Strong donation of the hydrogen bond of tyrosine during photoactivation of the BLUF domain

In the photoreaction of BLUF (sensor of blue light using FAD) domains, conserved Tyr/Gln nearby the FAD chromophore plays a crucial role in the formation of a red-shifted intermediate. The intermediate appears accompanying a hydrogen-bonding alteration of the Tyr/Gln. In order to reveal the hydrogen-bonding structure of the Tyr/Gln active center and FAD, FTIR spectroscopy was applied in the 4000-1800 cm -1 region, where X-H stretches appear. The present FTIR data show an unusually low O-H stretching vibration of tyrosine at 2800-2600 cm -1. This provides direct evidence of the light-induced switch of the hydrogen-bonding network, where the conserved Tyr (Tyr21 for AppA) donates an unusually strong hydrogen bond in the intermediate