Analysis of double cross-polarization rates in solid proteins

Abstract

The double cross-polarization experiment involving the sequential transfer of nuclear polarization among 1H-, 15N-, and 13C-spin systems can be used to detect the occurrence of 13C- 15N bonds in solid proteins. However, the complexity of most proteins requires simplifying approximations to make possible the determination of the concentrations of those 13C- 15N bonds. This paper demonstrates that a model involving only two 13C- 15N cross-polarization rates (determined from model compounds) and simple statistics is adequate to treat data from heavily 13C- and 15N-labeled proteins in soybean leaves. In the course of examining the applicability of model compounds to this problem, we have used a moment analysis of 15N NMR lineshapes to determine 13C- 15N dipolar couplings in two doubly labeled amino acids, glycine and asparagine.