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

Crystal structures of conformationally restricted (Z)- and (E)-6-styrylpurines with the beta-substituents involving hydrogen, chlorine, and bromine atoms as well as a methylthio group were studied as conformer models of N(6)-adenines in relation to active conformation of cytokinins. X-ray crystallographic analyses confirmed that all of the trans-isomers exist in an anti conformation, whereas the cis-isomers except the (E)-methylthio derivative adopt a syn conformation. The derivative with a bulky beta-substituent was found to be in an anti conformation in contrast to the other cis-isomers. The preferred anti conformation and potent cytokinin activity of the trans-isomers supports the anti-transoid form as the most plausible active conformation 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.