A N-15-H-1 dipolar CSA solid-state NMR study of polymorphous polyglycine (-CO-CD2-(NH)-N-15-)(n)

The solid-stare H-1 MAS (magic-angle spinning), H-2 static, N-15 CP (cross polarization)-MAS and N-15-H-1 dipolar CSA (chemical shielding anisotropy) NMR (nuclear magnetic resonance) spectra of two different modifications of C-alpha-deuterated N-15-polyglycine, namely PG I and PG II (-CO-CD2- (NH)-N -15-)(n) are measured. The data from these spectra are compared to previous NMR, infrared, Raman and inelastic neutron scattering work. The deuteratio n of C-alpha eliminates the largest intramolecular H-1-H-1 dipolar coupling . The effect of the remaining (N)H-(N)H interaction (similar to 5 kHz) is n or negligible compared to the N-15-H-1 coupling (about 10 kHz). Its effect on the dipolar CSA spectra, described as a two-spin system, is analyzed ana lytically and numerically and it is shown that those parts of the powder sp ectrum, which correspond to orientations with a strong dipolar N-15-H-1 int eraction, can be described as an effective two-spin system permitting the m easurement of the strength of the N-15-H-1 dipolar interaction and the orie ntation of the dipolar vector with respect to the N-15 CSA frame. While in the PG II system the N-15 CSA tensor is collinear with the amide plane, in the PG I system the CSA tensor is tilted ca. 16 degrees with respect to the (delta(11)delta(22)) CSA plane.