Structure Determination of Auxin Phytohormones.

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The potential energy surfaces of 5-Cl-IAA, 6-Cl-IAA, 7-Cl-IAA, and 5,6-Cl₂-IAA show a high similarity with that of unsubstituted IAA: the positions of the local minima are almost identical with differences of the torsion angle values within a few degrees, and the same is true for the transition states and the reaction paths. Furthermore the relative energy of corresponding stationary points is within a range of 1 kJ/mol, except for the increased energy gap between the global minimum (which is displayed above) and the respective minimum of second lowest energy, if a chlorine atom is substituted in position 5. In unsubstituted IAA, 6-Cl-IAA, and 7-Cl-IAA this energy gap is below 0.7 kJ/mol, whereas in 5-Cl-IAA and 5,6-Cl₂-IAA it is more than 1.9 kJ/mol.

The CHELPG routine was used to determine effective nuclear charges for the H-N-C-C-C-COOH fragment of all local minima. In this fragment, the atoms are only indirectly affected by the halogenation in different positions, therefore any charge differences in this part of the molecule therefore are no "trivial" effects. Also, the functional groups COOH and NH can be expected to play a key role in the binding to the auxin receptor. The values show minor variations for the atoms of the COOH group, but considerable variation for all other atoms. For the NH group, e. g., the nitrogen charge varies between -0.26 and -0.69 and that of hydrogen ranges from +0.32 and +0.42.

Two main conclusions can be drawn from the results of this study, both of which point into the same direction. The first is based on the striking similarity of the potential energy surfaces of these biologically diverse compounds and the contrasting situation in 4-Cl-IAA. The latter shows that the steric and electronic interference of the same substituent in a different position can change the situation completely. If the generally accepted assumption that all IAA derivatives bind to same receptor is correct, then steric and electronic effects of the halogen substituents during the binding process can be expected to be the main reason for their different biological activity. This may well be the explanation for the observation of Hatano et al. that the positions of chlorine atoms are more important for auxin activity than lipophilicity or resistence to peroxidase decomposition.

The second conclusion regards the wide range of effective nuclear charges that occurs for each of the compounds discussed here upon movement of the acetic acid side chain (which is the only flexible part of the molecule). This wide range shows that the electron density is quite flexible in large parts of the molecule and that it can easily re-orient itself during conformational changes. This ability can also be expected to be an important feature during the binding process.

4-Cl-IAA.
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4-, 5-, 6-Et-IAA.
 
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