Determination of molecular structure by NMR using low order orientation in bicelles

Abstract

NMR spectra of molecules oriented in liquid crystalline phase provide information on the structure and orientation of the molecules. Thermotropic liquid crystals used for such a purpose as orienting media result in the spectra of spins that are generally strongly coupled unless heteronuclei are involved. The analyses of such spectra are carried out by diagonalising the Hamiltonian numerically, adopting the least square techniques. Phospholipid bicelles on the other hand are weak orienting systems which provides well-resolved proton and carbon spectra of spins that are, in general, weakly coupled. A spin system which is of the type AA′BB′C in thermotropic liquid crystal behaves as AA′MM′X type in bicelles. Though the resultant spectrum is not always first order, nevertheless, the estimate of the proton–proton and proton–carbon dipolar couplings and the chemical shifts can be made from the spectrum directly. The use of such a media as an alternative to thermotropic liquid crystalline phase for the determination of the structure of small molecules is demonstrated in this paper. The molecule pyridine is chosen for such a purpose. The structure of pyridine, including 13C spins, in both bicelle and thermotropic nematic phase have been determined.