Fig. 1.

(A) A 1D ¹³C (MAS) NMR spectrum of a lyophilised sample of Gly-Pro-Gly-Gly peptide. The backbone carbons are indicated with coloured circles and their respective NMR signals assigned on the spectrum. Signals marked * are spinning sidebands, necessary artefacts of magic-angle spinning (see (C)). (B) Schematic of a 2D homonuclear correlation spectrum, e.g. ¹³C - ¹³C for the molecule at the top with nuclear (atomic) sites A, B, C, D, E. In this hypothetical spectrum, cross-peaks, indicated as contours (open circles), occur between signals from nuclei that are physically close in space, i.e. A-B, B-C, etc. The diagonal signals in the 2D spectrum (filled circles) are typical for this type of 2D spectrum and do not indicate that e.g. site As are close together with other site As. (C) Solid-state NMR spectroscopy typically requires magic-angle spinning (MAS) to remove the effects of anisotropic nuclear spin interactions. The strength of the effect of these interactions on the NMR signal frequency depends on the orientation of the molecule containing the nucleus with respect to the magnetic field applied in the NMR experiment. Thus in a sample where, e.g. there are collagen molecules with multiple orientations, there will be multiple, overlapping signals, resulting in a broad line (exemplified here with the lineshape resulting from shielding anisotropy - interaction of e.g. ¹³C nuclei with surrounding electrons in the collagen molecules) which limits resolution. Spinning the whole sample at the so-called magic-angle (54.74^∘) with respect to the magnetic field, removes the molecular orientation dependence of the chemical shift, resulting in a sharp line at the isotropic chemical shift for each nuclear site. If the spinning rate is slower than $\sim$width of the NMR signal, so-called spinning sidebands also appear in the spectrum, radiating out from the isotropic signal at the spinning frequency apart.