N-terminus FITC labeling of peptides on solid support: the truth behind the spacer

Fluorescein isothiocyanate (FITC) is an amine reactive derivative of fluorescein dye that has wide ranging application in biochemistry. It has been extensively used to label peptides and proteins. However, its use in solid phase peptide synthesis is restricted. Indeed, in acidic conditions required for linker cleavage, N-ter FITC-labeled peptides undergo a cyclization leading to the formation of a fluorescein with subsequent removal of the last amino acid. This can be avoided when a spacer such as amino hexanoic acid is used or if non-acidic cleavage is operated to release targeted peptide from the resin.