Probing Proton−Proton Proximities in the Solid State:  High-Resolution Two-Dimensional ¹H−H Double-Quantum CRAMPS NMR Spectroscopy

A new 1H DQ (double-quantum) CRAMPS (combined rotation and multiple-pulse sequence) solid-state nuclear magnetic resonance experiment incorporating DUMBO homonuclear 1H dipolar decoupling is presented. The major resolution enhancement enables DQ peaks corresponding to all 22 close (<3.5 A) proton-proton proximities in the dipeptide beta-AspAla to be observed. In particular, the DQ CRAMPS spectrum provides access to the alkyl region of the spectrum and yields a clear assignment of the two CH and two diastereotopic CH2 proton resonances.