Self-association of pyrimidine and some of its methyl derivatives in aqueous solution

The self-association of pyrimidine and some of its methyl derivatives (4- and 5-methylpyrimidine; 4-6-dimethylpyrimidine) was-studied in aqueous solution, at different pH values, by UV spectroscopy. The variation in molar absorptivity with the concentration of these compounds was measured for the pi --> pi* bands, at nearly 240 nm, and for the n --> pi* band, at nearly 270 nm, whenever this band was observable separately. Hyperchromic or hypochromic deviations from Beer's law with increasing concentration were detected. These effects were interpreted in terms of two processes of self-association: dimerization and polymerization. From the experimental data, self-association constants for dimerization (K-2) and polymerization (K-n) were calculated. The results obtained are discussed in relation to the structural features of each compound. In this work, the different behaviours of the pi --> pi* and n --> pi* bands in the same compound have been observed for the first time. These behaviours can be interpreted, according to the theory of hypochromism in polynucleotides, in terms of horizontal (colinear) interactions for the dimerization and vertical (stacking) interactions for the polymerization.