Comparison of epimeric methyl lithocholate and methyl iso-lithocholate molecules: single crystal X-ray structure of methyl lithocholate, ab initio HF/6-31G*optimized structures and experimental and calculated DFT/B3LYP C-13 NMR chemical shifts

摘要

C-13 NMR chemical shifts have been measured and assigned for epimeric methyl 3alpha/beta-hydroxy-5beta-cholan-24-oates (methyl lithocholate [3alpha-OH epimer] and methyl iso-lithocholate [3beta-OH epimer]). Their molecular dynamics simulations suggest that for both epimers there exists two predominant gas phase conformations, which have been further forwarded for ab initio/HF optimizations and DFT/GIAO based C-13 NMR chemical shift calculations. Excellent linear relationships have been observed between experimental and calculated C-13 NMR chemical shifts for both epimers. For methyl lithocholate (MeLC), the other minimum energy conformation equates very well with the single crystal X-ray structure (orthorhombic, space group P2(1)2(1)2(1), unit cell a = 7.14710(10) Angstrom, b = 11.9912(2) Angstrom. c = 26.4368(5) Angstrom). The crystalline packing of MeLC consists of continuous parallel intermolecular hydrogen bonded [3alpha-(OHO)-O-...=C24] head-to-tail polymeric chains, which are further cross-linked by many simultaneous weak C(sp(3))(HO)-O-...-type of interactions. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 30]