Molecule protection

The high activity of the Rh/y-Al203 system even at such a low Rh loading (0.1% w/w) can be rationalized on the basis that, as indicated by IR studies of adsorbed CO and EXAFS analysis, all the Rh atoms are zerovalent coordination by the NR3 molecules protects them from oxidation by the solid support [24,35]. 

The lag-phase measurement at 234 nm of the development of conjugated dienes on copper-stimulated LDL oxidation is used to define the oxidation resistance of different LDL samples (Esterbauer et al., 1992). During the lag phase, the antioxidants in LDL (vitamin E, carotenoids, ubiquinol-10) are consumed in a distinct sequence with a-tocopherol as the first followed by 7-tocopherol, thereafter the carotenoids cryptoxanthin, lycopene and finally /3-carotene. a-Tocopherol is the most prominent antioxidant of LDL (6.4 1.8 mol/mol LDL), whereas the concentration of the others 7-tocopherol, /3-carotene, lycopene, cryptoxanthin, zea-xanthin, lutein and phytofluene is only 1/10 to 1/300 of a-tocopherol. Since the tocopherols reside in the outer layer of the LDL molecule, protecting the monolayer of phospholipids and the carotenoids are in the inner core protecting the cholesterylesters, and the progression of oxidation is likely to occur from the aqueous interface inwards, it seems reasonable to assign to a-tocopherol the rank of the front-line antioxidant. In vivo, the LDL will also interact with the plasma water-soluble antioxidants in the circulation, not in the artery wall, as mentioned above. 

Experiment G [see Eq. (16)] shows that a stannylene may be prepared from a molecule protected by acetylation. 

Treating ovotransferrin and human serum transferrin with 170-400 molar excess of ethoxyformic anhydride resulted in complete ethoxy-formylation of histidines with complete loss in iron-binding activity (33). The binding of each iron (two iron-binding sites per protein molecule) protected two histidines from ethoxyformylation, and in both cases the proteins remained completely active. These results plus kinetic analyses of the inactivations indicated two essential histidines in each binding site. Ethoxyformic anhydride also may react with amino groups. 

Lys(Mob)-OH. The photolysis half lives for the amino acids containing polar side chains (Arg, Trp, and His) were noted to cleave slower than the remaining amino acids, but nevertheless cleaved within a reasonable length of time (Table 2). It is clear that the local environment of molecules protected with 2-nitrobenzyl groups, which is governed by both the molecule itself and the photolysis solvent, can greatly influence the relative rates of cleavage. 

The presence of carbohydrate on protein molecules protects them from dena-turation. For example, bovine RNase A is more susceptible to heat denaturation than its glycosylated counterpart RNase B. Several other studies have shown that sugar-rich glycoproteins are relatively resistant to proteolysis (splitting of polypeptides by enzyme-catalyzed hydrolytic reactions). Because the carbohydrate is on the molecule s surface, it may shield the polypeptide chain from proteolytic enzymes. 

Antioxidant molecules protect cell components from oxidative damage. Prominent antioxidants include GSH and the dietary components a-tocopherol, /J-carotene, and ascorbic acid. 

L-Ascorbate, a water-soluble molecule, protects membranes from oxidative damage by regenerating a-tocopherol from a-tocopheroxyl radical. The ascorbyl radical formed in this process is reconverted to L-ascorbate during a reaction with GSH. 

CogdeU RJ, Isaacs NW, Freer AA, Arrelano J, Howard TD, Papiz MZ, Hawthornthwaite-Lawless AM and Prince S (1997) The structure and function of the LH2 (B800-850) complex from the purple photosynthetic bacterium Rhodopseudomonas acidophila strain 10050. Prog Biophys Molec Biol 68 1-27 Connors RE, Burns DS, Farhoosh R and Frank HA (1993) Computational studies of the molecular structure andelectronic spectroscopy of Carotenoids. J Phys Chem 97 9351-9355 De Las Rivas J, Telfer A and Barber J (1993) Two coupled carotene molecules protect P680 from photodamage in isolated Photosystem 11 reaction centres. Biochim Biophys Acta 1142 155-164... 

De las Rivas J, Telfer A and Barber J (1993) 2-coupled / -carotene molecules protect p860 from photodamage in isolated Photosystemllreaction centers. BiochimBiophysActa 1142 155-164... 

Finally, one could envision the use of host structures as whole molecule protecting groups for specific guest molecules—an application that shonld be of great interest in the context of systems chemistryas it would allow for the reactivity of a specific guest to be switched... 

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