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as expected on the basis of the natural isotopic H/D ratio. The deuterium sat ellites constitute an ideal system for testing dynamic range and resolution of minor resonances very close to large peaks. Peaks di, d->, and d3 are at +7.033 Hz, -12.435 Hz, and -31.909 Hz, respectively, relative to the main resonance. The dynamic range factor D is 10,000, which is greater than the ratio of integrated intensities, because of a contribution from 13C-2H scalar relax ation to the line widths of the deuteri um satellites. On the basis of evidence presented elsewhere, we assign peaks ai and a2 of Figure 4a to C-l and C-5, respectively, of CH ;f -CO-CH 2 -(OH)C(CH 3 ) 2 , com monly called "diacetone alcohol," which is the aldol condensation dimer of acetone. Integrated intensities, cor rected for differences in NOE values, indicate that our sample of reagent grade acetone contained about 0.01% diacetone alcohol. This amount is 3 orders of magnitude less than the equilibrium proportion. Figure 4 suggests that minor compo nents with D values much lower than those of peaks ai, a2, and di-d3 can be observed if their separation from the major resonance is 40 Hz or more. We believe that D values of 105 are accessi ble if the signal-to-noise ratio is high enough. The example of Figure 4, how ever, involves a line width (of the strong resonance) of less than 0.1 Hz. For larger line widths, D values of 104 or more will require separations of much more than 40 Hz, as demonstrat ed below for line widths of about 1 Hz. Aldehyde and aldehydrol a n omers of glucose. The equilibrium composition of aqueous reducing sug ars has been investigated by carbohy drate chemists for the last 140 years. It is well known that each aldopentose or aldohexose in aqueous solution exists as an interconverting mixture of at least six isomeric species (anomers): the two pyranoses, the two furanoses, the acyclic aldehyde form, and the acy clic hydrated aldehyde (gem-diol or al dehydrol) form (27). The equilibrium proportions and rate constants of in terconversion are of considerable inter est because they are major determi nants of chemical and biochemical be havior. The equilibrium proportions of five of the six anomers mentioned above are known with varying degrees of accuracy for most of the aldopentoses and aldohexoses. The exception is the hydrated aldehyde form (27), which has only been reported for aque ous solutions of the two aldotetroses erythrose (28) and threose (28, 29), which contain about 10% aldehydrol at room temperature. The anomeric carbon resonance of the aldehydrol of one aldohexose, D-[l- 13 C]idose, has been detected recently (30), but the
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proportion of this anomer was not re ported. It is generally taken for granted that NMR is not suitable for detecting an anomer whose proportion is much less than one percent (27), even though there are isolated examples of NMR detection of much lower proportions, notably the recent report of 0.03% alde hyde in aqueous D-[l- 13 C]ribose (30). The detailed anomeric composition of aqueous D-glucose is undoubtedly the greatest challenge of the remaining gaps of knowledge about anomeric equilibria, not only because of the bio logical importance of glucose, but also because of the overwhelming domi nance of the two pyranose anomers (27). The reported proportions of the /3-furanose and aldehyde anomers are 0.14% (at 43 °C) and 0.002% (at 20 °C), respectively (27). The α-furanose and aldehydrol forms have never been de tected (27). We expect the anomeric carbon resonance of the aldehydrol to be in close proximity to the corre sponding resonances of the pyranose forms. In contrast, the anomeric carbon resonance of the aldehyde an omer is expected about 100 ppm downfield of the saturated anomeric carbon resonances (29, 30). With the use of our ultra-high resolu tion methodology, we have observed all six anomers of a sample of aqueous D-[l-13C]glucose, and we have obtained equilibrium proportions as a function of temperature. Figure 5 shows the an omeric carbon region in the 13C NMR spectrum of 1.4 M D-[l-13C]glucose in H 2 0. It is important at this point to comment about the peaks labeled X in Figure 5, which are instrumental arti facts. It is well known that very large signal-to-noise ratios (for the large res onances), obviously required for the study of minor components without "solvent" peak suppression, reveal spurious peaks caused by imperfect data accumulation and processing (31, 32). Although these artifacts are very small relative to the major resonances, they can be mistakenly identified as peaks of minor components. We are confident that these artifacts can be reduced to levels even lower than those in Figure 5 and that systematic proce dures can be developed for distinguish ing residual artifacts from real peaks. In the meantime, we have distin guished the resonances of minor an omers of glucose from artifacts by ob serving the temperature dependence of the ,3 C NMR spectra; the intensities of the peaks assigned to minor anomers changed with temperature, all by more than a factor of 4 when going from 27 to 82 °C. We assigned peaks 1, 2, 3, 4, and 5 of Figure 5 to the /î-furanose, α-furanose, /3-pyranose, α-pyranose, and linear gem-diol anomers, respectively, and we obtained the proportions of the an-
