Strong Donation of the Hydrogen Bond of Tyrosine during Photoactivation of the BLUF Domain

Abstract

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.