Chain lengthening

Studies show polymethine chain lengthening in highly asymmetrical dyes to be accompanied by strong quadratic increases in deviations (3,7,9,10,30,31). In contrast to polymethines, the deviations of the related asymmetrical polyenes are negative as the break of the symmetry leads to a decrease in bond alternation (32). 

The piC values of polymethine dyes depend on terminal group basicity (64) thus the protonation abHity diminishes if the basic properties of the residues decrease, passing from benzimidazole, quinoline, benzothiazole, to indolenine. On the other hand, the piC of higher homologues increases with chain lengthening. The rate constant of protonation is sensitive to other features, for example, substituents and rings in the chain and steric hindrance for short-chain dyes. 

Electron Level Position. One essential condition of spectral sensitization by electron transfer is that the LUMO of the dye be positioned above the bottom of the conduction band, eg, > —3.23 eV in AgBr or > —4.25 eV in ZnO (108). To provide the desired frontier level position respectively to the valence and conduction bands of the semiconductor, it is necessary to use a polymethine with suitable electron-donor abiHty (Pq. Increasing the parameter (Pq leads to the frontier level shift up, and vice versa. Chain lengthening is known to be accompanied by a decrease of LUMO energy and hence by a decrease of sensitization properties. As a result, it is necessary to use dyes with high electron-donor abiHty for sensitization in the near-ir. The desired value of (Pq can be provided by end groups with the needed topological index Oq or suitable substituents (112). 

These polymers are liquids which may usefully be cast or used for impregnation and caulking compounds. In addition they may be vulcanised by a variety of agents, ostensibly by a chain lengthening process. It should, however, be noted that these polymers normally contain small quantities of tri-chloropropane in the original monomer mix so that the three-dimensional chain extension will lead to cross-linking. 

Photopolymerization, in general, can be defined as the process whereby light is used to induce the conversion of monomer molecules to a polymer chain. One can distinguish between true photopolymerization and photoinitiation of polymerization processes. In the former, each chain propagation step involves a photochemical process [1,2] (i.e., photochemical chain lengthening process in which the absorption of light is indispensable for... 

Discovery of the chain-lengthening sequence was initiated by the observation of Heinrich Kiliani in 1886 that aldoses react with HCN to form cyanohydrins (Section 19.6). Emil Fischer immediately realized the importance of Kiliani s discovery and devised a method for converting the cyanohydrin nitrile group into an aldehyde. 

Much of the chemistry of monosaccharides is the familiar chemistry of alcohols and aldehydes/ketones. Thus, the hydroxyl groups of carbohydrates form esters and ethers. The carbonyl group of a monosaccharide can be reduced with NaBH4 to form an alditol, oxidized with aqueous Br2 to form an aldonic acid, oxidized with HNO3 to form an aldaric acid, oxidized enzymatically to form a uronic acid, or treated with an alcohol in the presence of acid to form a glycoside. Monosaccharides can also be chain-lengthened by the multistep Kiliani-Fischer synthesis and can be chain-shortened by the Wohl degradation. 

Rosanoff in 1906 selected the enantiomeric glyceraldehydes as the point of reference any sugar derivable by chain lengthening from what is now known as D-glyceraldehyde belongs to the D series, a convention still in use. 

Scheme 3. Regioselective step-by-step chain lengthening of the precursor 12 to [/j]pericyclines...

The formation of a dipeptide from two amino acids via elimination of water (as shown above) can only take place when energy is removed from the system thus, the starting materials must be converted to a reactive state. The principle is the same for the construction of tri- or tetrapeptides, as well as for the long amino acid chains in proteins. In a 1M solution of two amino acids at 293 K and a pH value of 7, only about 0.1% exists as the dipeptide, i.e., the equilibrium shown in Eq. 5.2 lies on the side of the free amino acids. The formation of a dipeptide requires more energy than chain lengthening to give higher peptides. 

DIRECTIONALITIES in the flow of information from DNA to RNA to protein. All new DNA or RNA chains grow by adding new nucleotides to a free 3 end so that the chain lengthens in the 5 to 3 direction. Protein is made by reading the RNA template starting at the 5 end and making the protein from the N to the C terminus. 

HC1 (18°C), recovery by rotary evaporation, and electrolytic reduction. Once isolated, the free dipetide ester (or acid) is available for chain lengthening (cf. Scheme 1) and Table VII gives examples of peptides prepared using this approach. 

Different Amino and Carboxyl Protecting-Group Combinations of L-Serine/L-Threonine Used for Glycopeptide Chain-Lengthening... 

All the normal acids show practically the same value of jB at a water-air interface, a value which gives the mean area per molecule of 24-3 0 3 Jl. This value is also given by i-butyric acid at a water-benzene interface. The values of B for the iso-acids are consistently a little larger than those for the normal acids. The values of x show that these acids resemble gases whose critical temperatures increase as the carbon chain lengthens. The iso-... 

Preisig-Mueller, R. et al.. Plant polyketide synthases leading to stilbenoids have a domain catalyzing malonyl-CoA C02 exchange, malonyl-CoA decarboxylation, and covalent enzyme modification and a site for chain lengthening. Biochemistry, 36, 8349, 1997. 

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