Subpicosecond fluorescence upconversion measurements of primary events in yellow proteins

The primary step in the photocycle of photoactive yellow protein (PYP) has been investigated by means of the subpicosecond fluorescence upconversion technique. The nature of the chromophore is found to affect the fluorescence decay dynamics: upon replacement of the coumaric acid chromophore by 7-hydroxy-coumarin-3-carboxylic acid, the fluorescence decay becomes much longer (with typical decay times larger than 60 ps), whereas the 700 fs and 3 ps components have disappeared. It is discussed that the obtained results are strong support for the idea that the initial step in the photocycle involves trans-cis isomerization inside the coumaric acid chromophore in native PYP.