Secondary structures peptide chains

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. 

Each polytripeptide chain is twisted around a threefold screw axis and exists in a secondary structure, analogous to the left-handed polyproline II-helix, i.e. with transposition of the peptide bond (pitch 8.4 A, 3 amino acids) (Figs. 2,3). 

The effect of C ,C -disubstituted amino acids (aaAAs) on peptide secondary structure has been studied in recent years.2a d While longer side-chain C ,C -di-n-alkyl amino acids promote extended peptide conformation,23 alicyclic aaAAs, in which the Ca carbon forms a cyclic bridge with itself, such a 1-aminocyclopentane-l-carboxylic acid (Ac5c) and 1-aminocyclohexane-l-carboxylic acid (Ac6c), have helix-forming characteristics similar to those of 1 -aminoisobutyric acid (Aib).2ax... 

Before analyzing in detail the conformational behaviour of y9-peptides, it is instructive to look back into the origins and the context of this discovery. The possi-bihty that a peptide chain consisting exclusively of y9-amino acid residues may adopt a defined secondary structure was raised in a long series of studies which began some 40 years ago, on y9-amino acid homopolymers (nylon-3 type polymers), such as poly(/9-alanine) 3 [14, 15], poly(y9-aminobutanoic acid) 4 [16-18], poly(a-dialkyl-/9-aminopropanoic acid) 5 ]19], poly(y9-L-aspartic acid) 6 ]20, 21], and poly-(a-alkyl-/9-L-aspartate) 7 [22-36] (Fig. 2.1). 

The recognition that short chain / -peptides can form regular secondary structures initially came from detailed conformational analysis of y9 -peptides 1 and 66 (which incorporates a central (2S,3S)-3-amino-2-methylbutanoic acid residue) by NMR in pyridine-d5 and CD3OH [10, 103, 164] and homooUgomers (as short as four residues) of trons-2-amino-cyclohexanecarboxyhc acid (trans-ACHC) (e.g. hex-amer 2 for the (S,S) series) by NMR and X-ray diffraction [6, 126, 159]. 

Seebach, D., Abele, S., Gademann, K., Guichard, G., Hintermann, T., Jaun, B., Matthews, J. L., and Schreiber, J. V. (1998). /32- and /3 -peptides with proteinaceous side chains Synthesis and solution structures of constitutional isomers, a novel helical secondary structure and the influence of solvation and hydrophobic interactions on folding. Helv. Chim. Acta 81, 932-982. 

Seebach, D., Ciceri, P. E., Overhand, M.,Jaun, B., Rigo, D., Oberer, L., Hommel, U., AmsUttz, R., and Widmer, H. (1996). Probing the helical secondary structure of short-chain /3-peptides. Helv. Chim. A da 79, 2043-2066. 

The stability of secondary structure is also influenced by surrounding structures (Fig. 2-3). Secondary structure may be stabilized by interactions between the side chains and by interactions of the side chains with other structures in the protein. For example, it is possible to arrange the amino acid sequence of a protein or peptide into a helix that has one face that is hydrophobic and one that is hydrophilic. The helix wheel shown in Fig. 2-3 illustrates how this is possible. View the helix as a long cylinder. The peptide backbone spirals up and around the cylinder. The... 

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