Branching terminal aliphatic chains

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

Figure 10. Effect of branching in the terminal aliphatic chains.

Figure 25. Effect of a methyl branch in the terminal aliphatic chain on mesophase formation.

Other glycolipids are carbohydrate esters of fatty acids (such as mycolic acids). Mycosides are also part of this group and they contain a branched aliphatic chain with hydroxyl groups esterified by fatty acids and terminated at one end by a phenol group to which the carbohydrate moiety is linked. 

It is considerably more difficult to inhibit oxidation in the gas phase than in the liquid phase. At the high temperatures of gas-phase oxidations the rates of the chain-propagating and branching reactions are increased to a greater extent than the rates of the chain-terminating reactions. Initiation by surfaces can also constitute a serious problem. The majority of liquid-phase antioxidants which are effective at high temperatures are too involatile to be useful in the gas phase. However, inhibition can be achieved with aliphatic amines, which are generally rather ineffective inhibitors of low temperature liquid-phase oxidations. The mechanisms by which the different types of antioxidants inhibit oxidation are briefly described below. 

Recent investigations provide new insight on the structural chemistry of dissolved organic matter (DOM) in freshwater environments and the role of these structures in contaminant binding. Molecular models of DOM derived from allochthonous and autochthonous sources show that short-chain, branched, and alicyclic structures are terminated by carboxyl or methyl groups in DOM from both sources. Allochthonous DOM, however, had aromatic structures indicative of tannin and lignin residues, whereas the autochthonous DOM was characterized by aliphatic alicyclic structures indicative of lipid hydrocarbons as the source. DOM isolated from different morphoclimatic regions had minor structural differences. 

The carboxypeptidases are released from their inactive precursors in the pancreatic juice of animals. The most studied example is bovine carboxypeptidase A, which contains one mole of zinc per protein molecular weight of 34 500. These enzymes cleave the C-terminal amino acid residue from peptides and proteins, when the side-chain of the C-terminal residue is aromatic or branched aliphatic of l configuration. At least the first five residues in the substrate affect the activity of the enzyme. The enzyme also shows esterase activity. Esters and peptides inhibit each other competitively, indicating that the peptidase and esterase sites overlap, even if they are not the same. 

Carboxypeptidase A catalyzes the hydrolysis of the C-terminal amino acids in peptides and proteins it shows a preference for aromatic and branched aliphatic side chains at the C-terminal end. The enzyme also acts as an esterase. The X-ray structure of the native form of carboxypeptidase A, was solved by Rees and Lipscomb to 1.5 A... 

Carboxypeptidase A is an exopeptidase which specifically hydrolyzes C-ter-minal aromatic and branched chain aliphatic amino acids from di- and polypeptides. Dipeptides in which the N-terminal amino group is free are hydrolyzed only slowly, whereas the corresponding N-acylated dipeptides are rapidly hydrolyzed. The presence of an N-methyl group on the a-amino nitrogen of either the first, or the second amino acid residue of a polypeptide greatly suppresses the rate of hydrolysis 33, 34). Hanson and Smith 143) and Abramowitz et al. 144) have shown that the identity of the side chain residue in polypeptides of up to five amino acid residues in length influences the magnitudes of ifm and cat-... 

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