Ligation chemistry "pseudo" - natural.

Method for synthesizing a desired polypeptide of formula: aaNH2-Q-aax-aay-W-aaCOOH where Q and W each denote the optional presence of one or more additional amino acid residues, aaNH2 denotes the amino acid residue N-terminal polypeptide; aax and aay indicate the remnants of internal adjacent amino acids, having side chains x and y, respectively, and wherein aay is one non-ribosomally specified and aaCOOH denotes the amino acid residue with C terminal amino acid of the polypeptide; the method comprising: (A) ligating a first peptide having the formula: aaNH2-Q-aax-COSR, wherein R is any compatible group with the thioester group, to a second peptide having the formula: Cys-W- aaCOOH to thereby form the polypeptide: aaNH2-Q-aax-Cys-W-aaCOOH; and (B) incubating said polypeptide in the presence of a reagent Raa-X, where Raa-X is selected from the group consisting of X-CH 2 COOH, X- (CH2) 2 COOH, X- (CH2) -C (O ) NH2, X-CH2OH, X-CHOHCH3, X-CH2CHOHCH3, X-CH2CH2OH, X- (CH2) 2NH-GP, X-CH2NH-C (NH2) 2, X- (CH2) 2NH-C (NH2) 2 , X-CH2CH3, X-CH2φ, X-φ-OH (para), X-CH2φ-OH (para), X-CH2φ-OPO3 (para), X-CH2-φ- (OH) 2 (meta para) , X-CH (COOH) 2, X-CH2-IM-GP, X-CH- (CH3) 2, X-CH2CH- (CH3) 2, X-CH (CH3) -CH2-CH3, X-CH2- CH (CH3) -CH2-CH3, wherein X is I or Br, and X-φ, wherein X is F, and X-CH2-iN, where X is F, I or Br, in the which φ denotes a benzyl group, IN denotes an indole group, IM denotes an imidazole group and GP indicates a protecting group; said incubation being under conditions sufficient to form said desired polypeptide.