Ultrafast Molecular Dynamics

Femtosecond Vibrational Spectroscopy

Focus of our research is to establish novel spectroscopic methods in the infrared (IR) spectral range, which resolve transient structures of molecular systems and the energy flow through them on a very fast femtosecond timescale. IR spectroscopy analyzes nuclear motions of molecular systems and hence directly those degrees of freedom, which are relevant for conformational changes of bio-macromolecules. A large variety of questions is addressed, ranging from complex problems such as protein folding, energy transport in biomolecules, to elementary structural processes in liquids like water and IR-driven "photochemistry".

2D-IRSpectroscopy

2D-IR spectroscopy is a novel spectroscopic tool to investigate structure and dynamics of solution phase systems with unprecedented detail. 2D-IR spectroscopy measures local contacts in a way similar to 2D-NMR, albeit with many orders of magnitudes higher time-resolution [5-7]. It thus combines essentially unlimited time resolution with significant structure resolution. We have recently introduced the method of transient 2D-IR spectroscopy, which extends conventional 2D-IR spectroscopy to the non-equilibrium regime, allowing us to make 'molecular movies' of fast conformational changes [1].

New Methods of Nonlinear Spectroscopy and Solution Phase Dynamics

We are constantly pushing the development of new spectroscopic methods. For instance, we are currently developing 3D-IR spectroscopy, both experimentally and theoretically [2], which would allow one to resolve in much more detail the non-trivial stochastic processes of, e.g., the complicated hydrogen-bond network of liquid water. More recently, we concentrate on the development of 2D Raman THz spectroscopy of water, which measures the low-frequency intermolecular degrees of freedom directly [9].    

Ultrafast Dynamics of Photoswitchable Peptides and Proteins

The ultimate goal of transient 2D-IR spectroscopy is to be able to investigate in unprecedented detail complicated structural processes as they occur, for example, during protein (or peptide) folding. To this end, we are developing various approaches to trigger such structural processes on ultrafast timescales by means of molecular switches covalently linked to a peptide or protein. We are then probing the response of the peptide by means of IR and 2D-IR spectroscopy, covering timescales from femtoseconds to microseconds [3]. The experimental work is complemented by computer (molecular dynamics) simulations. We recently started to use similar concepts to also investigate the conformational transition of allosteric proteins.

Artificial Photosynthetic Systems

Molecular sciences very likely will become central in tackling one of the most pressing problems of mankind, i.e., the supply of renewable energies via some form of an artificial photosynthetic system. With our toolset of vibrational spectroscopy, we will contribute to this field of research. Artificial photosynthetic systems are perfectly suited to be investigated with essentially unlimited time-resolution by optical spectroscopy, as they are naturally photo-triggerable. IR spectroscopy is intrinsically a very fast spectroscopy (in contrast to e. g. NMR spectroscopy), and it reports on chemical structure in a relative direct and interpretable manner (in contrast to e.g. electronic spectroscopy).

We investigate the electron transfer pathways between the various molecular components in homogeneous artificial photosynthetic systems developed in the groups of Roger Alberto  and Greta Patzke  [10]. As a new line of research, we will start to develop also surface sensitive IR spectroscopic technique, as any future artificial photosynthetic system most likely will work with catalytic surfaces rather than in homogeneous solutions.

Vibrational Energy Transport Mechanism

Biomolecules deal with energy in an extremely efficient way; as such a microscopic understanding of energy transport and energy loss mechanisms is required. Femtosecond IR-spectroscopy is also very well suited to study energy transport mechanism in peptides and proteins. To this end, we covalently attach a chromophor used as local heater to a peptide, and use certain vibrational modes as local thermometers at various distances from the heater. [8]

Matrix Spectroscopy and Elementary Reactivity of Small Molecules

Electronically driven photochemistry often is ultrafast, a discovery for which Ahmed Zewail received the Nobel prize in 1999. However, most (not all) chemistry is thermally driven and is happening on electronic ground state potential energy surfaces rather than through electronically excited states. We investigate IR-driven photochemical reactions in rare gas matrix environment, acting as a weakly interacting 'solvent', with the help of femtosecond IR spectroscopy to obtain a fundamental understanding of this important class of chemical reactions [4].

Vibrational Self-Trapping in Peptide Models and Model Peptides

The peculiarity of the hydrogen bonds which stabilize protein secondary structures lead to unconventional vibrational states such as vibrational solitons and polarons. These are investigated by ultrafast IR spectroscopy.

Selected Publications

[1] Ch. Kolano, J. Helbing, M. Kozinski, W. Sander, P. Hamm. "Watching Hydrogen-Bond Dynamics in a b-Turn by Transient 2D-IR Spectroscopy. Nature 444, (2006), 469

[2] P. Hamm, " Three-Dimensional-IR Spectroscopy: Beyond the Two-Point Frequency Fluctuation Correlation Function" J. Chem. Phys., 124, (2006), 124506

[3] J. Bredenbeck, J. Helbing, J. R. Kumita, G. A. Woolley. P. Hamm "a-Helix Formation in a Photoswitchable Peptide Tracked from Picoseconds to Microseconds by Time Resolved IR Spectroscopy"Proc. Natl. Acad. Sci. USA, 102, (2005), 2379

[4] R. Schanz, V. Botan, P. Hamm "A Femtosecond Study of the IR-Driven Cis-Trans Isomerization of Nitrous Acid (HONO)" J. Chem. Phys., 122, (2005), 04450

[5] S. Woutersen, P. Hamm "Nonlinear Two-Dimensional Vibrational Spectroscopy of Peptides" J. Phys-Condens. Mat. 14, (2002), R1035. Review Article on 2D Spectroscopy.

[6] S. Woutersen and P. Hamm "Structure Determination of Trialanine in Water Using Polarization Sensitive 2D Vibrational Spectroscopy" J. Phys. Chem. B 104, (2000), 11316-11320

[7] P. Hamm, M. Lim and R.M. Hochstrasser "The Structure of the Amide I Band of Peptides Measured by Femtosecond Nonlinear IR Spectroscopy" J. Phys. Chem. B 102, (1998), 6123-6138

[8] V. Botan, E.H.G. Backhus, R. Pfister, A. Moretto, M. Crisma, C. Toniolo, P.H. Nguyen, G. Stock, and P. Hamm " Energy transport in peptide helices" Proc. Natl. Acad. Sci USA, 104 (2007) 12749

[9] P. Hamm and J. Savolainen, 2D-Raman-THz spectroscopy of water: Theory, J. Chem. Phys. 136 (2012) 094516.

[10] B. Probst, C. Kolano, P. Hamm, R. Alberto, An efficient homogeneous intermolecular rhenium based photocatalytic system for the production of H2, Inorg. Chem. 48 (2009) 1836- 1843