“…In general, the derived magnitudes of the equilibrium hetero-association constants (K h ) have values in between the self-association constants of the interacting compounds [23, 27, 70, 85-87, 98, 99], except for a very few reports [27,84,100] which, however, were not linked to the physico-chemical properties of the interacting molecules. It was found that K h for the interaction of purine derivatives is, on average, higher than that for purine-pyrimidine which, in turn, is much greater than for pyrimidine-pyrimidine interactions [23,84,87,98]. Based on these results, it was concluded that hetero-association of derivatives of nucleic acid bases in water is largely governed by van der Waals interactions between the molecules in hetero-complexes, which are known to correlate with the dimensions of aromatic portion of the interacting molecules [11,86,87].…”