Terminal secondary C(sp3), aliphatic secondary C(sp2), aliphatic tertiary C(sp2), primary amine (aliphatic), ring secondary C(sp3) and imidazole numbers can be used to discriminate between cytokinins and cytokinin antagonists

Abstract

One of the most widely used approaches to study cytokinin mode of action involves the use of cytokinin antagonists (anticytokinins). Despite this, few studies have compared the chemical structure of cytokinins and anticytokinins. This formed the focus of the present study, which identified a range of molecular descriptors differentiating four cytokinins from 11 cytokinins antagonists. DRAGON software (version 5.5, 2007) and CambridgeSoft ChemOffice (version 12, 2010) tools (viz. ChemDraw and Chem3D) were used to calculate 212 molecular descriptors for the cytokinins and anticytokinins studied. Only 18 descriptors showed statistically significant differences between cytokinins and antagonists. The most marked molecular descriptor differences included the following: Cytokinins did not contain terminal secondary C(sp3), primary amines (aliphatic) or ring secondary C(sp3) which were present in several antagonists; cytokinins averaged 11 times more imidazoles, 2.2 times more secondary amines (aromatic), 1.8 times more rotatable bonds, 1.6 times more pyrimidines, 1.5 times more multiple bonds, 1.5 times aromatic bonds, 1.2 times non-H bonds and 1.2 times non-N atoms. The following molecular descriptors were also higher in cytokinins: rotatable bond fraction, sum of conventional bond orders (H-depleted), sum of Kier-Hall electrotopological states, molecular weight, and unsaturation index. The molecular descriptors (18) that differed significantly between cytokinins and anticytokinins were used to generate a dendrogram which correctly classified the two groups as two independent branches. The data presented support the classification of these 15 molecules as cytokinins or cytokinin antagonists and can be explored to determine their usefulness in finding new molecules of either class. Molecular descriptors differentiate cytokinins and anticytokinins.