Recognition of the folded conformation of plant hormone (auxin, IAA) conjugates with glutamic and aspartic acids and their amides

The molecular structure of the endogenous plant hormone (auxin) conjugate, N-(indol-3-ylacetyl)-L-glutamic acid, is deduced by comparison with N-2-(indol-3-ylacetyl)glutamine (IAA-Gln), N-2-(indol-3-ylacetyl)asparagine (IAA-Asn) and N-(indol-3-ylacetyl)-L-aspartic acid using X-ray structure analysis, H-1-NMR spectroscopy (NOE measurements) and molecular modelling. The significance of the overall molecular shape, and of the resulting amphiphilic properties, of the compounds studied an discussed in terms of possible implications for trafficking between cell compartments. Both in the solid state and in solution, the molecules are in the hair-pin (folded) conformation in which the side chain is folded over the indole ring. While extended conformations can be detected by molecular dynamics simulations, they are so short-lived that any major influence on the biological properties of the compounds studied is unlikely. (C) 2001 Elsevier Science B.V. All rights reserved.