Bacteriorhodopsin is a photosynthetic protein of halophilic (salt-liking) bacteria, the function of which is to move the protons across the cell membrane using the energy of light. The detailed functions of this protein are discussed in the context of photosynthesis. Here we will only briefly discuss the aspects of this protein that are necessary for the understanding of the experimental results.
Fig. 1) A: Energy scheme of retinal isomerization (model). The minimum of the excited state corresponds to cis molecular conformation. As the molecule moves towards this configuration in the excited state, the energy gap between the states is decreasing, and, at some point, thermally activated vibrational transitions promote the molecule back to ground state. B: Fluorescence decay kinetics at different wavelengths in bacteriorhodopsin. Reproduced from .
Light absorption in bacteriorhodopsin is performed by retinal – an elongated molecule, similar to carotene, attached to the protein via the Schiff base. The initial photoinduced event finally leading to the proton transfer is the trans-cis isomerization of this pigment. The lowest excited state of retinal is of p* type , which is an antibonding orbital, and, contrary to the lowest occupied state, the molecule is free to rotate around a double bond. In order for such rotation to occur, the energy of the excited state has to decrease, as the molecular configuration moves away from planar. At the same time, the energy of the ground state is increasing (the ground state has its energy minimum in all-trans configuration). Therefore the energy gap between the states is decreasing and, at some point the molecule returns to the ground state (Fig. 24A). If the conformation of the ground state accessible at that point is closer to cis, returning to the original trans state is impossible because of the energy barrier in the ground state, and retinal ends up being cis. The energy accumulated in the deformed chemical bonds of retinal is later used to transport the proton across the cell membrane.
For a long time, experimentalists could not determine the rate of the isomerization process: the time resolution of fluorescence experiments was insufficient and bacteriorodopsin was always faster than the available lasers. All the experiments were forced to conclude merely, that the isomerization rate is comparabale to or faster than time resolution of their experiments. Only in 1993, when first Ti:Sapphire lasers became available, M. Du and G. Fleming could finally measure the decay of excited state of bacteriorhodopsin. The kinetic traces measured in their fluorescence upconversion experiments are shown in Fig. 24B. It is obvious, that the excited state disappears extremely fast: most of the emission decays within the first picosecond. This, then, is the rate, at which bacteriorhodopsin isomerizes inside the protein. Figure 24B also shows that fluorescence is not lost in a single-exponential fashion: some light is still observed after 4 ps (see, for example the kinetic trace in Fig. 24B measured at 800 nm). This indicates that the protein dynamics observed is more complicated than a simple kinetics that can be described by a first-order differential equation. A model is needed to explain such inhomogeneous decay of fluorescence.
It is interesting to note that when retinal is dissolved in an organic solvet, its isomerization becomes a lot slower than inside the bacteriorhodopsin protein . This indicates that evolution has optimized bacteriorhodopsin protein to make the isomerization of retinal as fast as possible in order to use the energy of the absorbed photon most efficiently.