Structural Studies on Monohalogenated Derivatives of the Phytohormone Indole-3-acetic Acid (Auxin)

The physiological properties of the phytohormone (auxin) indole-3-acetic acid (IAA) and its ring substituted derivatives have so far been rationalized by a number of contradictory hypotheses based on incomplete structural data deduced mainly by inspection of molecular models. In order to give more evidence for structure-activity relationships of monohalogenated IAA's, the molecular structures of the natural auxin 4-Cl-IAA as well as 5-Cl-IAA, 6-Cl-IAA, 7-Cl-IAA and 5-Br-IAA have been compared, as revealed by X-ray analysis, and molecular mechanics and dynamics. The influence of the substitution site and the size of the halogen atom and bioactivity is discussed. The typical structural feature of the molecules studied is the slight distortion of part of the indole nucleus around C7: bond length C6-C7 1.368 (6) Å and C6-C7-C71 117.6(3)° (average values of five structures and seven molecules). The conformations of monohalogenated indole-3-acetic acid molecules, characteristic for auxins, are defined by rotations about two bonds only: one describes the relative orientation of a side chain towards the indole moiety and the second the orientation of the carboxylic group. The results of X-ray structure analysis, and molecular mechanics and dynamics revealed the folded shape of the molecules in all compounds studied. Ab initio calculations showed that the planar conformation can be adopted as well. Crystal data at 297 K for 4-Cl-IAA, 6-Cl-IAA, 7-Cl-IAA and 5-Br-IAA, and at 220 K for 5-Cl-IAA, using Mo Kα radiation (λ = 0.71073 Å), and Cu Kα (λ = 1.5418 Å) for 6-Cl-IAA, are as follows: 4-Cl-IAA, C₁₀H₈ClNO₂, M_r = 209.63, monoclinic, P2₁/c, a = 7.313 (4), b = 17.156 (4), c = 7.640 (4) Å, β = 92.71 (5)°, V = 957.5 (1) ų, Z = 4, D_x = 1.454 g cm^-3, μ = 3.7 cm^-1, F(000) = 432, R = 0.037, wR = 0.039 for 1040 symmetry-independent [I ≥ 3σ(I)] reflections; 5-Cl-IAA, C₁₀H₈ClNO₂, monoclinic, P2₁/c, a = 19.141 (4), b = 5.154 (2), c = 10.323 (3) Å, β = 116.23 (2)°, V = 913.5 (1) ų, Z = 4, D_x = 1.524 g cm^-3, μ = 3.8 cm^-1, F(000) = 432, R = 0.039, wR = 0.042 for 1184 symmetry-independent [I ≥ 3σ(I)] reflections; 6-Cl-IAA, C₁₀H₈ClNO₂, orthorhombic, Pbca, a = 61.08 (1), b = 12.115 (7), c = 7.674 (5) Å, V = 5679 (5) ų, Z = 24, D_x = 1.471 g cm^-3, μ = 33.9 cm^-1, F(000) = 2592, R = 0.052, wR = 0.052 for 3030 symmetry-independent [I ≥ σ(I)] reflections; 7-Cl-IAA, C₁₀H₈ClNO₂, monoclinic, P2₁/c, a = 20.244 (5), b = 4.829 (2), c = 10.728 (4) Å, β= 116.30 (1)°, V = 940 (1) ų, Z = 4, D_x = 1.481 g cm^-3, μ = 3.7 cm^-1, F(000) = 432, R = 0.042, wR = 0.029 for 889 symmetry-independent [I ≥ 3σ(I)] reflections; 5-Br-IAA, C₁₀H₈BrNO₂, M_r = 254.08, triclinic, P1, a = 5.645 (3), b = 9.713 (4), c = 10.019 (4) Å, α = 116.02 (3), β = 92.67 (5), γ = 100.12 (4)°, V = 481.2 (5) ų, Z = 2, D_x = 1.754 g cm^-3, μ = 42.0 cm^-1, F(000) = 252, R = 0.029, wR = 0.020 for 1865 symmetry-independent [I ≥ 3σ(I)] reflections.