Energy transfer in complexes of E. coli single-stranded DNA-binding protein with single-stranded poly-(2-thiouridylic acid)

Abstract

The complexes of point-mutated Escherichia coli single-stranded DNA-binding protein ( Eco SSB) with poly-(2-thiouridylic acid) (poly S 2U) have been studied by optical detection of magnetic resonance spectroscopy (ODMR). Previous work has determined that two of four tryptophan (Trp) residues in Eco SSB undergo stacking interactions with nucleic acid bases. Selective photoexcitation of S 2U bases was performed and subsequent triplet → triplet energy transfer from S 2U to nearby Trp residues in the protein took place. The zero-field splitting (ZFS) parameters and sublevel kinetics were determined for each Trp residue sensitized by S 2U. The sublevel lifetimes of the two sensitized residues are similar to those of normal Trp. The ZFS parameters, on the other hand, show a dramatic reduction relative to those of the uncomplexed protein, implying a more polarizable environment for the sensitized Trp residues and/or charge transfer interactions with the S 2U bases.