Effect of Temperature on the Stability of Association of Pyrimidine Bases with Serum Albumin: Proton NMR Study

Abstract

In a study of the effect of temperature on the interaction of 5-methylcytosine, 1,3-dimethyluracil, thymine, and 2′-deoxythymidine with bovine serum albumin (BSA) in aqueous solution, the proton magnetic resonance technique has been applied. An important broadening of proton resonances is observed in complexes that is due to a restricted mobility of the bonded molecule. A particular effect of BSA concentration on the pyrimidine ring signals or on the C-5 methyl protons of ligands shows the pyrimidine ring of 1,3-dimethyluracil and the methyl groups of 5-methylcytosine, thymine, and 2′-deoxythymidine as primary binding sites in the complex with albumin. The results reflect the existence of hydrophobic interactions between C-5 methyl groups of the ligand and hydrophobic domains of the serum albumin. It seems that in the complex of 1,3-dimethyluracil with BSA the pyrimidine ring plays a primary role in the binding. No strong chemical bonds are present. The temperature effect is characterized by a decrease of interaction at temperatures higher than 300 K and by the release of ligands from their binding sites on the unfolding albumin. The possible importance of direct interactions between pyrimidine bases and albumin has been discussed with respect to the problem of specific interactions between proteins and nucleic acids.