| Abstract
| - Crystal structures of conformationally restricted (Z)- and (E)-6-styrylpurines with the β-substituentsinvolving hydrogen, chlorine, and bromine atoms as well as a methylthio group were studied asconformer models of N6-adenines in relation to active conformation of cytokinins. X-ray crystallographic analyses confirmed that all of the trans-isomers exist in an anti conformation, whereasthe cis-isomers except the (E)-methylthio derivative adopt a syn conformation. The derivative witha bulky β-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 theanti-transoid form as the most plausible active conformation of N6-adenines. In addition, it is likelythat the syn-cisoid form of N6-adenines is also involved in receptor binding, by considering both thepreferred syn conformation of the cis-isomers and their moderate activity, although it does not playa major role compared to the anti-transoid form. Keywords: Cytokinin; 6-styrylpurine; crystal structure; conformation−activity relationship; tobaccocallus bioassay
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