6-N,N-dimethylamino-2,3-naphthalimide:: A new environment-sensitive fluorescent probe in δ- and μ-selective opioid peptides

new environment-sensitive fluorophore, 6-N,N-(dimethylamino)-2,3-naphthalimide (6DMN) was introduced in the delta-selective opioid peptide agonist H-Dmt-Tic-Glu-NH2 and in the mu-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 delta-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 delta receptors by confocal laser scanning microscopy.