Sum Frequency Generation (SFG) Spectroscopy
Prof. Dr. Mischa Bonn
Membranes constitute the highly active partition between living cells and the outside world. They regulate molecular transport, cell adhesion and intercellular signaling. A detailed understanding - and control - of the many biological processes that occur at the membrane surface, such as viral infection and targeted drug delivery, requires insights at the molecular level. Recent developments in experimental techniques have opened avenues for the study of intermolecular interactions and chemical processes at surfaces and interfaces with unprecedented time and spatial resolution, without the need for (fluorescent) labels. We employ Sum-Frequency Generation (SFG) to address important issues in biological (model) membranes.
SFG spectroscopy employs an ultrashort infrared pulse to excite vibrational resonances of the target molecules. This excitation is upconverted by a temporally and spatially overlapping visual (800 nm) pulse. Consequently, the vibrational spectrum can be read from the visual (600-700 nm) sum-frequency emission.
Since this second-order optical process is forbidden in centrosymmetric media, an SFG signal is only obtained from the surface, potentially including phospholipids, ligands and aligned water molecules.
The Biosurface Spectrosopy group uses four separate experimental setups to apply SFG spectroscopy to a wide range of materials, including interfacial water, phospholipids and other surfactants, proteins, and synthetic nanomaterials. Using laser systems with a pulse duration of down to 40 femtoseconds, the vibrational dynamics and reorientational motion of interfacial molecules can be monitored with high precision.
We have used time-resolved vibrational SFG spectroscopy to quantify the reorientational motion of water molecules at interfaces, and have found that interfacial water reorients ~3 times faster than water in bulk.
In a separate effort, we have used 2D-SFG to probe structural and energy relaxation dynamics of water at interfaces. In 2D-SFG, excitation and detection pulses are scanned independently in frequency space. Specific vibrational (sub-)groups can thus be excited, and the effect of that excitation is monitored on all groups within the frequency window. This approach allows for new insights into structure of water at different interfaces, but also the real-time flow of vibrational energy at the water surface.Future directions
Much of the past research on biological membranes has remained limited to investigations of the composition and static structure of membranes, whereas experimental information on the molecular dynamics in membranes is lagging, owing to the lack of appropriate experimental tools. It has been widely acknowledged that detailed information on the time-scales on which membrane biomolecular interactions occur is essential for a detailed understanding of these processes.
In the coming years, we aim to combine our knowledge in the field of membrane (static) spectroscopy with our expertise in ultrafast surface spectroscopy, to monitor the dynamics of membrane molecules in real time, from ~100 femtoseconds to seconds. By looking directly inside the lipids, the proteins and the water by means of their molecular vibrations, this novel approach circumvents the conventional usage of probe molecules (e.g. fluorescent probes) and is thus completely non-invasive.
"Ultrafast Reorientation of Dangling OH Groups at the Air-Water Interface Using Femtosecond Vibrational Spectroscopy"
"Ultrafast vibrational energy transfer at the water/air interface revealed by two-dimensional surface vibrational spectroscopy"