The primary photoreaction of photoactive yellow protein (PYP): anisotropy changes and excitation wavelength dependence

The absorption and stimulated emission changes in the first 535 ps of the PYP photocycle can be described by four life times of 0.7, 6.3, and 220 ps and long lived. Two intermediates, I0 and I0‡, were identified. We did not obtain indications for a significant excitation wavelength dependent primary photochemistry as found in low temperature absorption spectroscopy. The anisotropy of the primary photoproduct I0 and its successor I0‡ amounts to 0.3 – significantly lower than that of the bleached ground state (0.4). This distinctive change of the transition dipole moment orientation in the product state (24°) reflects changes of the chromophore geometry and electron density distribution caused by the photoisomerisation.