Abstract

A one step method for the attachment of a single fluorescent label to peptides through their N-terminus is described. The method employs fluorescein-5-isothiocyanate as the label of choice and a lower than normal derivatization buffer pH that discriminates against $\varepsilon$-amino derivatization in favor of $\alpha$-amino derivatization provided that this group may act as an efficient nucleophile. A kinetic study was performed in order to identify the optimal derivatization buffer pH for $\alpha$-amino group selectivity, using $\rm N\alpha$-t-BOC-L-lysine and $\rm N\varepsilon$-t-BOC-L-lysine as model compounds for derivatization through $\varepsilon$-amino and $\alpha$-amino groups, respectively. The optimal pH was found to be 8.5. Six small peptides, Arg-Lys, Lys-Trp-Lys, Lys-Lys, Lys-Lys-Lys, Lys-Lys-Lys-Lys and Arg-Pro-Lys-Pro, were derivatized at pH 8.5 in an effort to corroborate the results of the kinetic study. It was found that this method of single label derivatization via buffer pH control was successful for peptides with fewer than four lysines residues.