Biophysics
The VU-Biophysics group investigates elementary events in biology such as the transfer of excitons and electrons in photosynthesis, the transfer of protons in photosensing proteins, the transfer of hydrides in a photoactive enzymes etc. Furthermore, since these processes occur with a high efficiency they are generally coupled to specific protein dynamics. Following light absorption these events occur on an ultrafast timescale (10-14 10-8 s), which requires ultrashort laser pulses to study their evolution in real time.
Photosynthesis is the highly optimized process that plants and bacteria use to convert solar energy into biological energy. A system of light-harvesting pigments (chlorophylls, carotenoids) absorbs solar photons and the electronic excitation energy is rapidly ( < 10-12 s) transferred among these pigments until a special chlorophyll-protein complex is reached, the so-called photosynthetic reaction center, where an ultrafast electron transfer occurs, storing the energy in the form of a pair of separated charges. We study the energy transfer processes in the light-harvesting system and the electron transfer events in the reaction center by means of various time-resolved femtosecond laser-spectroscopic techniques. This research is embedded into two large European networks. Recently we have initiated studies concerning the application of principles of natural photosynthesis in solar cells.
Photosensing proteins detect a photon, which after absorption leads to a signal that triggers another event in the biological cell. An excellent example is the Photoactive Yellow Protein (PYP), a bacterial lightsensor, where the absorption of a blue photon leads to a signal that makes the bacterium swim away from blue light. We use ultrafast spectroscopy to study how in PYP an electronically excited state is converted into a protein structural change that initiates the response of the bacterium. We have recently constructed a femtosecond infrared experiment, which allows us to study the response of the protein to the absorption of light and thereby allows us to understand how electronic excitation may lead in a very specific manner to a protein response. Other photosensors that we study are flavoproteins involved in the regulation of gene expression, light-sensitive proteins that control the motion of plants (sunflowers!), etc.
Photoactive enzymes are proteins that perform a specific enzymatic reaction, but need light as a trigger. We study the photoactive enzyme Protochlorophyllide OxidoReductase (POR) that needs light to add two protons and two electrons to its substrate, protochlorophyllide. The reaction is an essential step in the formation of chlorophyll in plants. The process is of fundamental interest because there exist many proteins which perform a similar reaction in the dark, but then the timescale of the reaction is limited by the binding of the substrates, processes which occur in milliseconds. Thus we have the unique opportunity to ‘view’ the biological transition state in ‘real time’.
Femtosecond lightpulses can be precisely controlled in terms of their phase, spectrum, intensity and spatial shape. A number of such light pulses, precisely spaced in time, can be used to excite a sample and manipulate the subsequent electronic states. This kind of ‘control’ allows the investigator to become a player in the biological reaction, and in this way new states and new reaction pathways can be discovered. We investigate the complex reaction pathways of carotenoids, PYP, the green fluorescent protein and photosynthetic reaction centers using these techniques.
