Effect of nucleic acid binding on the triplet state properties of tetrapeptides containing tryptophan and 6-methyltryptophan: A study by phosphorescence and ODMR spectroscopy

Complexes of four peptides [KWGK, KGWK, K(6MeW)GK, KG(6MeW)K] with the nucl eic acids [poly(A), poly(C), poly(U), poly(I), and rG(8)] have been investi gated by phosphorescence and optically detected magnetic resonance (ODMR) s pectroscopy. The intrinsic spectroscopic probes used in these studies are t ryptophan (W) and 6-methyltryptophan (6MeW). Binding to the nucleic acids r esults in a red-shift of the phosphorescence 0,0-band (DeltaE(0,0)) of the aromatic residue as well as a reduction of its zero-field splitting paramet er (DeltaD). Results are compared with earlier studies of the HIV-1 nucleoc apsid protein, NCp7, that contains a single tryptophan residue (Trp37) with in a retroviral zinc finger sequence. Binding of poly(A) or poly(U) to the tetrapeptides induces larger DeltaE(0,0) and DeltaD than when bound to NCp7 , indicating stronger stacking interactions. Poly(I), on the ether hand, pr oduces larger shifts in Trp37 of NCp7. Binding of rG(8) produces sequence-d ependent effects in the peptides. When bound to NCp7, but in contrast with tetrapeptide binding, nucleic acids produce large changes in triplet state kinetics consistent with enhanced spin-orbit coupling. These results are di scussed in terms of three limiting types of tryptophan-base interaction: in tercalation, aromatic stacking, and edge-on interaction. These should have differing effects on the properties of the triplet state.