How photoreceptors detect photons

Living organisms must interact with their environment: they must sense the temperature, light quality, certain chemicals, etc. Once triggered, they initiate a cascade of signal transduction events leading to a response. Photoreceptors represent an important class of receptors because they can be triggered with light, allowing us to study their response over a time window from femtoseconds to seconds. Photoactive Yellow Protein (PYP) is a small cytosolic photoreceptor thought to be responsible for the negative phototactic response of its host organism Ectothiorhodospira halophila. The availablity of structural information for different functional states and the relatively simple photocycle make PYP an ideal “laboratory” for the detailed study of biological light detection and the relation of structural change to protein function. It also holds considerable promise for optical data storage and computing applications. Optical excitation of the intrinsic chromophore in PYP, p-coumaric acid, leads to the initiation of a photocycle that comprises several distinct intermediates. The dynamical processes responsible for the initiation of the PYP photocycle have been explored by the Biophysics group with several time-resolved techniques. Ultrafast electronic spectroscopies, such as pump-visible probe, pump-dump-visible probe, and fluorescence upconversion, are useful in identifying the timescales and connectivity of the transient intermediates, while ultrafast vibrational
spectroscopies link these intermediates to dynamic structures.

Contact: Marloes Groot, e-mail:


Detailed overview of the partial reactions in the photocycle of PYP.