Structural studies on monohalogenated derivatives of the phytohormone indole-3-acetic acid (auxin)

Abstract

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-CI-IAA as well as 5-CI-IAA, 6-CI-IAA, 7-CI-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-CI-IAA, 6-CI-IAA, 7-CI-IAA and 5-Br-IAA, and at 220 K for 5-CI-IAA, using Mo Kα radiation (λ = 0.71073 Å), and CuKα (λ = 1.5418 Å) for 6-CI-IAA, are as follows: 4-CI-IAA, C10H8ClNO2, Mr = 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) Å3, Z = 4, Dx = 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-CI-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) Å3, Z = 4, Dx = 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-CI-IAA, C10H8ClNO2, orthorhombic, Pbca, a = 61.08 (1), b = 12.115 (7), c = 7.674 (5) Å, V = 5679 (5) Å3, Z = 24, Dx = 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-CI-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) Å3, Z = 4, Dx = 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, Mr = 254.08, triclinic, P{\bar 1}, 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) Å3, Z = 2, Dx = 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.

Correlation of structural and physico-chemical parameters with the bioactivity of alkylated derivatives of indole-3-acetic acid, a phytohormone (auxin)

“…This computational approach based on molecular properties can also be used to predict the activities of new plant growth substances and herbicides. To provide more data on potential auxins and nonauxins we have systematically studied monochlorinated (Nigovic  et al, 1996) and dichlorinated IAAs (Antolic  et al, 1999), mono¯uorinated (Antolic  et al, 1996) and also nalkylated IAAs (alkyl = Me, Et, Pr n , Bu n substituted at position 5 of IAA; Kojic Â-Prodic  et al, 1991). This paper is focused on the quantitative structure±property relationships of alkylated IAAs [alkyl = Me and Et,(1)].…”

Structural and Physicochemical Studies of Olopatadine Hydrochloride Conformational Polymorphs

“…The values of the distances between C…O and H…O as well as the C-H…O(2) angle are as follows: 3.431 Å, 2.50 Å, and 158.5 and satisfy the criteria given for this type of hydrogen bonds. 34 In fact, this is the orientation of carboxylic group in respect to the center of tetramer (and of a chain of tetramers in general) which differs the most between the form I and the form II. In the form I, carbonyl oxygen of that group is oriented toward the inside of the tetramer, whereas in the form II, carbonyl oxygen of that group is oriented toward the outside of the tetramer.…”

Molecular structure and vibrational spectra of 4-, 5-, 6-chloroindole

“…Therefore we could not compare our calculated results with those of experimental values. The calculated geometric parameters for 4-, 5-, 6-Clin are summarized in Table 1, together with the definition of the natural coordinates and with the relevant data of 4-, 5-and 6-chloroindole-3-acetic acid [18].…”