A method for modifying a target protein or a target peptide at the amino functionality of the N-terminal amino acid comprising the steps of: a. modifying the target protein or target peptide so that the resulting protein or peptide contains a histidine amino acid at the position adjacent to the N-terminus amino acid; b. contacting the protein or peptide containing a histidine amino acid at the position adjacent to the N-terminus amino acid with an acylating agent at a pH lower than 6, to form a modified protein or peptide; wherein the acylating agent is of the following Formula: wherein R is H or SO 3 Na; L 1 is a linear or branched C 1 -C 30 alkyl or an unbranched oligo ethylene glycol moiety of Formula: or, wherein y is 0 to 34; R 1 is biotin or a fatty acid moiety; and n is 0 or 1; or wherein the acylating agent is of the formula: or and wherein the protein or peptide containing a histidine amino acid at the position adjacent to the N-terminus amino acid is prepared by replacing the two amino acids at the N-terminus of said target protein or target peptide with an amino acid sequence XH- wherein H is histidine and X is the amino acid A; or the protein or peptide containing a histidine amino acid at the position adjacent to the N-terminus amino acid is prepared by replacing the three amino acids at the N-terminus of said target protein or target peptide with an amino acid sequence XHX'- wherein H is histidine and X and X' are the amino acid A; or the protein or peptide containing a histidine amino acid at the position adjacent to the N-terminus amino acid is prepared by appending a two amino acid sequence of Formula XH or a three amino acid sequence of Formula XHX'- at the N-terminus of said target protein or target peptide, wherein H is histidine and X and X' are A. a. modifying the target protein or target peptide so that the resulting protein or peptide contains a histidine amino acid at the position adjacent to the N-terminus amino acid; b. contacting the protein or peptide containing a histidine amino acid at the position adjacent to the N-terminus amino acid with an acylating agent at a pH lower than 6, to form a modified protein or peptide; R is H or SO 3 Na; L 1 is a linear or branched C 1 -C 30 alkyl or an unbranched oligo ethylene glycol moiety of Formula: or, wherein y is 0 to 34; R 1 is biotin or a fatty acid moiety; and n is 0 or 1; or wherein the acylating agent is of the formula: the protein or peptide containing a histidine amino acid at the position adjacent to the N-terminus amino acid is prepared by replacing the two amino acids at the N-terminus of said target protein or target peptide with an amino acid sequence XH- wherein H is histidine and X is the amino acid A; or the protein or peptide containing a histidine amino acid at the position adjacent to the N-terminus amino acid is prepared by replacing the three amino acids at the N-terminus of said target protein or target peptide with an amino acid sequence XHX'- wherein H is histidine and X and X' are the amino acid A; or the protein or peptide containing a histidine amino acid at the position adjacent to the N-terminus amino acid is prepared by appending a two amino acid sequence of Formula XH or a three amino acid sequence of Formula XHX'- at the N-terminus of said target protein or target peptide, wherein H is histidine and X and X' are A.