Buffer-dependent pH sensitivity of the fluorescent chloride-indicator dye SPQ

Area of Science:

• Biophysical Chemistry
• Analytical Chemistry
• Membrane Transport

Background:

• 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) is a fluorescent probe used to study ion fluxes.
• Its fluorescence is known to be quenched by chloride (Cl-) and gluconate ions.
• The influence of buffer composition and pH on SPQ fluorescence requires further investigation.

Purpose of the Study:

• To investigate the impact of pH and buffer composition on SPQ fluorescence.
• To determine the suitability of SPQ for simultaneous measurement of chloride and proton fluxes.
• To establish accurate methods for quantifying ion fluxes using SPQ.

Main Methods:

• SPQ fluorescence intensity was measured in various buffer solutions (HEPES, MES, Tris) across a range of pH values.
• Titration curves were used to determine buffer pK values and quenching constants.
• Stern-Volmer analysis was applied to quantify the relationship between fluorescence quenching and ion concentrations.

Main Results:

• SPQ fluorescence decreased with increasing Cl- or gluconate concentration as expected.
• Contrary to expectations, SPQ fluorescence increased as pH decreased, independent of gluconate.
• Different buffer types (morpholine vs. piperazine derivatives) exhibited distinct quenching behaviors (KQbase and KmQ).

Conclusions:

• The apparent Cl-Stern-Volmer constant (KappCl) is dependent on both pH and buffer composition.
• SPQ is unsuitable for studying simultaneous transmembrane fluxes of Cl- and H+ without accounting for pH and buffer effects.
• Accurate interpretation of SPQ fluorescence data necessitates careful consideration of experimental conditions.