. For the excited singlet state, the decay of anisotropy for porphyrin cytochrome c immobilized in a viscous solution indicates that the proton transfer rate is about 2 nsec.
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