Pleated 1-sheet

Secondary structure (Section 27 19) The conformation with respect to nearest neighbor ammo acids m a peptide or pro tern The a helix and the pleated 3 sheet are examples of protein secondary structures... 

Section 27.19 Two secondary structures of proteins are particularly prominent. The pleated (3 sheet is stabilized by hydrogen bonds between N—H and C=0 groups of adjacent chains. The a helix is stabilized by hydrogen bonds within a single polypeptide chain. 

Proteins, or sections of proteins, sometimes exist as random coils, an arrangement that lacks the regularity of the a helix or pleated 3 sheet. 

The frequency of the amide I peak observed in the lens is sensitive to protein secondary structure. From its absolute position at 1672 cm-1, which is indicative for an antiparallel pleated 3-sheet structure, and the absence of lines in the 1630-1654 cm-1 region, which would be indicative of parallel (1-sheet and a-helix structures, the authors could conclude that the lens proteins are all organized in an antiparallel, pleated 3-sheet structure [3]. Schachar and Solin [4] reached the same conclusion for the protein structure by measuring the amide I band depolarization ratios of lens crystallins in excised bovine lenses. Later, the Raman-deduced protein structure findings of these two groups were confirmed by x-ray crystallography. 

The primary structure of a peptide is its amino acid sequence. We also speak of the secondary structure of a peptide, that is, the conformational relationship of nearest neighbor amino acids with respect to each other. On the basis of X-ray crystallographic studies and careful examination of molecular models, Linus Pauling and Robert B. Corey of the California Institute of Technology showed that certain peptide conformations were more stable than others. Two arrangements, the a helix and the pleated (3 sheet, stand out as secondary stractural units that are both particularly stable and commonly encountered. Both of these incorporate two important features ... 

Silk fibers, which are obtained from the secretion of the silkworm, are double filaments that are enclosed by a coating of a gum (sericin) as they are secreted [40]. The amino acid sequence of the silk protein was shown to be (glycine-serine-glycine-alanine-glycine-alanine) . The polypeptide chains are bound into antiparallel pleated (3-sheet stmctures by hydrogen bonding [31,39,42]. The structures are also held together by van der Waal forces [31, 38]. 

Another form of the ordered structure is the /3 sheet. In this structure, an extended chain makes interstrand rather than intrastrand H-bondings with one or two of the neighboring parallel or antiparallel strands. The peptide backbone in the /3 sheets has a geometry that approaches the most extended chain conformation allowed by normal bond lengths and angles. The displacement of each amino acid residue in the pleated /3 sheet is 3.47 A. 

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