Peptide parallel chain

Fig. 10.5. (a) A schematic representation of the vibrational modes of the parallel-chain pleated sheet the arrows represent the components of the transition moments of peptide groups in the plane of the paper. The plus and minus signs represent the components of the transition moments perpendicular to the plane of the paper, the former pointing upward, and the latter pointing downward, (b) A schematic representation of the vibrational modes of the antiparallel-chain pleated sheet (Miyazawa, 1960c). (From Schellman and Schellman, 1964.)... 

Despite these successes serious side reactions can result from the action of alkali on cysteine or cystine-containing peptides. Open-chain unsymmetrical cystine derivatives [92, 103, 166] and some cyclic molecules including oxytocin [153] and calcitonin [24, 25] are known to undergo rapid base-catalyzed disulphide interchange yielding symmetrical disulphides or mixtures of polymer and parallel and antiparallel dimers [107, 153]. Furthermore the possibility of /3-elimination of trityl, benzhydryl, and benzyl mercaptide ion from the corresponding peptide bound cysteine residue is known [154] the extent of... 

Pleated p sheet (Section 27.19) Type of protein secondary structure characterized by hydrogen bonds between NH and C=0 groups of adjacent parallel peptide chains. The individual chains are in an extended zigzag conformation. 

Fig. 2.27 The two types of extended /1-peptide strands with conformation requirements around the C(a)-C(/1) bonds. (A) Parallel and antiparallel polar sheets with antiperiplanar conformations around the C(a)-C fl) bond are promoted by unlike-fi -ami-no acids with alkyl side-chains. Antiperiplanar side-chains at C(a) and C(/3) occupy positions approximately perpendicular to the amide planes. (B) Extended strands formed by alternating +)-sc and (-)-sc conformations...

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. 

Fig. 2 (a) Antiparallel and (b) parallel P-sheet structures of two peptide chains connected by hydrogen bonds... 

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