Selective labeling of α- or ε-amino groups in peptides by the Bolton-Hunter reagent

Incorporation of N-succinimidyl-3(4-hydroxyphenyl) propionate (Bolton-Hunter reagent) or its ¹²⁵I-labeled derivative into peptides can be selectively directed towards either α- or ε-amine functions by modifying the pH of the reaction. Acylation of α-amino groups is favored at pH 6.5 whereas ε-amino groups react more readily at pH 8.5. We have taken advantage of this result to prepare two new ¹²⁵I-labeled analogues of substance P and neurotensin that bind selectively and reversibly to their respective receptors. The method described here is of general interest and can be used to incorporate various reporter groups into peptide structures.