Rapid pH and ΔμH⁺ jump by short laser pulse

The excited state (S₁) of sulfononaphthols has a pK value well below that of the ground state, consequently intensive illumination of their aqueous solution should lead to acidification. In this study the second harmonics of a ruby laser pulse (10 MW, 30 ns at 347.2 nm) were used for excitation of a sulfonophthol solution, resulting in a lowering of the pH from 8 to 4. The change in pH was demonstrated by spectral changes of the pH indicators Bromocresol Green, Bromothymol Blue, Bromocresol Purple and Phenol Red. The magnitude of the pH change was calculated from the kinetics of the changes in the indicators' absorbance and from fluorescence intensity of naphtholate. Sulfononaphthols, due to their hydrophylic nature, cannot permeate across phospholipid membranes. Taking advantage of this property, liposomes containing sulfononaphthol were prepared and irradiated by the laser pulse. Evidence is given that under such conditions the change in pH was limited to the space enclosed in the liposomes. The resulting proton-motive force (ΔμH⁺ = 180-240 mV) is adequate for perturbing the energy-coupled reactions of oxidative phosphorylation. Possible applications of this technique in chemical physics, chemistry, biochemistry and bioenergetics are discussed.