Number and Position of Chains

In order to exhibit provitamin A activity, the carotenoid molecule must have at least one unsubstituted p-ionone ring and the correct number and position of methyl groups in the polyene chain. Compared to aU-trans P-carotene (100% provitamin A activity), a-carotene, P-cryptoxanthin, and y-carotene show 30 to 50% activity and cis isomers of P-carotene less than 10%. Vitamin A equivalence values of carotenoids from foods have been recently revised to higher ratio numbers (see Table 3.2.2) due to poorer bioavailability of provitamin A carotenoids from foods than previously thought when assessed with more recent and appropriate methods. 

Independent of the contact geometry, the calculations also demonstrated that the introduction of gauche defects resulted in a decrease of the bridge conductance by a factor of 10, as compared to an all-trans alkanedithiol chain (see Fig. 14b, triangles). Due to variations in the number and positions of gauche defects, as well as various contact geometries, the molecular junctions can exhibit conductance values up to two orders of magnitude below the conductance values of an all-trans conformation of the alkyl chain. 

Vegetable oils or triglycerides obtained from the seeds of various plants are the source of a wide variety of fatty acid esters and derivatives (fatty acids and alcohols) with different molecular structure (chain length, number and position of... 

Histochemical demonstration of most of the O-acetylated sialic acids is possible because substituents on the side chain of Neu hinder periodate oxidation of this part of the molecule to an extent dependent on the number and position of the O-acetyl groups already mentioned. Correspondingly, removal of these ester groups by alkaline treatment (0.5% KOH in 70% ethanol91) may increase the staining reaction of a sialic acid. For example, the presence of O-acylated sialic acids has been demonstrated in colonic, epithelial mucin of man and various mammals (summarized in Ref. 91), in healthy and diseased, human small-intestine,188-190 in bovine submandibular gland,182 in mouse and rat erythrocyte-membranes,191 and in human lymphocytes.192... 

FIGURE 21-12 Routes of synthesis of other fatty acids. Palmitate is the precursor of stearate and ionger-chain saturated fatty acids, as well as the monounsaturated acids palmitoleate and oleate. Mammals cannot convert oleate to linoleate or a-linolenate (shaded pink), which are therefore required in the diet as essential fatty acids. Conversion of linoleate to other polyunsaturated fatty acids and eicosanoids is outlined. Unsaturated fatty acids are symbolized by indicating the number of carbons and the number and position of the double bonds, as in Table 10-1. 

From this point, the primary structure was fully determined. The primary structure of an enzyme, or other protein, is the number, length, and composition of the polypeptide chains, the linear arrangement of their amino acids, and the number and position of (he cross-links between chains. (The geometrical configuration of the molecule, which is usually a three-dimensional coiled and folded structure, and the side chains and their interactions were not determined.)... 

In normal-phase HPLC on a silica column, separation is based on the number and position of methyl substituents on the chromanol ring. In reversed-phase HPLC on a Cl8 column, separation is based on the structure of the side chain and the number of methyl substituents. It is therefore difficult to completely separate [3-and y-tocopherol and (3- and y-tocotrienols by reversed-phase HPLC, because both have the same side-chain structure and number of methyl substituents on the chromanol ring. Only six peaks are usually found in the reversed-phase HPLC method. Thus, reversed-phase HPLC is recommended for samples from animal tissues, which contain little or no P and y vitamers. 

Tocopherols comprise a methyl-substituted chroman-6-ol ring attached at C-2 to a saturated iso-prenoid side chain. Tocotrienols are analogous structures whose side chains contain three trans double bonds. The tocopherols and tocotrienols are designated as a, (S, y, and 8, according to the number and position of the methyl substituents in the chromanol ring (Fig. 5). 

Nomenclature Fatty acids are named according to the number of carbon atoms in the chain and the number and position of any double bonds. Some of the more common fatty acids are palmitate (06 0), stearate (08 0), oleate (08 1), linoleate (08 2), linolenate (08 3) and arachidonate (C20 4). The double bonds in a fatty acid are usually in the cis configuration. 

Additional factors controlling the rate of decomposition include carbon chain unsaturation, which increases rate of decomposition, number, and position of Cl atoms on an aromatic ring. For example, 2,4-dichlorophenoxyacetic acid is degraded readily, whereas 2,4,5-trichlorophenoxyacetic acid is more resistant. Finally, the position of attachment of a side chain alters the decomposition rate of aromatic compounds. For... 

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