Rapid pH and deltamuH+ jump by short laser pulse

Area of Science:

• Photochemistry
• Physical Chemistry
• Bioenergetics

Background:

• The excited state (S1) of sulfononaphthols exhibits a lower pK compared to the ground state.
• Illumination of aqueous sulfononaphthol solutions is expected to cause acidification.
• Sulfononaphthols' hydrophilic nature prevents their passage across phospholipid membranes.

Purpose of the Study:

• To investigate the acidification of sulfononaphthol solutions upon laser excitation.
• To demonstrate pH changes using pH indicators.
• To explore the confinement of pH changes within liposomes and assess the resulting proton-motive force.

Main Methods:

• Excitation of sulfononaphthol solutions using the second harmonic of a ruby laser pulse (347.2 nm).
• Monitoring pH changes via spectral shifts in pH indicators (Bromocresol Green, Bromothymol Blue, Bromocresol Purple, Phenol Red).
• Quantifying pH changes using indicator absorbance kinetics and naphtholate fluorescence.
• Preparing and irradiating liposomes containing sulfononaphthol.

Main Results:

• Laser excitation lowered the pH of sulfononaphthol solutions from 8 to 4.
• Spectral changes confirmed the pH shift, observable with multiple pH indicators.
• pH changes were localized within liposomes when sulfononaphthol was encapsulated.
• A proton-motive force (deltamuH+) of 180–240 mV was generated within liposomes.

Conclusions:

• Laser-induced excitation of sulfononaphthols effectively acidifies solutions.
• Encapsulation in liposomes confines the pH change, creating a significant proton-motive force.
• This technique holds potential applications in chemical physics, chemistry, biochemistry, and bioenergetics, particularly for studying energy-coupled reactions.