Vitamins physicochemical properties

Singh, H., McCarthy, O.J. and Lucey, J.A. (1997) Physicochemical properties of milk, in Advanced Dairy Chemistry, Vol. 3 Lactose, Water, Salts and Vitamins, (ed. P.F. Fox), Chapman Hall, London, pp. 469-518. 

The mode of action of the sulfonamides as antagonists of 4-aminobenzoic acid (PAB) is well documented, as is the effect of physicochemical properties of the sulfonamide molecule, e.g. pK, on potency (B-81MI10802). Sulfonamides compete with PAB in the biosynthesis of folic acid (44), a vital precursor for several coenzymes found in all living cells. Mammalian cells cannot synthesize folic acid (44), and rely on its uptake as an essential vitamin. However, bacteria depend on its synthesis from pteridine precursors, hence the selective toxicity of sulfonamides for bacterial cells. Sulfonamides may compete with PAB at an enzyme site during the assembly of folic acid (44) or they may deplete the pteridine supply of the cell by forming covalently-bonded species such as (45) or they may replace PAB as an enzyme substrate to generate coupled products such as (46) which are useless to the cell. 

Examples of simultaneous determinations are shown in Table 14. Few such methods have been reported, partly because different physicochemical properties prohibit a common extraction procedure for certain vitamins. Other reasons are a lack of sensitivity due to relative differences in concentration and differences in the optima] absorption wavelengths for each vitamin. 

The water-soluble vitamins are a highly diverse group of compounds with differing physicochemical properties. A single vitamin generally consists of several vitamers, or chemical species, each of which exhibits the same biological activity in vivo. Individual vitamers with the same biological functions often exhibit vastly different physicochemical properties. This necessitates unique extraction and separation procedures for each vitamin. As a result, each vitamin is considered individually in this chapter. A section on methods that determine multiple vitamins simultaneously is also included. 

The discussion of the physicochemical properties of each vitamin will highlight those properties having a significant impact on HPLC analysis. Recent methodological reviews will be listed for each vitamin. 

Because the vitamins occur in food in trace quantities, detection sensitivity is often an issue. Ultraviolet absorbance is the most common detection method. Fluorescence and electrochemical detection are used in specific cases where physicochemical properties permit and where increased sensitivity and selectivity are desired. Refractive index is seldom used, due to its lack of specificity and sensitivity. 

In foods, the simultaneous HPLC analysis of several vitamins is feasible only under special conditions, for example, determination of fortified, free vitamins rather than endogenous ones, which are often bound to other food components, or determination of vitamins in relatively simple foodstuffs, such as fruit juice. The HPLC separation per se of multiple vitamins is not difficult it is their simultaneous, nondegradative extraction from foods that is problematic. Developing a single set of extraction conditions that satisfies the diverse physicochemical properties and stability requirements of several vitamins is a challenge. 

Facilitated diffusion involves carrier-mediated transport down a concentration gradient. The existence of the carrier molecules means that diffusion down the concentration gradient is much greater than would be expected on the basis of the physicochemical properties of the drag. A much larger number of substances are believed to be transported by facilitated diffusion than active transport, including vitamins such as thiamine, nicotinic acid, riboflavin and vitamin B6, various sugars and amino acids. 

The medicinal chemistry of vitamins is fundamental not only to the therapeutics of nutritional problems but also to the understanding of the biochemical actions of other medicinal agents that directly or indirectly affect the metabolic functions of vitamins and coenzymes. Accordingly, this chapter includes a brief sununary of the basic biochemislry of vitamins, structure-activity relationships, physicochemical properties and some stability considerations, nutritional and therapeutic applications, and brief characterizations of repre.sentative pharmaceutical products. 

The terms used above for the substances that are actually considered as vitamins are trivial names, mostly group names, used for more than one derivative of a compound with similar biological activity. The function of vitamins in cell metabolism is just as varied as their chemical constitution. By virtue of their lipid solubility, fat-soluble vitamins generally affect physicochemical properties in various cell membranes. Furthermore, they act at the gene level as inductors of protein biosynthesis and as redox agents. The water-soluble vitamins act in many ways as coenzymes and thus enable the catalytic function of hundreds of enzymes. 

Table 1 Some important physicochemical properties of vitamins... 

The physicochemical properties of the water-soluble vitamins are extensively utilized in chemical methods. A method for quantitative vitamin C (ascorbic acid, AA) measurement in food and physiological samples is based on a reaction of the keto groups in dehydroascorbic acid (DHA) with o-phenylenediamine (OPD) to give a fluorescent quinoxaline. This method involves the oxidation of AA to DHA, followed by the measurement of total AA in the sample. The reductive capabilities of AA can especially be utilized for direct electrochemical (amperometric or coulometric) measurement when coupled with HPLC separation. 

Calcification involves the seeding of the calcium apatite crystal and the growth of the crystals. It would be more convenient to review the second of these mechanisms first. This can best be done by assuming for the moment that the bone matrix, connective or cartilaginous, is bathed in a medium supersaturated in calcium phosphate. Intrinsic to the matrix, there is a mechanism that precipitates calcium phosphate in the form of tiny apatite crystals. If there is no interference with the process of calcification, either by deficient absorption of calcium (vitamin D deficiency) or by active dissolution of the calcium crystals (parathormone), most of the process of mineralization can probably be explained by the physicochemical properties of the apatite crystals. Thus, the divalent cations and anions, PO4 and calcium, penetrate the hydration shell reaching the surface of the crystals where they are crystallized, thereby increasing the size of the crystals. As mineralization proceeds, the amount of bound... 

The introduction of one double bond with cis configuration can alter in the vitamin Kg series some physicochemical properties as well as the biological activity markedly, as shown by comparison of some mono-cts forms with the corresponding all-irons compounds. 

Tocopherols and tocotrienols appear to unite all necessary physicochemical properties to make them the ideal analytes for a liquid chromatographic separation and quantitation. They are nonpolar, nonvolatile, unstable, and easily detectable owing to their favorable UV, fluorescent, and electrochemical characteristics. Their nonpolar nature together with the absence of silanol sensitive functional groups minimizes unwanted chromatographic phenomena such as peak tailing and low efficiency. Unlike GC, LC of vitamin E proceeds at room temperature and does not require derivatization to improve its chromatographic properties or... 

More than 100 years ago a fluorescent compound was isolated first fi om whey, and later from different biological materials. When it Ijecame clear that the isolated yellow pigments, named lactochrome, ovoflavin, or lactoflavin, had a common structure, the new compound was named riboflavin (vitamin B2) (for historical review see 2). In the years between 1933 and 1935 the structure and the main chemical reactions of riboflavin were studied and the chemical synthesis was performed. Soon afterward, the coenzyme forms, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), were isolated in pure form, and the structures were determined. In the last 50 years many flavoproteins were isolated and their physicochemical properties were studied. Succinate dehydrogenase was the first enzyme found with the prosthetic group (FAD) covalently bound to the protein. About 20 flavoproteins are now known to contain covalently bound coenzyme (mainly via carbon atom 8a) (3). In mammalian tissue, the number of covalently bound flavoproteins appears to be limited. 

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