Photoreceptive proteins absorb light by chromophore molecule, and convert light into energy or signal. Ultrafast photoreactions such as electron transfer and isomerization allow efficient transitions from the electronically excited state of the chromophore, and stored light energy is utilized for each function. In case of light-energy conversion, light energy is finally stored as membrane potential, where proton is a principal component. Quinone pool facilitated by proton-coupled electron transfer in photosynthetic reaction center is a smart system to gain proton motive force. In contrast, light-driven ion pumping rhodopsins directly translocate protons, cations and anions. Ion-transporting rhodopsins are key tools in optogenetics, which revolutionize life sciences by control with light.
Animal retinas possess photoreceptor cells specialized to receive photons and convert them into neural signals. There are two types of visual cells, called rods and cones. Rods function in dim light conditions, and their photosensitivity is sufficiently high to respond to a single photon. Cones show rapid photoresponse, and the combination of cones having different wavelength sensitivity enables color vision. The transduction cascade in visual cells is initiated by the activation of G protein by photoactivated visual pigment, and this eventually results in the hyperpolarization of the cell. Recent progress in the study of the molecular basis of vision is reviewed.
After the gene cloning and development of GFP color variants about 20 years ago, fluorescent and luminescent proteins have become indispensable tools for biological research. Their genetically encodablity and light-emitting property has revolutionized our research ability by allowing the visualization of variety of living specimen ranging from biochemical events, proteins, cells, and organisms. Detailed understanding of the physicochemical mechanisms responsible for light generation has helped drive performance improvements and application development. Here we will cover basics of light-emitting proteins, as well as the use of them for bioimaging and biomanipulation.
Since the discovery of channelrhodopsins in 2002-2003, the light is now engineered to manipulate physiological states of a cell, a tissue or a body with the progress of three kinds of researches: (1) molecular biology of photoreceptive proteins, (2) cause-effect relationships in the biological system and (3) fabrication of opto-electronics or nanodevices. The near-infrared light, which conveys energy deep into the tissues, is expected to be used for optogenetics with the creation of proteins that change their conformations with such energy or the improvement of nanodevices that transfer the energy to photoreceptive proteins.
The history of optical microscopy was started by van Leeuwenhoek, who discovered microorganisms for the first time in the world. Even at that time, the most essential powers of the optical microscopy were high spatial resolution, high sensitivity, and detection of motion. In this small review, we will recall how these three powers of optical microscopy have extended biophysical sciences. Detection and analysis of the individuality, distribution, dynamics, and history in the biological scales from single molecules to cells and tissues were the major roles of the optical microscopy. These roles inevitably lead optical microscopy to proceed toward the computational sciences.
Ultrashort optical pulses enable us to observe many fascinating dynamics in proteins. In this review, we will first illustrate the progress of laser technology and then describe application of the laser pulses to observation of ultrafast protein dynamics. Developments of laser technology for the ultrashort pulse generation significantly improved the time-resolution of the measurements. The developments also allow us to observe vibrational and electronic coherence of proteins, which provides greatly increased understanding of protein dynamics.
Advent of synchrotron radiation completely changed X-ray experiments on biological samples. Protein crystallography data collection has now become a routine work for most of researchers. Protein small-angle scattering measurements are also popular. Time resolved experiments that make best use of high brilliance of synchrotron X-rays are not many in number but have been producing many biophysically interesting results. As synchrotron radiation facilities are still evolving toward lower emittance, new ideas of experiments using the more brilliant X-ray beam are anticipated.