Short and long range 2D 15 N- 15 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) 15 N- 15 N correlations on uniformly 15 N-labeled GB1 powders. A major feature exploited in these 15 N- 15 N correlations was AL FRESCO's remarkably low RF power demands, which enabled seconds-long mixing schemes when establishing direct correlations. These 15 N- 15 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, 15 N spin-diffusion experiments. An indirect approach producing long-range 15 N- 15 N correlations was also tested, relying on short (ms-long) 1 H N - 1 H N mixings schemes while applying AL FRESCO chirped pulses along the 15 N channel. These indirect mixing schemes produced numerous long-distance N i -N i±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 1 H N - 1 H N counterparts.