Stereoisomerism in Plant Growth Regulators of the Auxin Type

Abstract

The relation between chemical structure and physiological action of the plant growth regulators of auxin type has been the object of extensive studies for three decades. Many interesting hypotheses have been suggested to ex plain the available data, but no simple explanation apt to embrace all groups of active substances has as yet been presented. The problem has been the object of several reviews (e.g., 59, 85, 105, 109, 110). Two main alternatives can be discerned in the current discussion.(a) The auxin molecules become attached to a receptor site, presumably at a protein surface, by several weak bonds (e.g., van der Waals forces, electrostatic attraction, hydrogen bridges) . The bonding is thought to be of the same type as that occurring between enzyme molecules and their sub strates. This "multipoint attachment" concept has been developed mainly by Veldstra. The "three-point attachment" hypothesis of Wain and co-work ers, assuming a bonding by the ring system (or other flat part of the mole cule) , the carboxyl group and an a-hydrogen atom in the proper steric posi tion can be regarded as a variant of the multipoint attachment concept.(b) In sharp opposition to this concept, the "two-point attachment" hypothesis of Hansch and Muir assumes a covalent bonding of the auxin molecul e to a protein receptor. The carboxyl group is thought to form a pep tide bond with the protein, while the SH-group of a cysteinyl unit reacts through a nucleophilic attack at an ortho position of the ring system with elimination of two hydrogen atoms. These reactions can hardly be assumed to proceed spontaneously, and the assumption of enzymatic catalysis would apparently bring this hypothesis in close proximity to the multi-1 The survey of literature pertaining to this review was concluded in June 1964. 2 The following abbreviations will be used: POA (phenoxyacetic acid); POB [a (phenoxy)-n-butyric acid]; POC [a-(phenoxy)-n-caproic acid]; POiB [a-(phenoxy) iSO-butyric acid]; POP [a-(phenoxy)-propionic acid]; POV [a-(phenoxy)-n-valeric acid]; substituents in the phenyl nucleus are indicated by Me (methyl), MeO (meth oxy), and by the chemical halogen symbols. Thus, 2,4-CbPOA [2,4-dichlorophenoxy acetic acid (2,4-D)], and so on. The same system is followed for the substances re lated to phenyl-imino-acetic acid (PNA); phenylacetic acid (PA); 3-indoleacetic acid (IAA); 1-and 2-naphthylacetic acid (1-and 2-NAA); 1-and 2-naphthylmethyl acetic acid (1-and 2-NMA); 3-and 2-thianaphthenylacetic acid (3-and 2-TnA); l and 2-naphthoxyacetic acid (1-and 2-NOA); 1-and 2-naphthylthioacetic acid (l and 2-NSA); and 2-naphthylselenoacetic acid (2-NSeA). 53Annu. Rev. Plant. Physiol. 1965.16:53-72. Downloaded from www.annualreviews.org Access provided by University of Nebraska -Lincoln on 04/05/15. For personal use only.Quick links to online content Further ANNUAL REVIEWS

Plant Growth Regulator Activity of Optical Isomers of DMPA

“…However, a conservative estimate of the optical purity of the isomers is 95% for l-DMPA and 91% for d-DMPA4. With INTRODUCTION T HE difference in activity between optical isomers of plant growth regulators containing asymmetric carbon was recently reviewed (5). The influence of asymmetry in tetracovalent organophosphorus compounds on cholinesterase inhibition is a recently recognized phenomenon (1,3,8).…”