By using only half of the total evolution time for dephasing pulses, C{N} rotational-echo double resonance (REDOR) for clusters of C spins (RDX) results in the same universal REDOR behavior as observed for isolated C-N pairs. RDX combines Hahn echoes with solid echoes to suppress interference from scalar J couplings. This is crucial for long evolution times. The modified version (which we call RDX24) makes RDX quantitative for C clusters. We apply this scheme to human embryonic kidney cells labeled in culture by L-[C -N]-glutamine. We quantitatively characterize three separate nitrogen isotopic enrichments for: (i) the alpha nitrogens of glutamine residues in proteins (including the residues of the five amino acids synthesized from glutamine); (ii) the alpha nitrogens of the five amino-acid residues synthesized from glucose, together with those of the nine essential amino acids added to the growth medium; and (iii) the side-chain nitrogens of glutamine (and of asparagine derived from glutamine).
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| http://dx.doi.org/10.1016/j.jmr.2021.107043 | DOI Listing |
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