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In Helen Tweeddale's article,[1] they labelled metabolites of E. coli with [U-$^{14}$C]glucose. What sort of glucose is it? I understand they used isotope 14, which is radioactive but what does the U stand for?

  1. Helen Tweeddale, Lucinda Notley-McRobb, and Thomas Ferenci, J Bacteriol. 1998, 180(19), 5109–5116. PMID: 9748443 (pdf)
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According to the IUPAC organic chemistry nomenclature 2013, section P-83 ISOTOPICALLY LABELED COMPOUNDS, it has to do with uniformly labeled compounds:

P-83.5 GENERAL AND UNIFORM LABELING

P-83.5.1 In the name of a selectively labeled compound in which all positions of the designated element are labeled, but not necessarily in the same isotopic ratio, the symbol ‘G’ is used in place of locants to indicate a ‘general’ labeling.

Examples:

  1. Mixture of isotopically substituted compounds (selective labeling) $\ce{CH3-CH2-CH2-^{14}COOH}$, $\ce{CH3-CH2-^{14}CH2-COOH}$, $\ce{CH3-^{14}CH2-CH2-COOH}$, $\ce{^{14}CH3-CH2-CH2-^{14}COOH}$, etc., when added to $\ce{CH3-CH2-CH2-COOH}$, is designated as [G-14C]butanoic acid.
  2. D-Glucose in which all six positions are labeled with 14C, but not necessarily uniformly, is designated as D-[G-14C]glucose.

P-83.5.2 In the name of a selectively labeled compound in which all positions of the designated element are labeled in the same isotopic ratio, the symbol ‘U’ is used in place of locants to denote ‘uniform’ labeling.

Examples:

  1. Mixture of isotopically substituted compounds (uniform labeling) $\ce{CH3-CH2-CH2-^{14}COOH}$, $\ce{CH3-CH2-^{14}CH2-COOH}$, $\ce{CH3-^{14}CH2-CH2-COOH}$ and $\ce{^{14}CH3-CH2-CH2-COOH}$, in equal amounts, when added to $\ce{CH3-CH2-CH2-COOH}$, is designated as [U-14C]butanoic acid.
  2. D-Glucose in which 14C is equally distributed among the six positions is designated as D-[U-14C]glucose.

    Note: In the case of radioactive nuclides, ‘same isotopic ratio’ means ‘same specific radioactivity’.

(Please note that the D- stereodescriptor should be in smallcaps)

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