COMBINED AB-INITIO SCF AND MOLECULAR MECHANICS STUDIES OF PROPIONIC AND ISOBUTYRIC ACIDS AND THEIR INDOLE-DERIVATIVES RELATED TO THE PHYTOHORMONE AUXIN (INDOLE-3-ACETIC-ACID)

Detailed conformational analyses of propionic and isobutyric acids wer e performed to contribute to a better understanding of the stereochemi cal characteristics of biologically active indole-3-aliphatic acids. T he studies are based on ab initio SCF (RHF/6-31G) and molecular mecha nics (force fields used: MM2, MM3, CFFS1, AMBER, CVFF, ESFF) methods. The results obtained with the CFFS1 and MM3 force fields revealed the best agreement with the experimental values and those from ab initio c alculations. Normal mode frequencies in the harmonic oscillator approx imation were calculated for the geometry optimized conformers with C-s symmetry of these compounds as well as of indole-3-acetic acid (IAA) and some of its biologically important derivatives (4-Cl-IAA, 6-Cl-IAA , 7-Cl-IAA, 4-Me-IAA) and indole-3-isobutyric acid (IIBA). The influen ce of the indole ring on the C=O and O-H stretching frequencies was an alyzed. A small decrease of the C=O frequency was determined in the in dole-3-acetic acid derivatives and a larger one in indole-3-isobutyric acid.