Pen residues

In contract to these eompoynds, the myre conformatlonally constrained [d Pen, D-Cys ]-, [D-Pen. Cys ]-, [0 -Pen, D -Pen ]-, and C)-Pen. Pen ]-enkephalins exhibit exceptional 6 receptor selectivity. Conformational differences associated with the Pen residues appear to be responsible for the high receptor selectivity. Finally, a series of cyclic retro-Inverso enkephalins have been designed to incorporate favorable conformational restriction and Increased resistance to enzymatic degradation. 

While conformation II (Fig. 2.34) of Uke-y -amino acids is found in the 2.614-helical structure, conformation I, which similarly does not suffer from sy -pen-tane interaction, should be an appropriate alternative for the construction of sheet-like structures. However, sheet-like arrangement have not been reported so far for y-peptides composed of acyclic y " -amino acid residues. Nevertheless, other conformational biases (such as a,/9-unsaturation, cyclization between C(a) and C(y)) have been introduced into the y-amino acid backbone to restrict rotation around ethylene bonds and to promote extended conformation with formation of sheets in model peptides. Examples of such short chain y-peptides forming antiparallel (e.g. 152 [208]) and parallel (e.g. 153-155 [205, 208]) sheet-hke structures are shown in Fig. 2.38. 

While stirred, 1-heptene (9.8 g, 0.1 mol) was added at 20°C to a suspension of phosphorus pentachloride (41.7 g, 0.2 mol) in dimethyl-dicholorsilane (100 ml). The reaction mixture was stirred at 20°C for 3 h and cooled to -10°C. Sulfur dioxide dried over phosphorus pen-toxide was bubbled into the reaction mixture until complete dissolution of the intermediate complex was observed. After evaporation of solvent, the residue was vacuum distilled to give pure 2-chlorohept-l-ylphosphonic dichloride (22.1 g, 88%). 

The most common partition coefficient encountered in environmental work (Sect. 2.1.4) is the octanol water partition coefficient (K0VI) and the solid phase carbon-water partition coefficient (Koc). A partition coefficient for dissolved organic matter-water (i. e., Kd ()M) or dissolved organic carbon-water (i. e., Kd oc) occasionally appears in the literature. In the case of PCBs, Boyd and Sun [378] defined a partition coefficient for residual transformer oil and water as Kd oil, while Sun and Boyd [379] defined a coefficient for PCB dielectric fluid-water as Kd pen-These authors [378,379] identified a total partition coefficient that com-... 

Pen tammino-aluminium chloride if heated in an atmosphere of dry hydrogen melts at 880° C. and boils at 450° C., leaving a white powdery residue of monammino-aluminium chloride, [A1(NH3)]C13. If the distillation is carried out without hydrogen the residue consists of a mixture of pentammino- and monammino-chloride. 

Purify of the crude residue by flash chromatography on silica gel using pen-tane Et20 (96 4) as eluent to obtain pure 1-methoxy-4,4-dimethyl-2-phenyl-2-... 

In an effort to decrease the flexibility of the disulfide-bridged enkephalins, penicillamine residues (C, C -dimethylcysteine, Pen) have been introduced in place of the cysteine residues. The D-Pen2-D-Cys5 peptide exhibited substantial 8 receptor activities (Mosberg et al., 1982). Even... 

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