A15N-1H dipolar CSA solid-state NMR study of polymorphous polyglycine (-CO-CD2-15NH-)n

The solid-state1H MAS (magic-angle spinning),2H static,15N CP (cross polarization)-MAS and15N-1H dipolar CSA (chemical shielding anisotropy) NMR (nuclear magnetic resonance) spectra of two different modifications of Cα-deuteratedl5N-polyglycine, namely PG I and PG II (-CO-CD2-l5NH-)n are measured. The data from these spectra are compared to previous NMR, infrared, Raman and inelastic neutron scattering work. The deuteration of Cα eliminates the largest intramolecular1H-1H dipolar coupling. The effect of the remaining (N)H-(N)H interaction (∼5 kHz) is not negligible compared to the15N-1H coupling (about 10 kHz). Its effect on the dipolar CSA spectra, described as a two-spin system, is analyzed analytically and numerically and it is shown that those parts of the powder spectrum, which correspond to orientations with a strong dipolar15N-1H interaction, can be described as an effective two-spin system, permitting the measurement of the strength of the15N-1H dipolar interaction and the orientation of the dipolar vector with respect to the15N CSA frame. While in the PG II system the15N CSA tensor is collinear with the amide plane, in the PG I system the CSA tensor is tilted ca. 16° with respect to the (δ11δ22) CSA plane.