Crystal structure and conformation of beta-substituted (Z)- and (E)-6-styrylpurines as conformer models

Crystal structures of conformationally restricted (Z)- and (E)-6-styrylpuri nes with the beta -substituents involving hydrogen, chlorine, and bromine a toms as well as a methylthio group were studied as conformer models of NG-a denines in relation to active conformation of cytokinins. X-ray crystallogr aphic analyses confirmed that all of the trans-isomers exist in an anti con formation, whereas the cis-isomers except the (E)-methylthio derivative ado pt a syn conformation. The derivative with a bulky beta -substituent was fo und to be in an anti conformation in contrast to the other cis-isomers. The preferred anti conformation and potent cytokinin activity of the trans-iso mers supports the anti-transoid form as the most plausible active conformat ion of N-6-adenines. In addition, it is likely that the syn-cisoid form of N-6-adenines is also involved in receptor binding, by considering both the preferred syn conformation of the cis-isomers and their moderate activity, although it does not play a major role compared to the anti-transoid form.