Tryptophan Intercalation In G, C Containing Polynucleotides: Z to B Conversion Of Poly [d(G-5M C)] In Low Salt Induced By A Tetra Peptide

Summary

Binding of a tetra peptide, lysyl tryptophenyl glycyl lysine O - ter butyl ester (KWGK) with duplex forms of G, C containing polynucleotides, Poly [d(G-C)], Poly [d(G-5M C)], Poly (dG), Poly (dC) and E.coli DNA were studied under low salt conditions using UV absorption, fluorescence, and circular dichroic (C.D) spectroscopy. On addition of the peptide (upto a P/N mole ratio of 0.5), the Poly [d(G-5M C)] under low salt (1 mM Na CI) conditions, was converted from Z to B-form as shown by the inversion of C.D spectra. The two binding constants (K₁ and K₂) were determined from fluorescence spectroscopy of which K₂estimates the intercalation of the tryptophenyl side chain between the base pairs of DNA and K₁ estimates the electrostatic interactions between the lysyl side chains and phosphate groups. The strength of intercalation is : Z-form of Poly [d(G-5M C)] ≫ B form of Poly [d(G-5M C)] ≫ E.Coli DNA > Poly (dG). Poly (dC). This means that peptide seems to have strong preference for Z compared to B-form and for alternating over non-alternating G, C Sequences. This suggests that tryptophan intercalation may act as a discriminating factor in recognizing Z and B-forms and may have a potential role in Protein-Nucleic acid interactions that are important for transcription.