Abstract: A method of forming an amino acid thiohydantoin from an N-protected amino acid or the C-terminal amino acid of an N-protected peptide. The amino acid is activated by reaction with a ketenimine, and the activated ester is converted to the thiohydantoin by reaction with silyl or pyridine isothiocyanate. The ketenimine is generated by treating an N-substituted isoxazolium compound, such as Woodwards Reagent K with a base, preferably in the presence of the amino acid. Also disclosed is a solid phase support having a derivatized N-substituted isoxazolium or ketenimine group for use in the method.

Abstract: A method of C-terminal peptide sequencing. The peptide is reacted with a mixed anhydride of isothiocyanic acid and a carboxylic, carbonic, or sulfonic acid, under basic conditions, to produce a C-terminal peptidyl thiohydantoin. The C-terminal amino acid can be identified by cleaving the thiohydantoin from the peptide and identifying the free amino acid thiohydantoin.

Abstract: A method of C-terminal peptide sequencing. The peptide is reacted with an activated support derivatized with a mixed anhydride of isothiocyanic acid and carboxylic or carbonic acid, under basic conditions. The peptidyl thiohydantoin which forms is separated from the solid support and further reacted with a cleaving agent carried on a second solid support, to release a free C-terminal amino acyl thiohydantoin from the peptide. The free thiohydantoin is analyzed to determine the C-terminal peptide residue. The residual peptide can be recycled through the supports for successive C-terminal residue determinations.

Abstract: A peptide is sequenced from its carboxyl terminal by coupling with an isothiocyanate having a chemical formula R(.sub.x)Si(NCS).sub.y where x=0-3 and y=1-4. R may be alkyl or aryl. The coupling reaction is preferably conducted at a temperature range of 50.degree.-90.degree. C. for a period to approximately sixty (60) minutes or less. A weak acid anhydride, preferably a carboxylic acid anhydride, or a weak acid chloride, preferably a carboxylic acid chloride, may be mixed with the isothiocyanate. An organic base catalyst selected from a group consisting of tertiary amines including aromatic amines such as pyridine may also be utilized.