Noncovalent interactions in polyamine/nucleoside (or diaminocarboxylate) systems studied by potentiometric and NMR techniques

Abstract

Noncovalent interactions and formation of molecular complexes have been found to occur between a number of polyamines and adenosine or cytidine. Since these observed interactions affect the acid–base character of particular components of the systems the determination of overall stability constants of the complexes was possible on the basis of a computer analysis of potentiometric data and then equilibrium constants of the reactions were calculated. The comparison of the constants and the changes in the position of ¹³C NMR signals provided grounds for a conclusion that mainly two (or three) –NH_x⁺ groups of amine and electron-rich centres as well as the system of π electrons from purine and pyrimidine bases constitute sites for the interactions. The tendency of particular polyamines to form adducts depends on the number of nitrogen atoms as well as the length of the methylene chains in the molecules. Distributions of molecular complexes as a function of pH were calculated. The ranges of the occurrence of PA/Nuc molecular complexes overlap with those of nucleoside deprotonation and polyamine protonation, which confirms the validity of the assumed model of interactions. It was found that in the polyamine systems with diaminocarboxylates no adducts were formed.