Sander Woutersen

Resonant intermolecular transfer of vibrational energy in liquid water

Many biological, chemical and physical processes involve the transfer of energy. In the case of electronic excitations, transfer between molecules is rapid, whereas for vibrations in the condensed phase, resonant energy transfer is an unlikely process because the typical timescale of vibrational relaxation (a few picoseconds) is much shorter than that of resonant intermolecular vibrational energy transfer. For the OH-stretch vibration in liquid water, which is of particular importance due to its coupling to the hydrogen bond, extensive investigations have shown that vibrational relaxation takes place with a time constant of 740 ± 25 femtoseconds (ref. 7). So for resonant intermolecular energy transfer to occur in liquid water, the interaction between the OH-stretch modes of different water molecules needs to be extremely strong. Here we report time-resolved pump-probe laser spectroscopy measurements that reveal the occurrence of fast resonant intermolecular transfer of OH-stretch excitations over many water molecules before the excitation energy is dissipated. We find that the transfer process is mediated by dipole–dipole interactions (the Förster transfer mechanism) and additional mechanisms that are possibly based on intermolecular anharmonic interactions involving hydrogen bonds. Our findings suggest that liquid water may play an important role in transporting vibrational energy between OH groups located on either different biomolecules or along extended biological structures. OH groups in a hydrophobic environment should accordingly be able to remain in a vibrationally excited state longer than OH groups in a hydrophilic environment.

Hydrogen-bond lifetime measured by time-resolved 2D-IR spectroscopy: N-methylacetamide in methanol

Abstract 2D vibrational spectroscopy is applied to investigate the equilibrium dynamics of hydrogen bonding of N-methylacetamide (NMA) dissolved in methanol-d4. For this particular solute–solvent system, roughly equal populations are found for two conformers of the solute–solvent complex, one of which forms a hydrogen bond from the CO group of NMA to the surrounding solvent, and one of which does not. Using time-resolved 2D-IR spectroscopy on the amide I band of NMA, the exchange between both conformers is resolved. Equilibration of each conformer is completed after 4.5 ps, while the formation and breaking of the hydrogen bond occurs on a slower, 10–15 ps time scale. This interpretation is supported by classical molecular-dynamics simulations of NMA in methanol. The calculations predict a 64% population of the hydrogen-bonded conformer and an average hydrogen-bond lifetime of ≈12 ps.

Peptide conformational heterogeneity revealed from nonlinear vibrational spectroscopy and molecular-dynamics simulations

Nonlinear time-resolved vibrational spectroscopy is used to compare spectral broadening of the amide I band of the small peptide trialanine with that of N-methylacetamide, a commonly used model system for the peptide bond. In contrast to N-methylacetamide, the amide I band of trialanine is significantly inhomogeneously broadened. Employing classical molecular-dynamics simulations combined with density-functional-theory calculations, the origin of the spectral inhomogeneity is investigated. While both systems exhibit similar hydrogen-bonding dynamics, it is found that the conformational dynamics of trialanine causes a significant additional spectral broadening. In particular, transitions between the poly(Gly)II and the αR conformations are identified as the main source of the additional spectral inhomogeneity of trialanine. The experimental and computational results suggest that trialanine adopts essentially two conformations: poly(Gly)II (80%) and αR (20%). The potential of the joint experimental and computational approach to explore conformational dynamics of peptides is discussed.

Mechanism for vibrational relaxation in water investigated by femtosecond infrared spectroscopy

We present a study on the relaxation of the O–H stretch vibration in a dilute HDO:D2O solution using femtosecond mid-infrared pump-probe spectroscopy. We performed one-color experiments in which the 0→1 vibrational transition is probed at different frequencies, and two-color experiments in which the 1→2 transition is probed. In the one-color experiments, it is observed that the relaxation is faster at the blue side than at the center of the absorption band. Furthermore, it is observed that the vibrational relaxation time T1 shows an anomalous temperature dependence and increases from 0.74±0.01 ps at 298 K to 0.90±0.02 ps at 363 K. These results indicate that the O–H⋯O hydrogen bond forms the dominant accepting mode in the vibrational relaxation of the O–H stretch vibration.

Isotope-edited two-dimensional vibrational spectroscopy of trialanine in aqueous solution

Two-dimensional vibrational spectroscopy is applied to the amide I mode of trialanine and two of its isotopomers dissolved in heavy water. We use site-directed 13C isotope substitution to change the individual frequencies of the coupled oscillators, and hence to modify specific matrix elements of the molecular Hamiltonian. It is found that all of the results can be well described by an excitonic model for the amide I band, using the same coupling strength and dipole–dipole angle for all three isotopomers. This demonstrates that these two spectral parameters are determined by the secondary structure of the peptide, which remains unchanged upon isotope substitution.

Subpicosecond conformational dynamics of small peptides probed by two-dimensional vibrational spectroscopy

The observation of subpicosecond fluctuations in the conformation of a small peptide in water is demonstrated. We use an experimental method that is specifically sensitive to conformational dynamics taking place on an ultrafast time scale. Complementary molecular-dynamics simulations confirm that the conformational fluctuations exhibit a subpicosecond component, the time scale and amplitude of which agree well with those derived from the experiment.

Nonlinear two-dimensional vibrational spectroscopy of peptides

In this overview, we discuss theoretical and experimental aspects of nonlinear two-dimensional infrared (2D-IR) spectroscopy. With this technique both peptide conformation and conformational flexibility can be probed. The quantitative relation between the experimental 2D-IR spectrum and the peptide conformation is discussed, and examples of how the conformation of a peptide and the timescale of its fluctuations are derived from its (time-resolved) 2D spectrum are presented.

Inhomogeneous dynamics in confined water nanodroplets

The effect of confinement on the dynamical properties of liquid water was studied by mid-infrared ultrafast pump–probe spectroscopy on HDO:D2O in reverse micelles. By preparing water-containing reverse micelles of different well defined sizes, we varied the degree of geometric confinement in water nanodroplets with radii ranging from 0.2 to 4.5 nm. We find that water molecules located near the interface confining the droplet exhibit slower vibrational energy relaxation and have a different spectral absorption than those located in the droplet core. As a result, we can measure the orientational dynamics of these different types of water with high selectivity. We observe that the water molecules in the core show similar orientational dynamics as bulk water and that the water layer solvating the interface is highly immobile.

Influence of ions on the hydrogen-bond structure in liquid water

The orientational-correlation time of water molecules in ionic solutions has been measured with femtosecond pump–probe spectroscopy. It is found that the addition of ions has no influence on the rotational dynamics of water molecules outside the first solvation shells of the ions. This shows that the presence of ions does not lead to an enhancement or a breakdown of the hydrogen-bond network in liquid water.

Time-resolved two-dimensional vibrational spectroscopy of a short α-helix in water

Nonlinear two-dimensional (2D) vibrational spectroscopy has been used to investigate the amide I band of an alanine-based 21-residue α-helical peptide in aqueous solution. Whereas the linear absorption spectrum consists of a single, broad amide I band, the 2D vibrational spectrum clearly reveals that this band is composed of two amide I transitions, which are assigned to the A and E1 modes. The A–E1 frequency splitting is found to be approximately 10 cm−1. We find that the amide I band is inhomogeneously broadened due to conformational disorder of the helix. The 2D line shapes can be well described using distributions of the dihedral angles (φ,ψ) around their average values with a width of 20°, confirming previous molecular-dynamics studies. Time-resolved 2D measurements show that the conformation fluctuates on a time scale of picoseconds.