Molecular alignment of proteins in bicellar solutions: quantitative evaluation of effects induced in 2D COSY spectra

Partial molecular alignment leads to an incomplete averaging of anisotropic magnetic interactions such as magnetic dipole interaction or chemical shift anisotropy. In the present contribution we quantitatively describe and evaluate the effects induced by the addition of magnetically oriented lipid bicelles in homonuclear two-dimensional (2D) NMR correlation (COSY) spectra of proteins. It is shown that 2D COSY experiments allow the measurement of H(N)-H(alpha) residual dipole couplings of positive sign which can be used for structure refinement. In contrast to the double- and triple-resonance experiments previously proposed, these measurements can be carried out even on nonisotope-enriched samples.