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 crystal solvent results in measurable splittings of several hyperfine-shifted proton lines, without excessive broadening. These splittings are due to residual 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 used is essentially correct. Advantages of this strategy with respect to the milder orientation obtained by using phospholipid mixed micelles (usually called bicelles) were that the induced splittings were so large that they could be measured in homonuclear proton spectra of paramagnetic proteins, in particular for the heme signals in heme proteins, whose 13C enrichment may not always be trivial. The sizable broadening induced by the strong orientation effect is not a problem as long as well-resolved hyperfine-shifted signals are considered.