[Model studies of DNA-protein interaction. I. Effect of aminocarboxylic and amide groups of amino acids on DNA thermostability and conformation].

To elucidate interactions of amino-carboxylate dipole and amide group of amino acids with DNA, glycine and glutamine, concentration dependences of the melting curves and CD spectra of calf thymus DNA at low ionic strength (10(-4) M) Na-citrate have been studied. A considerable increase of the melting temperature delta t1/2 and a decrease of the temperature interval of melting delta t with the rise of glycine concentration were observed without changes in the CD spectrum. A comparison was made between the influence of dipolar glycine ion and isolated amino and carboxylate ions of ammonium acetate. The data obtained indicated the predominance of electrostatic interaction of glycine with DNA phosphates until the ligand concentration was approximately 0.6 M and, apparently, specific interactions of carboxylate ion with guanine at higher glycine concentrations. Destabilizing effect of glutamine on DNA at a concentration of 5.10(-3) M was observed, whereas at higher concentrations two-phase increase of delta t1/2 was shown. Small changes in DNA CD spectrum under the action of glutamine were registered. The comparison data for glutamine and acrylamide showed that DNA destabilizing effect was due to the amide group. The destabilizing effect of amide group can be explained by its interaction with the bases in single-stranded regions of DNA with the formation of two H-bonds. It is possible that the increase of DNA delta t1/2 is the result of the interaction of phosphates both with aminocarboxylate and amide groups of glutamine.