Cross-polarization schemes for peptide samples oriented in hydrated phospholipid bilayers.

Continuous-wave, ramped amplitude, and frequency modulated cross-polarization schemes (abbreviated as CWCP, RACP, and FMCP, respectively) are evaluated for static samples in anisotropic phases, such as peptides oriented in lipid environments. It is shown experimentally that both RACP and FMCP give rise to 20% higher polarized signal intensity in comparison to CWCP. The CP matching bandwidths for CWCP and RACP are about the same. Because of its adiabaticity, FMCP has a much broader CP matching bandwidth than CWCP and RACP. In addition, the (15)N RF amplitude used at the center of the FMCP matching profile is much lower than that of the CWCP and RACP matching profiles. A sample of [(15)N]Leu(4) labeled gramicidin A oriented in lipid bilayers was used to demonstrate these experiments.