Polarization portraits of lightharvesting antennas: from single molecule spectroscopy to imaging

Sammanfattning: Popular Abstract in English Plants, algae and some bacteria are able to harvest solar energy to run chemical reactions by a complex process known as photosynthesis. Crucial molecules in this process are the so-called light-harvesting antennas. These antennas contain large amounts of pigments, such as chlorophyll, that are used for solar light absorption. Moreover, these pigments are also involved in the efficient transport of the absorbed energy towards special sites termed reaction centres. This occurs through a process referred to as excitation energy transfer. Light-harvesting antennas are not only present in natural photosynthetic organisms but also in any device that uses solar energy as “fuel” for its operation. Therefore the understanding of these molecules can help us to create new and more efficient solar based devices. This thesis reports the study of natural and artificial light harvesting antennas. We are particularly interested in the way the pigments are arranged in these molecules and how efficient the exchange of energy between them is. To obtain this information we used microscopy techniques that are able to study just one light-harvesting antenna at a time. This allowed us to investigate how different copies of an antenna behave. Many times important information in a system are hidden behind what is called the ensemble average. For example, consider that you want to study how human beings look like. One way to do this study would be to go to a large sports event and take a picture of the whole crowd. Then by analysis of the average appearance of humans in this picture you might be able to conclude that they have two arms, two legs and a head. However, the presence of a few red-haired persons might pass completely unnoticed. To characterize the orientation of the pigments in a single antenna and the excitation energy transfer between these pigments we used light polarization. Our methods are based on the fact that single pigments absorb and emit light polarized along a specific direction relative to its chemical structure. You might be familiar of the term polarization from sunglasses that use this principle to selectively block sunlight reflected from surfaces, such as still water. Nowadays, this property is also used in movie theatres and TV screens to display 3D movies. The polarization of light has to do with the orientation of its electric field vector. For example, in linearly polarized light the oscillating electric field vector is confined to a plane along the propagation direction of the light. By this principle, we can use the way a single antenna absorbs light to obtain information about the orientation of the pigments in its structure. If the pigments are randomly oriented then the antenna would absorb all polarization orientations equally. On the contrary, if the pigments in the antenna are preferentially aligned, then light polarized along a specific direction is preferentially absorbed by the antenna. Further, we can use the relationship between the polarization of the light absorbed by the antenna and the polarization of the emitted fluorescence to study energy transfer processes between differently oriented pigments. For example, consider an antenna that is excited by light polarized at angle α. As a result, pigments that are oriented along this direction are preferentially excited. Therefore, the emission should also be preferentially polarized at angle α in the absence of energy exchange between different pigments. On the other hand, if pigments that are differently oriented exchange energy, then the emission of this antenna would be polarized at a different angle. Using our polarization sensitive single molecule technique called two dimensional polarization imaging we were able to measure the excitation energy transfer efficiency of individual light-harvesting antennas. This can help us to evaluate the “quality” of an antenna before using it for the construction of a solar based device. Furthermore, we showed that our methodology can also be used as a new fluorescence imaging microscopy that uses the energy transfer sensitivity as imaging contrast. This opens new exciting applications for our technique in the study of systems relevant for biology, such as the aggregation of proteins involved in the causes of various diseases.