Partition Coefficients of Biologically Important Pyrimidines and Purines [1]

Distribution (partition) of an organic substance between two liquid phases, usually water and an or­ ganic solvent, plays an important role in extraction, partition chromatography, and various other separa­ tion processes used in many areas of chemistry. Partition (distribution) coefficient, P, which is the quantity used to characterize the distribution of a solute between two immiscible solvents (one of which is normally water), is defined as P = C0/Cw, where CG is concentration of the extractable species in the organic solvent and Cw is concentration of the extractable species in the aqueous layer [3]. In most cases, partitioning can be treated by methods of clas­ sical thermodynamics. Partition coefficients repre­ sent a useful criterion for the selection of a suitable solvent and the choice of optimum conditions for extraction. A lso, distribution of biomolecules be­ tween water and an organic phase often parallels their biological activity. As a continuation of our systematic studies of groundand excited-state physical properties of biologically important pyrimidines and purines [4, 5], we have decided to investigate the solubility of sixteen pyrimidines and purines in several organic solvents and to determine the corresponding parti­ tion coefficients. The pyrimidines and purines in­ cluded in our study were nucleic acid components (uracil, thymine, cytosine, adenine, guanine), al­ kaloids (hypoxanthine, theobromine, theophylline, caffeine), and cancerostatic drugs or otherwise biologically active compounds (5-fluoro-, 5-bromo-, and 5-iodouracil, 6-chloroand 6-mercaptopurine) (Scheme 1).