Alternate 13C-12C labeling for complete mainchain resonance assignments using C alpha direct-detection with applicability toward fast relaxing protein systems.

Experiments that use direct (13)C detection and take advantage of the slower relaxation of (13)C magnetization compared to (1)H offer an attractive strategy for extending the limits of NMR to include larger, highly dynamic, or paramagnetic proteins. Because carbonyl carbons ((13)C') suffer from serious relaxation enhancement as a consequence of their large chemical shift anisotropy, deuterated alpha carbons are the preferred nuclei for (13)C detection in large and/or fast relaxing systems. However, direct detection of (13)C alpha is not straightforward owing to the presence of one-bond (13)C-(13)C couplings with (13)C' and (13)C beta that split the signals into multiples and hence reduce the sensitivity. Here we present the use of (13)C enrichment at alternating carbon sites and deuteration at the C alpha position to overcome these difficulties. The desired labeling pattern is achieved by expressing the protein in E. coli in D(2)O with either [2-(13)C] or [1,3-(13)C] glycerol as the carbon source. With this labeling strategy, we show that complete assignment of the main chain (including prolyl residues) can be achieved with a single CaN HSQC experiment. This approach offers advantages for the detection of NMR signals from sites with fast nuclear relaxation and offers promise for investigations of larger proteins and/or protein complexes that are inaccessible by proton-detected experiments.