The NMR-derived structure of estrogen (17b-estradiol, E2), the drug of choice for postmenopausal women, was compared with a recent literature crystal x-ray structure of Fab-bound E2.1 H and 13 C NMR spectra of E2 were acquired in DMSO-d 6 . Assignments were obtained from an analysis of DQF-COSY, TOCSY, HETCOR, HMQC and HMBC 2D NMR spectra. The 1 H and 13 C NMR assignments are the first reported for E2 in DMSO-d 6 . Two solution structures, S1 and S2, were obtained with molecular modeling using NOE constraints. S1 overlaps with the crystal structure for all rings. S2 shows prominent differences in the C-ring (C9-C11-C12-C13) segment, which deviates from a chair conformation, and excellent overlap in the A-, B-and D-rings of E2. The C-ring in S2 adopts a boat conformation as opposed to a chair conformation in the x-ray and S1 structures. The S2 structure is about 6• more twisted than the bound x-ray and S1 models. The S1, S2 and x-ray structures had ring bowing values of 10.1 ± 0.3, 11 ± 1 and 10.37• , respectively. Of the 100 solution conformers generated, 83 had S1 conformation and 17 had S2 conformation, with average internal energies of 112 ± 2 and 141 ± 2 kcal mol −1 , respectively. The 100 S1-and S2-derived conformers showed a r.m.s.d. of 0.72Å for all atoms. The x-ray, S1 and S2 C18 -O17 distances were 2.93, 2.92 ± 0.01 and 2.93 ± 0.01Å, respectively, and the O3 -O17 distances were 11.06, 11.18 ± 0.12, and 10.89 ± 0.05Å, respectively.

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