Figure 4. This schematic diagram begins at the bottom left, where a sup 13 C label is shown in bold type at the C2 position of glucose-6-phosphate ([2-sup 13 C]G6P). Naturally abundant carbon,12C, has no magnetic moment, and is nuclear magnetic resonance (NMR)-invisible. The position of13C in the next step of glycolysis is shown immediately to the bottom right. The label remains in the C2 position after phosphoglucose isomerase converts G6P into [2-sup 13 C]fructose-6-phosphate ([2-sup 13 C]F6P); however, when [2-sup 13 C]G6P traverses the pentose phosphate pathway, decarboxylation puts the13C label in the C1 position, causing the formation [1-sup 13 C]F6P. Distinctly different chemical shifts characterize the13C-NMR resonance peaks for these two isotopomers, i.e., these two molecules that are identical except for the location of an isotope label. Measured relative to the chemical shift for [2-sup 13 C]glucose at 74.1 ppm, the chemical shift for the3C resonance of [1-sup 13 C]F6P is at 63.75 ppm, whereas that for [2-sup 13 C]F6P is at 76–82.59 ppm. Therefore, it is possible to distinguish F6P formed via the pentose phosphate pathway from F6P formed by glycolysis.