A COMPARATIVE PICOSECOND-RESOLVED FLUORESCENCE STUDY OF TRYPTOPHAN RESIDUES IN IRON-SULFUR PROTEINS

The fluorescence intensity and anisotropy decays of the intrinsic tryp tophan emission from six Fe/S proteins (ranging from the very simplest ones to enzyme complexes containing one, two or more Trp residues) we re measured. All proteins were examined in the reduced and the oxidize d state. In either redox state each protein exhibits ultrarapid trypto phan fluorescence decay on the picosecond timescale contributing up to 93% of the total decay. Correlation times in the range of 1 ns or les s were found for all six iron-sulfur proteins reflecting internal Trp motion. In addition, some proteins exhibit longer correlation times re flecting segmental motion and overall protein tumbling. The ultrarapid fluorescence decay in iron-sulfur proteins indicates efficient radiat ionless energy transfer between distant tryptophan residues and iron-s ulfur clusters. Such an energy transfer mechanism can be accounted for by referring to the three-dimensional structures of rubredoxin and fe rredoxin in calculating the transfer efficiency of the single tryptoph an-iron-sulfur couple.