Ascorbic acid, chemical structure

L-Ascorbic acid, better known as vitamin C, has the simplest chemical structure of all the vitamins (Figure 18.30). It is widely distributed in the animal and plant kingdoms, and only a few vertebrates—humans and other primates, guinea pigs, fruit-eating bats, certain birds, and some fish (rainbow trout, carp, and Coho salmon, for example)—are unable to synthesize it. In all these organisms, the inability to synthesize ascorbic acid stems from a lack of a liver enzyme, L-gulono-y-lactone oxidase. 

Since many essential nutrients (e.g., monosaccharides, amino acids, and vitamins) are water-soluble, they have low oil/water partition coefficients, which would suggest poor absorption from the GIT. However, to ensure adequate uptake of these materials from food, the intestine has developed specialized absorption mechanisms that depend on membrane participation and require the compound to have a specific chemical structure. Since these processes are discussed in Chapter 4, we will not dwell on them here. This carrier transport mechanism is illustrated in Fig. 9C. Absorption by a specialized carrier mechanism (from the rat intestine) has been shown to exist for several agents used in cancer chemotherapy (5-fluorouracil and 5-bromouracil) [37,38], which may be considered false nutrients in that their chemical structures are very similar to essential nutrients for which the intestine has a specialized transport mechanism. It would be instructive to examine some studies concerned with riboflavin and ascorbic acid absorption in humans, as these illustrate how one may treat urine data to explore the mechanism of absorption. If a compound is... 

The primary chemicals of interest in chilies are capsaicinoids, namely capsaicin (0.02%) and dihydrocapsaicin (figure 8.11). Also found are flavonoids, carotenoids (capsanthin), steroid saponins (capsicidin), and ascorbic acid or vitamin C (0.2%). Capsaicin has a vanilloid chemical structure. Mechanisms of Action... 

Less frequently used at present is electron spin resonance spectroscopy, which is based on the use of spin probes as model componnds or covalent spin labeling of drugs. Microviscosity and micropolarity of the molecnlar environment of the probe can be derived from electron spin resonance spectra. Moreover, the spectra allow us to differentiate isotropic and anisotropic movements, which result from the incorporation of the probe into liposomal structures. Quantitative distribution of the spin probes between the internal lipid layer, the snrfactant, and the external water phase is to be determined noninvasively. On the basis of the chemical degradation of drugs released from the lipid compartment, agents with reductive features (e.g., ascorbic acid) allow us to measure the exchange rate of the drugs between lipophilic compartments and the water phase [27,28]. 

Interest has been shown in using derivatives of ascorbic acid as antioxidants. One such compound is ascorbyl palmitate. In the structure of ascorbyl palmitate, the 2- and 3-positions are occupied by hydroxyl groups the 6-position contains the substituted fatty acid. Other derivatives, synthesized by Seib and associates were ascorbate-2 phosphate and ascorbate 2-triphosphate (27). Both of these compounds were reported to inhibit lipid oxidation in ground meat as measured by chemical means (2). Ascorbate-2-phosphate was also found to inhibit MFD in beef as measured by sensory means (2). The ascorbate-2-triphosphate was not tested as an inhibitor of MFD in this study, but... 

Figure 11.2 Chemical structures of ascorbic acid and its derivatives.

Ascorbic acid ( Vitamin C ). The crystal structure of this substance cannot be said to be established with the certainty and precision we associate with those already described nevertheless, there is no reason to doubt that the structure suggested by Cox and Goodwin (1936) on the basis of p, limited study of the X-ray reflections is essentially correct. The work is described here because this crystal structure presents some very interesting and instructive features. It is also historically interesting because a preliminary study by optical and X-ray methods played a part in the elucidation of the chemical structure of this biologically important substance. (Cox, 1932 a Cox, Hirst, and Reynolds, 1932 Cox and Hirst, 1933.)... 

Food processing operations can be optimized according lo the principles used for other chemical processes if the composition, Ihernio-phvsical properties, and structure of the food is known, However, the complex chemical composition and physical structures of most foods can make process optimization difficult. Moreover, the quality of a processed product may depend more on consumer sensory responses than on measurable chemical or physical attributes. Retention levels of ascorbic acid. CoHnO. or thiamine can often be used as an indicator of process conditions. 

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... 

Fig. 9.5 Chemical structures of low molecular weight antioxidants Melatonin (a) ascorbic acid (Vitamin C) (b) glutathione (c) lipoic acid (d) a-tocopherol (Vitamin E) (e) and a-tocotrienol (f)...

The analytical utility of near-infrared spectroscopy can be demonstrated by an analysis of mixtures composed of glucose, lactate, urea, alanine, ascorbate, and triacetin in a pH 6.8 aqueous phosphate buffer.6 The chemical structures of these test compounds are presented in Figure 13.2. These components were selected to represent different classes of molecules expected in typical biological matrices. Glucose represents carbohydrates lactate represents small organic acids urea is a... 

Compounds known to behave in this way in vivo are listed in recent reviews in this Series.1 2 The structures of some of the /3-D-glucopyranosiduronic acids isolated from urine have been proved by chemical synthesis.3 A few similar derivatives of flavones and triterpenes have been isolated from plants. D-Glucuronic acid also occurs in mammalian tissues as a constituent of acid mucopolysaccharides (aminodeoxypolysaccharides, containing uronic acid), such as hyaluronic acid, chondroitinsulfate, and heparin,4 and it is a direct precursor of L-ascorbic acid in plants and mammals.6 It is present in many of the plant polysaccharides classified as hemicelluloses6 and gums,7 and it has also been found in certain bacterial polysaccharides.4... 

The structure of ascorbic acid resembles an alpha-hydroxy acid, which is generally not appreciated. Ascorbic acid is present in most fruits, and may underlie some of the effects attributed to fruit extracts. Vitamin C has pronounced HA-stimulating effects in the fibroblast assay. But its antioxidant activity confounds the effects it may induce. The deposition of HA is stimulated when Vitamin C is added to cultured fibroblasts. The most profound changes occur in the compartmentalization of HA. The preponderance of the enhanced HA becomes cell-layer instead of being secreted into the medium.240,261 The chemical reactions catalyzed by ascorbic acid that bind HA to cell or matrix components are not known. 

FIGURE 29.2 Chemical structure of vitamin C (ascorbic acid, (f )-5-[(5)-l,2-dihydroxyethyl]-3,4-dihydroxy-5 -furan-2-on). 

Answer The properties of the vitamin—like any other compound—are determined by its chemical structure. Because vitamin molecules from the two sources are structurally identical, their properties are identical, and no organism can distinguish between them. If different vitamin preparations contain different impurities, the biological effects of the mixtures may vary with the source. The ascorbic acid in such preparations, however, is identical. 

Fig. 8.29 Industrial, chemical synthesis of L-ascorbic acid. Experimental conditions have been taken from [145]. Structures (except those of the acetone derivatives) are in open-chain Fischer projection to make the...

Figure 10. Chemical structures of L-ascorbic acid, erythorbic acid, and ascorbyl palmitate.

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