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Quantum chemical methods AM1 and PM3 and chromatographic methods were used to qualitatively characterize pathways of bacterial production of indole-3-acetic acid (IAA). The standard free energy changes (DG8 H sum ) for the synthesis of tryptophan (Trp) from chorismic acid via anthranilic acid and indole were calculated, as were those for several possible pathways for the synthesis of IAA from Trp, namely via indole-3-acetamide (IAM), indole-3-pyruvic acid (IPyA), and indole-3-acetonitrile (IAN). The DG8 H sum for Trp synthesis from chorismic acid was ± 402 (± 434) kJ´mol ±1 (values in parentheses were calculated by PM3). The DG8 H sum for IAA synthesis from Trp were 2 565 (± 548) kJ´mol ±1 for the IAN pathway, ± 481 (± 506) kJ´mol ±1 for the IAM pathway, and ± 289 (± 306) kJ´mol ±1 for the IPyA pathway. By HPLC analysis, the possibility was assessed that indole, anthranilic acid, and Trp might be utilized as precursors for IAA synthesis by Azospirillum brasilense strain Sp 245. The results indicate that there is a high motive force for Trp synthesis from chorismic acid and for IAA synthesis from Trp, and make it unlikely that anthranilic acid and indole act as the precursors to IAA in a Trp-independent pathway.

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