The quantum yield of bacteriorhodopsin

The conflicting reports on the quantum yield of the primary reaction in the photocycle of bacteriorhodopsin led us to reinvestigate the quantum yield under various experimental conditions. Quantitation of the molecules in the photocycle was done by measuring the number of molecules being in the M intermediate state 1 ms after laser flash excitation via determination of the absorption change at 602 nm. It was shown that this procedure provides to the lower limit of the quantum yield. A value of 0.64 ∓ 0.04 at pH 7.0 and room temperature (19°C) was obtained, independent from the wavelength of excitation in the range 500–600 nm and the ionic strength between 1 mM and 2 M salt.