Conformation ascorbic acid

The crystal structure of D-gulono-1,4-lactone (2) has been determined,42 and is closely approximated by formula 38. Interestingly, the conformation of the side chain in 38 differs from that in crystalline L-ascorbic acid,1 very probably because of different steric interactions between the 3- and 5-hydroxyl groups in 2 (that is, 38) and 6. Bridgman43 measured the compressibility of gulono-1,4-lactone crystals. 

It has been proved by X-ray analysis that, in the solid state, 1 is the tautomer present, but the claim has been made that,68 in solution, L-ascorbic acid exists as 2. Structures 3,4, and 5 were readily eliminated by a study of the l3C-n.m.r. spectrum,88,87 and, on the basis of the chemical shifts of the carbon resonances for C-l, C-2, and C-3, and the known chemistry of L-ascorbic acid, 1 is favored over 2 in solution. Berger98 claimed that the proton-carbon-coupled spectrum of L-ascorbic acid is consistent only with structure 1. Ogawa and coworkers87 studied the conformation of L-ascorbic acid and L-ascorbic acid-5-d in deuterium oxide by 13C-n.m.r. spectroscopy, and concluded that, in... 

Figure 27. Dependence of specific rate constant for pre-filamentary center growth upon bromide ion concentration, showing conformity with the Butler-Volmer law of electrode kinetics. Developing agent, 10 m ascorbic acid [107].

The objectives of this work were to verify the assignments of carbon-13 resonances in L-ascorbic acid (I), to use the carbon-13 chemical shifts to assign positions of substitution in derivatives of I, and to determine the conformational preference of I and its sodium salt (II) in aqueous solution. 

NMR Spectroscopy and the Conformations of L Ascorbic Acid and D-Isoascorbic Acid in Aqueous Solution... 

In 1977 NMR spectroscopy was used to assign a conformation to the side-chain of L-ascorbic acid (I) in water (21). That report stimulated us to measure the 600-MHz NMR spectra of I and D-iso-ascorbic acid (in) and their 4- and 5-deutero derivatives. The 600-MHz NMR spectra were analyzed, and the vicinal coupling constants were used to predict preferred conformations. 

DiflFerent coupling constants between H5-H6 and H5-H6 can occur only if there is a preferred conformation for L-ascorbic acid around the C5-C6 bond. The C4-C5 bond of I also exists in a preferred conforma-... 

When a standard for a food product has been established wherein the use of ascorbic acid is permitted, the standard should be consulted to ensure that the labeling of the food product conforms with the labeling specifications of the standard. 

Much progress has been made in recent years in the chemistry of ascorbic acid. At least six new syntheses of L-ascorbic acid have been devised since 1971. One of those methods was used to prepare specifically labeled L-ascorbic acid to investigate its biosynthesis in plants. Proton magnetic resonance at 600.2 MHz has shown that the side chain of L-ascorbic acid and its sodium salt in aqueous solution adopt the same conformation as crystalline L-ascorbic acid. The conformation of crystalline sodium L-ascorbate, on the other hand, is different. 

Figure 3.8 Structure of L-ascorbic acid molecule showing preferred conformations...

The H n.m.r spectrum of L-ascorbic acid is particularly interesting and careful analysis has enabled the conformations of the molecule in aqueous solution to be studied. When the spectrum is run in D2O, the four OH protons are exchanged and do not appear as separate signals. The remaining four protons (H-6, H-6, H-5, and H-4) comprise an ABMX system, the two protons on C-6 being non-equivalent because of the chirality of C-5. 

The magnitude of the proton coupling constants, J, obtained from such spectra enables the preferred conformation of L-ascorbic acid in aqueous solution to be deduced. For example, Jh4, hs is found to be 1.8 Hz which is the predicted value for the conformation shown in... 

Figure 3.13 Preferred C-4—C-5 conformations of aqueous L-ascorbic acid...

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