Selective labeling of histidine by a designed fluorescein-based probe

The synthesis of a novel fluorescent probe, 3-epoxypropoxy fluorescein (EPF), and its properties for labeling of histidine are described. The probe contained a fluorescein fluorophore with long-wavelength response and an active epoxy labeling group. In alkaline media EPF reacted selectively with histidine, rather than with other amino acids, causing a large increase in fluorescence intensity and thereby allowing a selective detection of histidine. This fluorescence increase resembled that of the fluorescein diaion with the increase of the media basicity, suggesting that the addition reaction of histidine with the epoxy group provides the fluorophore moiety with a basic molecular environment. As an application of this probe, fluorescent labeling of histidine in human serum was attempted and the obtained results were in agreement with those given by using histidine–nickel complex adsorptive voltammetry. Further, the relative S.D. of the method was 1.8% for 10 replicate determinations of 0.55 μM histidine. When 10 μM of EPF was used, the linear range for histidine was 0.007–10 μM with a detection limit (S/N=3) of 0.001 μM.