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Molecular Mechanics and SCF MO Conformational Analysis of Indol-3-ylacetic Acid, Phytohormone

Conformational analysis of indol-3-ylacetic acid (IAA)-plant growth hormone has been performed using molecular mechanics and the ab initio SCF MO theory. The equilibrium geometry of IAA has been determined. Relative energies of alternative conformations, their charge distribution, dipole moment and energy barriers between them have been calculated. The position of the carboxyl group relative to the indole ring depends on two torsion angles, Tl(C2-C3-C8-C9) and T2(C3-C8-C9=02). Rotational barriers for these two angles were explored and it emerged that both rotations (about the C3-C8and C8-C9 bonds) can be accomplished in a reasonable time period at room temperature (the barrier height is about 4.6-10.9 (TI) and 1.7-3.8 (T2) kJ/mol respectively, according to ab initio calculations. Ab initio (GAMESS)and molecular mechanics (DISCOVER (CVFF and cff91), SYBYL(TRIPOS) and MM2(87), calculations revealed qualitatively the same shape of potential energy surface (E =f(Tl, T2)). However, energy differences between various conformations depend on the basis set (ab initio calculations) and force field (molecular mechanics) used.

X-ray structure ; indol-3-ylacetic Acid ; molecular modelling

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