Side-chain cleavages, peptides

The mild acidolytic cleavage procedure noted above is used to deprotect various side chain protected peptides synthesized in solution or on chlorotrityl-resin with tyrosine O-sulfate synthons as listed in Table 4. The overall yields are significantly superior to those obtained by postsynthetic sulfation of the purified peptides, since they are typical for synthetic peptides after the final deprotection and purification step. The additional main advantage of this approach derives from a facile analytical characterization, since sulfonated byproducts at the tyrosine and tryptophan level, as well as oxidation of the methionine residues resulting from postsynthetic sulfation of tyrosine peptides are avoided. 

Cleavage of peptide from the resin (with side-chain deprotection) peptides were cleaved by a mixture of TFA/H20/TIS (90 3 7 10mL-g-1 resin) for 1 h. The resin was filtered off and washed with TFA (3 x). The collected filtrates were taken to dryness in vacuo and the peptide was precipitated by ice-cold Et20. The precipitate was collected by centrifugation, resuspended in Et20 and collected by centrifugation twice. The precipitate was dried and lyophilized. 

For peptides containing trityl-protected side chains cleavage conditions 4, 5, 6,6A, 7, or 7A should be used. 

However, interpretation of, or even obtaining, the mass spectrum of a peptide can be difficult, and many techniques have been introduced to overcome such difficulties. These techniques include modifying the side chains in the peptide and protecting the N- and C-terminals by special groups. Despite many advances made by these approaches, it is not always easy to read the sequence from the mass spectrum because some amide bond cleavages are less easy than others and give little information. To overcome this problem, tandem mass spectrometry has been applied to this dry approach to peptide sequencing with considerable success. Further, electrospray ionization has been used to determine the molecular masses of proteins and peptides with unprecedented accuracy. 

Cleavage of a peptide bond is an example of a nucleophilic attack. The nucleophile in the reaction is either an activated water molecule or part of the side-chain of an amino acid, and peptidases are described as having either a water nucleophile or a protein nucleophile. Peptidases with a water nucleophile either utilize one or two metal ions as ligands for the water molecule, in which case the peptidase generally acts... 

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