10811
J. Phys. Chem. 1991,95, 10811-10816
Solution Conformation of Gibberellic Acid As Determined by NMR and Molecular Mechanics Calculations N. Marcbettini, Y. Yang; and C. Rossi* Department of Chemistry, University of Siena, Pian dei Mantellini, 44 53100 Siena, Ztaly (Received: May 28, 1991; I n Final Form: July 29, 1991)
The proton and carbon assignments of the plant hormone gibberellic acid (GA,) in DMSO-d6were determined. Dynamical and conformational properties of GA, in solution were studied on the basis of proton and carbon dipolar relaxation contributions. A new experimental method for the simultaneous detection on surrounding nuclei of labile protons of direct and cross-relaxation contributionswas proposed and applied to study the conformation of GAS. Finally, the GASstructuresobtained from experimental NMR parameters, energy minimization theory, and X-ray crystal diffraction are compared.
Introduction Growth and development of higher plants are regulated by phyt0hormones.l Five classes of phytohormones of unrelated chemical structure are known: auxins, gibberellins, cytokinins, abscissic acid, and ethylene. These small molecules can easily cross cell walls to interact with receptor sites and stimulate specific metabolic reactiow2 In spite of the fact that there are only five classes of phytohormones, their biological activity as plant regulators is far from being understood. In the present paper the conformation and dynamical properties in solution of a phytohormone, gibberellic acid (GAJ, were analyzed by nuclear magnetic resonance spectroscopy and molecular mechanics calculations. Gibberellins are known to stimulate the production of hydrolytic enzymes (particularly a-amylase) that digest the fat, carbohydrate, and protein reserves of storage tissues. These events constitute the first steps of the germination process.3 It is well-known that the definition of the structural properties of a biomolecule in solution is fundamental for the identification of bioactive mo1e;cular moieties, for structureactivity correlation, and for studying receptor sites. The proton and carbon N M R assignments of GA3 in pyridine-ds and CDC13have been reported in previous paper^."^ After preliminary investigation of the NMR parameters in DMSO-d, solution, the dynamical and structural properties of GA3 were determined by proton and carbon relaxation investigations.68 Structural information obtained by NMR in solution was finally used as constraints in molecular mechanics calculations for defining the solution structure of gibberellic acid. This structure was compared with the structure obtained by X-ray investigation in the solid statee9
TABLE I: Proton Relaxation Parameters Measured for a 0.1 mol dm-j DMSO-8,Solution of Gibberellic Acid (T= 298 K)
6
R,”,s-’
12.51 6.30 5.71 5.54 5.10 4.83 4.83 3.85 3.05 2.47 2.14 1.05
0.95 1.75 1.65 0.95 3.95 1.97 1.55 2.05 5.60 3.11
R I se 7 s-I
Rl nsf R l x
1.25 1.16
1.40 1.42
2.70
1.46
1.38 1.10 1.47 5.20
1.43 1.41 1.40 1.07
NOEBB = (I, - Zo)/Zo, where I, and I, are the line integrals measured under broad-band and gated proton decoupling conditions, respectively.I0 The theoretical NOEth,,, = (YH/ Y ~ ) ( ~ ~ assumes ~ / u ~a constant ~ ) value of 1.998 in extreme narrowing conditions w,r,
