Cross-polarization in 14N overtone NMR spectroscopy of solids

Several versions of cross-polarization (CP) sequences are analyzed in order to improve the sensitivity of 14N overtone NMR of biological solids. Optimum conditions are determined from experiments on a single crystalline sample of a model peptide, N-acetyl-d,l-valine, for an efficient CP from 1H to 14N overtone frequency. Experimental results infer that a CP sequence consisting of an amplitude modulated spin-locking of 14N overtone frequency during the CP period followed by a composite 90° postpulse is efficient in transferring the proton dipolar order to the observable overtone signal.