Close range interactions between nucleotide bases and tryptophan residues in an Escherichia coli single-stranded DNA binding protein-mercurated poly(uridylic acid) complex. A study by optically detected magnetic resonance spectroscopy.

Optically detected triplet state magnetic resonance spectra are reported for the complex formed between mercurated poly(Urd) and Escherichia coli single-stranded DNA binding protein. Upon forming a complex, the triplet state properties of Trp residue(s) in the protein are perturbed by the heavy mercury atom and are characterized by a shortened triplet state lifetime and the appearance of a strong D + E slow passage optically detected magnetic resonance signal. These features, which signal an external heavy atom effect, provide direct evidence for a close range interaction between mercurated nucleotide bases and Trp residues owing to the requirement of a van der Waals contact between the perturbed molecule and the heavy atom perturber. The amplitude-modulated phosphorescence microwave double resonance technique selectively displays the phosphorescence spectrum of the heavy atom-perturbed Trp triplet states. A van der Waals contact manifested through a stacked structure of the mercurated uridine base and the indole moiety of Trp is strongly suggested as the most plausible mode of interaction from steric considerations, since other approaches of the mercury atom are blocked by the covalent attachment of 2-mercaptoethanol to mercury. The magnitude of the heavy atom perturbation also is consistent with Hg approach to the pi-system from above or below the indole aromatic plane, and is at least an order of magnitude larger than effects expected from an edge-on approach.