A new environment-sensitive fluorophore, 6-N,N-(dimethylamino)-2,3-naphthalimide (6DMN) was introduced
in the ä-selective opioid peptide agonist H-Dmt-Tic-Glu-NH2 and in the í-selective opioid peptide agonist
endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH2). Environment-sensitive fluorophores are a special class of
chromophores that generally exhibit a low quantum yield in aqueous solution but become highly fluorescent
in nonpolar solvents or when bound to hydrophobic sites in proteins or membranes. New fluorescent
ä-selective irreversible antagonists (H-Dmt-Tic-Glu-NH-(CH2)5-CO-Dap(6DMN)-NH2 (1) and H-Dmt-Tic-
Glu-Dap(6DMN)-NH2 (2)) were identified as potential fluorescent probes showing good properties for use
in studies of distribution and internalization of ä receptors by confocal laser scanning microscopy
