New Auxin Analogs. Possible Probes for Auxin Receptors Elvia Reynoso-Herrera, Carlos Rius-Alonso, Martha Albores-Velasco* Facultad de Qirimica, U.N.A.M. 04510. Mexico, D. F. Fax: (5) 6223774. E-mail: malbores@servidor.unam.mx * Author for correspondence and reprint requets Z. Naturforsch. 54c, 1042-1048 (1999); received March 29/August 3, 1999 Auxin. Auxin Receptors, Molecular Modeling and 2,4-D Analogs Based on structure-activity relationship studies, auxin analogs that can be covalently bound to a polymeric support are proposed. Molecular modeling studies were carried out by com paring different parameters of substituted phenoxyacetic acids with their auxin activity. A good correlation of the activity with the size and shape of the HOMO orbital of the acids was found. Accordingly, analogs with a substituent in the 5 position of the aromatic ring, capable to be bound to a polymeric matrix were synthesized and their auxin activity was evaluated with the wheat coleoptile elongation test. Compounds with a hydroxymethyl- and with a carboxymethyloxy- substituent were active in this test. Their use as probes for the 2,4- D receptor is proposed. Introduction Theories on auxin structure-activity correlations have been widely reviewed, (Muir and Hansch, 1955; Katekar, 1979; Edgerton et al., 1994). Hypo thetical receptor sites on specific cellular recogni tion proteins are proposed, which bind the hor mone and initiate a sequence of events (signal transduction), that culminate in a characteristic physiological or biochemical response. Three approaches have been used for auxin structure-activity correlation studies: the activity measurement of adequate series of compounds and estimation of the molecular characteristics which determine their activity; binding studies with suitable compounds on isolated receptors and lastly, molecular modeling studies. Studies by Porter and Thimann (1965), Wain and Fawcett (1969), Kaethner (1977), Lehmann (1978) and Katekar and Geissler (1983), resulted in models of auxin binding sites and had the long- Abbreviations: 2,4-D, 2,4-dichlorophenoxyacetic acid; ABP, auxin binding protein; ABP1, auxin binding pro tein 1 from maize; CNDO, complete neglect of dif ferential overlapping; HOMO, highest occupied molecu lar orbital; IAA. indoleacetic acid; INDO, intermediate neglect of differential overlapping; Kd, dissociation con stant; Log/3 , logarithm of the partition coefficient; LUMO, lowest unoccupied molecular orbital; N2AA, 2- Naphthylacetic acid: NAA, Naphthyl acetic acid; PMR. Proton magnetic resonance; s, strong; vs, very strong. term purpose of knowing the chemical nature of receptors, which might assist in their isolation and characterization. On the other hand, a large number of auxin binding proteins located in more than one type of cellular membranes have been reported (Jones et al., 1998; Libbenga et al., 1986). They are charac terized as ABP’s (auxin binding proteins) by their susceptibility to covalent photolabeling by triti- ated azido-indole-3-acetic acid (Venis and Napier, 1985). The multiplicity of auxin binding sites opened the problem of auxin site modeling for re investigation. ABP1 is one of the best-studied binding pro teins. Ray (1977), measured dissociation constants of 45 auxin analogs with ABP1, his data on auxin binding represent the only intensive characteriza tion of a single auxin-binding site analyzed in vitro published to date. With these data, Edgerton et al. (1994) modeled the auxin-binding site of' the maize’ ABP1. These researchers proposed an auxin phytophore that incorporates the key struc tural features of the three compounds that bind the protein with the highest affinity: NAA (naph- thylacetic acid), N2AA (2naphthylacetic acid) and IAA (indoleacetic acid). The ABP1 binding site according with these authors, consists of a plat form to which the indole or naphthalene rings bind and a somewhat flexible region, which interacts with the carboxylic acid group. 0939-5075/99/1200-1042 $ 06.00 © 1999 Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com • D This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution-NoDerivs 3.0 Germany License. On 01.01.2015 it is planned to change the License Conditions (the removal of the Creative Commons License condition “no derivative works”). 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