STRUCTURAL COMPARISON OF BIOLOGICALLY-ACTIVE AND INACTIVE CONJUGATES OF ALPHA-AMINO-ACIDS AND THE PLANT-GROWTH HORMONE (AUXIN) INDOLE-3-ACETIC-ACID

The crystal structures of the naturally occurring biologically active conjugate N-(indol-3-ylacetyl)-L-valine [N-(IAA)-L-Val] and the synthe tic inactive N-(indol-3-ylacetyl)-alpha-aminoisobutyric acid [N-(IAA)- alpha-Aib] have been determined. The growth-promoting activity of thes e compounds is discussed on the basis of their stereochemical properti es. Conformational analysis of the amino-acid side chains has been per formed by molecular mechanics and dynamics, and a comparison has been made with the conformations detected in the solid state. N-(IAA)-L-Val , C15H18N2O3, M(r) = 274.32, monoclinic, P2(1), a = 7.185 (2), b = 10. 850 (6), c = 9.600 (4) angstrom, beta = 105.80 (4)-degrees, V = 720 (1 ) angstrom3, Z = 2, D(x) = 1.265 g cm-3, Cu Kalpha radiation, lambda = 1.5418 angstrom, mu = 6.902 cm-1, F(000) = 292, T = 297 K, R = 0.052, wR = 0.059 for 1452 reflections with I greater-than-or-equal-to 3sigm a(I). N-(IAA)-alpha-Aib, C14H16N2O3, M(r) = 260.30, monoclinic, P2(1)/ c, a = 11.134 (7), b = 7.689 (2), c = 15.548 (10) angstrom, beta = 95. 62 (3)-degrees, V = 1324 (1) angstrom3, Z = 4, D(x) = 1.311 g cm-3, Mo Kalpha radiation, lambda = 0.71073 angstrom, mu = 0.995 cm-3, F(000) = 552, T = 297 K, R = 0.042, wR =0.036 for 1442 reflections with I gre ater-than-or-equal-to 3sigma(I). The indole ring system and the C atom of the adjacent methylene group are coplanar whereas the CONR residue adopts a folded conformation. In the structure of N-(IAA)-L-Val the p eptide H faces the indole ring. In the structure of N-(IAA)-alpha-Aib the peptide group lies astride the ring plane and is involved in an in tramolecular hydrogen bond to the carboxylic group. In both crystal st ructures packing is via N-H...O of the indole N atom and the carboxyli c group, and O-H...O of the carboxylic group and peptide O atom.