Proton magnetic resonance studies of the binding of oligopeptides containing tryptophan to polyribonucleotides poly A, poly U and poly C

The binding of oligopeptides Lys-Trp-Gly-Lys OtBu, Lys-Gly-Trp-Lys OtBu and Lys-Trp-Lys to Polyadenylic, Polycytidylic and Polyuridylic acid has been studied by Proton NMR at 90 MHz and 500 MHz at oligopeptide/Polynucleotide ratios ranging from 0.01 to 0.20 at 275-365 K. Downfield shift of 0.01-0.2 ppm at 296 K of the H2, H8 and H1' resonances of Poly A due to binding with oligopeptides is accompanied by a marked narrowing of resonance lines of Poly A. The ring protons of tryptophan shift upfield by 0.3-0.6 ppm at 296 K on binding to Poly A. Changes in chemical shift of both adenine and tryptophan protons on binding are much smaller at 355 K than that at 275 K. These observations are ascribed to intercalation of the tryptophan ring in the adenine bases resulting in partial destacking of adenine bases in Poly A. Using the magnetic anisotropy ring current shifts, an overlap geometry of tryptophan ring in the adenine has been proposed. Addition of oligopeptides to Poly C and Poly U, on the other hand, suggests that tryptophan ring does not stack in Poly U and Poly C.