Purine stacking: Effects of alkyl substituents

Abstract

The association behavior of various alkyl-substituted purines in aqueous solution was investigated by nuclear magnetic resonance and vapor pressure osmometry. The data suggested that stacking was highly dependent on the nature and location of the alkyl substituent. Larger alkyl groups enhanced the degree of association, and stacking was highly dependent on the nature and location of the substituent. Differential chemical shifts of the ring protons suggest that the association process became less random as the size of the alkyl group was increased. Results are reported for 2-, 6-, 8-, and 9- tert.-butylpurine, 8-isopropylpurine, and 9-methyl-, 9-ethyl-, and 9-isopropylpurine. Synthetic procedures are given for the tert.-butylpurines and isopropylpurines.