Partial orientation of cytochrome c in a lyotropic liquid crystal: Residual H-H dipolar coupling

It is shown that dissolving a paramagnetic protein in a lyotropic liquid cr ystal solvent results in measurable splittings of several hyperfine-shifted proton lines, without excessive broadening. These splittings are due to re sidual dipolar coupling of neighboring geminal protons and are interpreted in terms of an orientation tensor, whose magnitude corresponds to about 1% orientation. The reasonable agreement obtained between observed splittings and their calculated absolute values shows that the simplified analysis use d is essentially correct. Advantages of this strategy with respect to the m ilder orientation obtained by using phospholipid mixed micelles (usually ca lled bicelles) were that the induced splittings were so large that they cou ld be measured in homonuclear proton spectra of paramagnetic proteins, in p articular for the heme signals in heme proteins, whose C-13 enrichment may not always be trivial. The sizable broadening induced by the strong orienta tion effect is not a problem as long as well-resolved hyperfine-shifted sig nals are considered.