Mitochondria Porphyrins form an important, naturally occurring, yet not well-explored family of molecules. Various kinds of porphyrins take part in a multitude of biochemical processes, including those taking place in our body.
They are found in our blood stream, as the main building block of haemoglobin and a part of the vitamin D12 complex. The significance of porphyrins and the role they play is strongly connected to their structure and dynamics, in particular to the mobile hydrogen atoms that constantly change their position within the molecule. This makes the molecule switch continuously between two chemical structures, a process known in chemistry as tautomerization. The goal of the project was to elucidate the nature of the tautomerization reaction: how fast it is in individual molecules, how sensitive it is to the local environment and how it relates to the structure of the molecule itself.
To this end we used a set of sophisticated experimental techniques that combine specially polarized laser excitation with fluorescence detection from individual molecules. The obtained fluorescence patterns provided direct information about the speed of tautomerization. For some specific porphyrins it was possible to determine the presence of two tautomeric forms by monitoring fluctuations of the emission spectra of individual molecules in time. Moreover, together with our collaborators from the Institute of Photonic Sciences in Barcelona (ICFO) we developed a special experimental approach, termed stimulated emission microscopy. We demonstrated its capabilities by studying ultrafast charge dynamics in individual quantum dots and set up a solid basis for studying chemical reactions at a single molecule level.
The experiments revealed that the speed of the double proton transfer spans many orders of magnitude – the reaction can be extremely fast (occurs millions of times per second) and it can be very slow (timescale of seconds). Intriguingly, the tautomerization rate is not constant for a particular molecule, but changes dramatically in time. It can be very fast in one moment and extremely slow just a moment later. These observations are a consequence of the interactions between the porphycene molecule and other molecules in the direct surroundings. Tautomerization is very sensitive to the local environment – small movements, translations, changes of conformation of the surrounding molecules may slightly change the state of the porphycene in a way that it speeds up or slows down the proton transfer. Consequently, porphycenes can be used as probes of the local nanoscale dynamics of the surrounding medium. Porphycenes are considered as potential building blocks for complex artificial molecular structures capable of performing certain operations using light or charge. From this technological point of view our results indicate that we can design and shape the local environment in order to induce specific tautomerization dynamics in a particular porphycene molecule. Finally, we learned that tautomerization is not very sensitive to the structure of the molecule itself – for porphycenes with different structures these are still interactions with the local nano-environment,which determine tautomerization dynamics, and not the exact structure of the molecule.
This project gave us much needed insights into tautomerization, one of the most fundamental chemical processes. We now better understand the mechanism behind the hydrogen transfer and hydrogen bond rearrangement, with quantum tunnelling being one of the key factors. Moreover, the proposed experiments form a solid basis for studying hydrogen transfer and hydrogen bonding dynamics in complex biomacromolecules.
How did you benefit from the POLONEZ fellowship?
The POLONEZ fellowship had an important impact on my career and future plans. Apart from expanding my transferable skills, the successful realization of the project and, in particular, my participation in a number of conferences and symposia improved my visibility as a researcher on a national and international level and gave me the opportunity to set up new collaborations.
Dr hab. Eng. Łukasz Piątkowski – graduated from the Faculty of Technical Physics at the Poznan University of Technology. He received his PhD in physics at the University of Amsterdam in 2012. In the years 2012-2016 he completed a postdoctoral internship at the ICFO Institute of Photonic Sciences in Barcelona, where he used microscopic methods to deal with ultrafast processes in single molecules of biological importance. Since 2019, he has been running his own research team at the Institute of Physics at Poznan University of Technology. Author of over 30 scientific papers, including in Nature Chemistry and Science.