The design and synthesis of a new polarity-sensitive fluorescent probe, 3-(4-chloro-6-p-maleimidylphenoxyl-1,3,5-triazinylamino)-7-dimethylamino-2-methylphenazine, is reported for characterizing the local polarity and structure, such as the thiol domain, of a protein. The probe comprises a polarity-sensitive fluorophore (neutral red moiety) and a thiol-specific labeling group (maleimidyl moiety). The probe exhibits a sensitive response of shift of fluorescence maximum emission wavelength to solvent polarity, but not to pH and temperature, which makes the probe suitable for determining the local polarity change of a protein denatured by pH or temperature. The application of this kind has been first demonstrated for the polarity detection of the Cys121 domain of β-lactoglobulin. It is found that the polarity of the Cys121 domain corresponds to a dielectric constant of 17.3, and this value hardly alters after heat treatment, which may be attributed to the improved thermal reversibility by the Cys121 modification. The simple mixture of the probe and the protein at pH 5.6 is also studied, revealing that the free probe prefers to bind to an outer hydrophobic region. Heat treatment of the mixture causes the modification of Cys121 residues; this modification does not completely destroy the calyx but results in the opening of a channel for the probe to enter the calyx of β-lactoglobulin. These results show that Cys121 plays an important role not only in the thermal reversibility but also in the accessibility of the calyx to a ligand. The strategy presented here further indicates that the combination of polarity-sensitive fluorescence probe with site-specific labeling may serve as a powerful means for elucidating structures and properties of proteins.
