Quantitative structure-activity relationships in cytokinin agonistic and antagonistic pyrido[2,3-d]pyrimidine derivatives: insights into receptor topology

2-(Methylthio)pyrido[2,3-d]pyrimidines having various alkylamino and anilino substituents at the 4-position were prepared and tested for their cytokinin agonistic and antagonistic activities by the tobacco callus bioassay. The alkyl series of compounds showed anticytokinin activity, whereas the anilino derivatives exhibited both cytokinin and anticytokinin activities depending on the structure and position of the benzene substituents. Quantitative structure-activity analyses were carried out for each class and for the combined set of compounds with use of physicochemical parameters and regression analysis, indicating that the quality of activity, agonistic or antagonistic, as well as the extent of activity, is significantly affected by the steric features of the molecule. On the basis of the present results and previous quantitative analyses on cytokinins and other classes of anticytokinins, a dimensional map for the cytokinin receptor site can be drawn, which can serve as the basis for the design of novel agonists and antagonists.