Partial 13C isotopic enrichment of nucleoside monophosphates: useful reporters for NMR structural studies

Analysis of the 13C isotopic labeling patterns of nucleoside monophosphates (NMPs) extracted from Escherichia coli grown in a mixture of C-1 and C-2 glucose is presented. By comparing our results to previous observations on amino acids grown in simi- lar media, we have been able to rationalize the labe- ling pattern based on the well-known biochemistry of nucleotide biosynthesis. Except for a few notable absences of label (C4 in purines and C30 in ribose) and one highly enriched site (C10 in ribose), most car- bons are randomly enriched at a low level (an average of 13%). These sparsely labeled NMPs give less com- plex NMR spectra than their fully isotopically labeled analogs due to the elimination of most 13C-13C scalar couplings. The spectral simplicity is particularly advantageous when working in ordered systems, as illustrated with guanosine diphosphate (GDP) bound to ADP ribosylation factor 1 (ARF1) aligned in a liquid crystalline medium. In this system, the absence of scalar couplings and additional long-range dipolar couplings significantly enhances signal to noise and resolution.