Configurational isomerism within the peptide bond

The amide group shows restricted flexibility because its central —NH—CO— bond has some double-bond character due to resonance stabilisation [—NH—CO— 

An early term, s-values (Zimm and Bragg, 1959) for these tendencies, is rarely used now. 

Arg+ indicate the presence of a charge on the side-chain as a result of protonation or of deprotonation. 

The Chou-Fasman rules in brief (Fasman, 1985), are A cluster of FOUR a-helix-promoting amino-acid residues (Glu+, Ala, Leu, His+, Met, Gin, Trp, Val, or Phe) in a run of SIX amino acids will initiate an a-helix to form, until sets of a-helix-breaking amino acids (those with s-values less than 1.00 in this table) are encountered. Proline cannot occur in an a-helix in the inner part of a polypeptide chain, or in a helix in the C-terminal end of a polypeptide chain, but can occur within the last three residues in the TV-terminal end of an a-helix. A cluster of THREE (3-s true tare-forming amino-acid residues out of a run of FIVE amino adds will initiate the formation of a (3-sheet, which will end when a set of four (3-sheet-breaking amino acids is reached.  

The a-helix is one of the best-known regular conformational features as a subheading within the secondary structure of polypeptides and is frequently adopted in chains of six or more helicogenic amino acids (see Table 2.1 for a definition of terms and examples). The (3-sheet is another classic conformational structure that has been detected from the earliest days of X-ray crystallography of proteins. Local 

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