Short and long range 2D N-N NMR correlations among peptide groups by novel solid state dipolar mixing schemes.

A recently developed homonuclear dipolar recoupling scheme, Adiabatic Linearly FREquency Swept reCOupling (AL FRESCO), was applied to record two-dimensional (2D) N-N correlations on uniformly N-labeled GB1 powders. A major feature exploited in these N-N correlations was AL FRESCO's remarkably low RF power demands, which enabled seconds-long mixing schemes when establishing direct correlations. These N-N mixing schemes proved efficient regardless of the magic-angle spinning (MAS) rate and, being nearly free from dipolar truncation effects, they enabled the detection of long-range, weak dipolar couplings, even in the presence of strong short-range dipolar couplings. This led to a connectivity information that was significantly better than that obtained with spontaneously proton-driven, N spin-diffusion experiments. An indirect approach producing long-range N-N correlations was also tested, relying on short (ms-long) H-H mixings schemes while applying AL FRESCO chirped pulses along the N channel. These indirect mixing schemes produced numerous long-distance N-N (n = 2 - 5) correlations, that might be useful for characterizing three-dimensional arrangements in proteins. Once again, these AL FRESCO mediated experiments proved more informative than variants based on spin-diffusion-based H-H counterparts.