Hydrogen bonds and proton transfer in the photoisomerization of Photoactive Yellow Protein

Photoactive yellow protein (PYP) is the protein responsible for the negative phototaxis of Halorhodospira halophila. The absorption of blue light of its chromophore, p-coumaric acid, triggers a photocycle in PYP, initiated by trans-cis isomerization of its chromophore. Pump-probe spectroscopy in the visible reveals that proline at position 68 is responsible for the high efficiency of isomerization in PYP, and mutation of this residue with other neutral residues reduces the quantum yield of the reaction. We studied the dynamics of this photoreaction for wt PYP and P68A, P68V and P68G mutants, by dispersed ultrafast pump-dump-probe spectroscopy, where the photocycle can be started and interrupted with appropriately tuned and timed laser pulses. The quantum yield of the wt, having proline at position 68 is the highest, followed by the yields of P68V, P68A and P68G. The environment of the chromophore is crucial for the interplay of transitions to the ground state, the ground state intermediate and the product. Mutations provide access of water molecules around the chromophore and additional hydrogen bonds to the chromophore thus enabling an increase in the Manuscript in preparation