Structure of a biologically active conjugate of auxin: N-indol-3-ylacetyl-L-norleucine at 297 and 133 K

Abstract

CI6H2oN203, M r = 288.35, monoclinic, P2), Z=2, MoKa, a=0.71073A, F(000)=308. At 297 K: a = 9.350 (6), b = 9.271 (2), c = 9.695 (12), fl = 108.38 (3) °, I, = 798 (1)/~3, Dx = 1.201 gcm -3, /J, = 0.782 cm- l, R = 0.072, wR = 0.070 for 851 reflec- tions with I> 1.5tr(/). At 133 K: a = 9.474 (6), b = 9.163(4), c=9.524(6) A, fl=107.83(3) ° , I/= 787.1 (8)/~3, Dx = 1.217 g cm -3, Iz = 0.792 cm -l, R = 0.053, wR = 0.046 for 986 reflections with I> 1.5tr(/). The indole ring system and the C atom of the adjacent methylene group are coplanar, whereas the CONR residue adopts a folded conformation. The peptide H is orientated towards the aromatic nucleus. The crystal packing involves hydrogen bonds, N--H...O between the indole N atom and the carboxylic acid group, and O---H.-.O between the carboxylic acid group and the peptide O atom. Introduction. Indole-3-acetic acid (IAA, auxin) is a plant growth hormone which regulates many physio- logical functions such as cell division and enlargement, development differentiation, and the synthesis of specific proteins (Thimann, 1977; Davies, 1987). Biogenesis and metabolism of the plant hormones are, as yet, incompletely understood and there is no general agreement on the molecular mechanism of their action. A special regulatory func- tion has been attributed to the bound auxins, or auxin conjugates (Cohen & Bandurski, 1982; Magnus, 1987). They are involved in hormone trans- port and serve as long- and short-term storage forms of the hormone. In addition, some of amino-acid conjugates are biologically active per se (Magnus, Hangarter & Good, 1992). Investigations of molecu- lar recognition of auxins have so far been based on a few structural and physico-chemical parameters of the compounds studied: relative orientation of side chains towards the indole nucleus, intramolecular distance between the active sites (indole nitrogen and O==C), distribution of hydrophobic and hydrophilic regions, and electronic properies of the aromatic system. In order to recognize essential parameters of * To whom correspondence should be addressed.