Photochemical processes in biological chromophores and clusters.
Goal of the research
The goal of our projects is to understand photochemical processes in biological systems, such as DNA or proteins. Unfortunately, the large size and structural complexity of such biological molecules hampers the direct experimental and theoretical characterization.
We therefore use a bottom-up approach and first investigate the inherent properties of the isolated chromophores by experiment and theory. Next, we explore how the photochemical processes are influenced by a local environment by studying molecular clusters of increasing size. Like assembling a puzzle, the systematic study of all relevant interactions can lead to the description of complex biological systems through knowledge of all relevant local properties. While it would be overly optimistic to hope for a complete understanding of photochemical processes in biology, this approach clearly offers fundamental insight into the chemical and physical properties of increasingly large systems.
Spectroscopy of molecular clusters
The investigation of small molecular clusters containing one or several biological chromophores allows the controlled introduction of directed molecular interactions, e.g. hydrogen bonding or base stacking in DNA bases. Our experimental tools include femtosecond time-resolved electron and mass spectroscopy. Time resolved pump-probe spectroscopy allows the observation of excited state processes in real time. Electron spectroscopy allows the direct assignment of electronic structure. The combination of pump-probe and electron spectroscopy thus allows to follow the evolution of electronically excited states along a (photochemical) reaction coordinate. Mass spectroscoy diagnoses the composition of the investigated cluster species. Together with the fs pump probe technique this offers a direct correlation of cluster composition and excited state processes. With the simultaneous detection of electrons and ions in the low signal limit (<< 1 ionization event per laser shot), we can perform time-resolved electron-ion coincidence experiments and assign the electronic structure of selected cluster species.
Experiments in a liquid-jet photoelectron spectrometer allow the time-resolved electron spectroscopy of solvated species. With the liquid jet, we currently investigate the formation and evolution of solvated electrons. In the future, we hope to extrapolate from microsolvated molecular clusters to aqueous solutions.
The fundamental problem of bottom-up science
There is a fundamental problem with the bottom-up approach to investigate increasingly larger and more complex systems: The complexity of quantum-systems scales not linearly, but rather exponentially with size. Hence, there is little hope to extend scientific observations and models to systems of significantly larger size simply by performing more experiments.
The example of aminopyridine shown when you move the mouse over the figure to the right is a misleading example, because the symmetry of the corresponding dimer allowed us to characterize a photochemical process with the basic tools of time-resolved mass- and electron spectroscopy. For real DNA base pairs, which may adopt a large number of tautomeric, and topomeric structures, corresponding experiments cannot yield enough information. To resolve this problem, we move to correlated measurements: With the correlated measurement of two observables, the spectroscopic information content of corresponds to the product of the information content obtained by two independent measurements. This is readily obvious if we consider the two dimensional plot of a correlated measurement as compared to the linear axis of a conventional measurement: The plane spanned by the two corelated spectroscopic axes has an area corresponding to the product of the axis lengths of vertical and horizontal axis. With the more favorable scaling law of corelated measurements and the careful selection of correlated observables (see "CRASY" below), we hope to extend our experimental investigations to systems of much larger size and complexity.
The information content of spectroscopic experiments is fundamentally limited by Heisenbergs uncertainty limit: the spectroscopic resolution is proportional to the observation time. For the investigation of fast processes, this creates a big problem, because the observation time is forcibly short. This effect limits the information content in our time-resolved experiments to a degree, that we cannot routinely determine the structure of the molecules or clusters under investigation. If we have a heterogeneous sample (an impure or instable compound, or multiple isomers of a molecule / cluster), the we cannot assign the observed properties to a structure. Because theoretical predictions are always connected to well-defined structures, this also limits our ability to compare experiment and theory.
To overcome this problem, we observe multiple, correlated observables in correlated rotational alignment spectroscopy (CRASY). An infrared alignment pulse creates a coherent rotational wavepacket, which is probed by delayed excitation and ionization. Because the molecular excitation dipoles rotate, we observe a temporal modulation of the signal intensity as a function of time. The fourier transformation of the temporal modulation recovers all underlying frequencies, i.e. generates a rotational Raman spectrum. In mass-CRASY experiments, we can correlate masses of molecules and molecular fragments with the structure of the initiaoly photoexcited species. In electron-CRASY experiments, we obtain structure-selective electron spectra. Finally, we can perform dynamics-CRASY experiments, where excited state lifetimes are assigned to particular molecular structures.