New approach for the espectroscopic measurements of pKa values of the pH indicator 4-Heptadecyl-7-hydroxicoumarin in microheterogenous systems

TYMCZYSZYN E.,; ZUBIRI D.; BERNIK D.

Brasil

Congreso; IV Congreso de Biofísica del Cono sur; 2000

The probe 4-heptadecyl-7-hidroxicoumarin has been used for measuring the surface potential at the interface of microheterogeneous systems like films, micelles and model membranes. The apparent pKa values allow the estimation of the interfacial potential by means of a modified Boltzmann’s law. The pKa values of the probe were obtained trough the fluorescent spectra recorded at different pH in the titration process. However, due to the high intrinsic pKa value of the probe in those systems, the pH that must be reached is too high to be compatible with the normal functioning of most of natural proteins and enzymes. This fact limits to use of this probe to the studies on model systems. Herein, we characterized the extinction coefficients of the acidic and basic species of the coumarin. Therefore, we calculated the pKa of the probe with the absorption spectra measured within a very narrow pH range. This approach allows the calculation of surface potential in biological membranes avoiding denaturation processes.