Structure Determination of Auxin Phytohormones.

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Each of the potential energy surfaces of 4-, 5-, and 6-ethylindole-3-acetic acid (Et-IAA) contains four symmetry unique energy minima. The global minima, which are shown below this paragraph, are always characterized by an out-of-plane orientation of the ethyl group. The acetic acide side chain, however, is coplanar with the indole nucleus in 5- and 6-Et-IAA, whereas it is tilted in 4-Et-IAA. In the latter case, the two side chains point towards opposite sides of the indole nucleus; as a consequence of this conformation, a weak hydrogen bond C-H···O=C is formed between the methylene group of the ethyl side chain and the carbonyl oxygen atom, with a hydrogen bond distance of 2.552 Å and a bond order of 0.021.

Comparison with the results obtained earlier for unsubstituted IAA shows that ethylation in position 5 or 6 introduces only minor changes of the PES, i.e., the positions of the local minima in T1/T2-space correspond more or less to those of unsubstituted IAA. Also, the reaction paths related to the acetic acid side chain are basically identical. The same has been found for 5- and 6-Cl-IAA. In the case of 4-Et-IAA, the deviations from unsubstituted IAA are much larger, but despite these deviations, the pattern of T1/T2-reaction paths of the IAA PES is also present in that of 4-Et-IAA. This is a remarkable contrast to the situation in 4-Cl-IAA, where some local minima appear at significantly different positions in T1/T2-space, and the reaction paths are completely different. Hence it appears that the different PES of 4-Cl-IAA is an effect that is specifically related to the chloro substituent at position 4. Interestingly, the qualitative picture, which emerges from the PES of 4-Cl-IAA, IAA, and 4-Et-IAA, is well in accord with the measured biological data.

5-Cl-, 6-Cl-, 7-Cl-, 5,6-Cl₂-IAA.
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4-Me-IAA.
 
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