Excited-state intramolecular proton transfer as a fluorescence probe for protein binding-site static polarity

A fluorescence probe is introduced for protein conformation and binding-site monitoring as the proton-transfer (PT) tautomer fluorescence by using 4-hydroxy-5-azaphenanthrene (HAP) as a prototype. A typical grossly-wavelength-shifted PT fluorescence for HAP is observed in the 600-nm spectral region for this UV-absorbing molecule (absorption onset, 400 nm), for which case PT occurs even in protic solvents. It is shown that PT fluorescence of HAP can serve as a protein-binding-site static-polarity calibrator, shifting from a lambda max of 612 nm in cyclohexane to 585 nm in ethanol at 298 K, contrary to the usual dispersion red shift. A small mechanical solvent-cage effect is noted in ethanol at 77 K, but solvent dielectric relaxation is not apparent from the fluorescence spectrum. Thus, HAP serves to distinguish static solvent-cage polarity from dynamical solvent dielectric relaxation and other solvent-cage effects (mechanical restriction of molecular conformation). HAP as a PT-fluorescence probe is applied to human serum albumin (HSA) and beaver apomyoglobin.