Abstract: The present invention provides a method for effectively introducing sulfonic acid groups into the N-terminus of a protein or a peptide. The method comprises a modification step to react a N-terminus in a protein or peptide with a compound A which includes disulfide group and a cleavage step to cleave a disulfide bond of the disulfide group to convert into a sulfonic acid group. The present invention also provides a method of analyzing proteins or peptides easily and effectively on mass spectrometry and an intermediate that can be used to effectively derivatize proteins or peptides as sulfonic acid derivatives.

Abstract: The present invention provides a method for effectively introducing sulfonic acid groups into the N-terminus of a protein or a peptide; a method capable of analyzing proteins or peptides easily and effectively on mass spectrometry; an intermediate that can be used to effectively derivatize proteins or peptides as sulfonic acid derivatives. A method for derivatizing a protein or peptide to a sulfonic acid derivative, comprising steps of: modification step to react a N-terminus in a protein or peptide with a compound A which includes disulfide group, to obtain a protein or peptide modified with the compound A at the N-terminus; and cleavage step to cleave a disulfide bond of the disulfide group to convert into a sulfonic acid group, thereby converting the modified protein or peptide into a sulfonic acid derivative. A method for analyzing the amino acid sequence of a protein or peptide, wherein the sulfonic acid derivative of a protein or a peptide obtained by the above method is subjected to mass spectrometry.