4-Dimethylamino-beta-nitrostyrene, a fluorescent solvatochromic probe to estimate the apparent dielectric constant in serum albumin: Experimental and molecular dynamics studies

Abstract

The spectroscopic properties of 4-dimethylamino-beta-nitrostyrene (DANS) were theoretically and experimentally correlated with the polarity of organic solvents and applied to estimate the overall apparent dielectric constant in the binding sites of bovine serum albumin (BSA). The linear correlation between McRae's solvent polarity function and the wavelength of maximum emission (R2 = 0.8899) showed the suitability of DANS as a fluorescent solvatochromic probe. The interaction between BSA and DANS was evaluated (Ka = 3.1 × 104 mol−1 L), and based on the DANS/BSA emission, the apparent dielectric constant of BSA was estimated (ε = 5.5 ± 0.8). The experimental and theoretical absorption and emission maxima were correlated using density-functional theory. Molecular dynamics was used to calculate the mean electrostatic energy between DANS and the organic solvent molecules. These results were associated with the experimental fluorescence and applied to the BSA/DANS system, resulting in electrostatic energy (−37 kJ mol−1). The application of this model generated a dielectric constant of 5.8 ± 1.3 to BSA, which was consistent with the values obtained by McRae's solvent polarity function. The mean electrostatic energy of DANS in BSA binding sites was calculated directly from molecular dynamics simulations and resulted in −40 ± 2, −47 ± 3, and −48 ± 3 kJ mol−1 for sites I, II, and III. These results showed that the environment offered by the site I presented the value of electrostatic energy closest to the obtained with the solvatochromic model. In conclusion, DANS can be used to estimate the apparent polarity of albumins.