The Hydrated Excess Proton in the Zundel Cation H5 O2 (+) : The Role of Ultrafast Solvent Fluctuations.

The nature of the excess proton in liquid water has remained elusive after decades of extensive research. In view of ultrafast structural fluctuations of bulk water scrambling the structural motifs of excess protons in water, we selectively probe prototypical protonated water solvates in acetonitrile on the femtosecond time scale. Focusing on the Zundel cation H5 O2 (+) prepared in room-temperature acetonitrile, we unravel the distinct character of its vibrational absorption continuum and separate it from OH stretching and bending excitations in transient pump-probe spectra. The infrared absorption continuum originates from a strong ultrafast frequency modulation of the H(+) transfer vibration and its combination and overtones. Vibrational lifetimes of H5 O2 (+) are found to be in the sub-100 fs range, much shorter than those of unprotonated water. Theoretical results support a picture of proton hydration where fluctuating electrical interactions with the solvent and stochastic thermal excitations of low-frequency modes continuously modify the proton binding site while affecting its motions.

Anharmonicities and coherent vibrational dynamics of phosphate ions in bulk H2O.

Phosphates feature prominently in the energetics of metabolism and are important solvation sites of DNA and phospholipids. Here we investigate the ion H2PO4(-) in aqueous solution combining 2D IR spectroscopy of phosphate stretching vibrations in the range from 900-1300 cm(-1) with ab initio calculations and hybrid quantum-classical molecular dynamics based simulations of the non-linear signal. While the line shapes of diagonal peaks reveal ultrafast frequency fluctuations on a sub-100 fs timescale caused by the fluctuating hydration shell, an analysis of the diagonal and cross-peak frequency positions allows for extracting inter-mode couplings and anharmonicities of 5-10 cm(-1). The excitation with spectrally broad pulses generates a coherent superposition of symmetric and asymmetric PO2(-) stretching modes resulting in the observation of a quantum beat in aqueous solution. We follow its time evolution through the time-dependent amplitude and the shape of the cross peaks. The results provide a complete characterization of the H2PO4(-) vibrational Hamiltonian including fluctuations induced by the native water environment.

Anharmonic Backbone Vibrations in Ultrafast Processes at the DNA-Water Interface.

The vibrational modes of the deoxyribose-phosphodiester backbone moiety of DNA and their interactions with the interfacial aqueous environment are addressed with two-dimensional (2D) infrared spectroscopy on a femto- to picosecond time scale. Beyond the current understanding in the harmonic approximation, the anharmonic character and delocalization of the backbone modes in the frequency range from 900 to 1300 cm(-1) are determined with both diagonal anharmonicities and intermode couplings on the order of 10 cm(-1). Mediated by the intermode couplings, energy transfer between the backbone modes takes place on a picosecond time scale, parallel to vibrational relaxation and energy dissipation into the environment. Probing structural dynamics noninvasively via the time evolution of the 2D lineshapes, limited structural fluctuations are observed on a 300 fs time scale of low-frequency motions of the helix, counterions, and water shell. Structural disorder of the DNA-water interface and DNA-water hydrogen bonds are, however, preserved for times beyond 10 ps. The different interactions of limited strength ensure ultrafast vibrational relaxation and dissipation of excess energy in the backbone structure, processes that are important for the structural integrity of hydrated DNA.

Ultrafast phosphate hydration dynamics in bulk H2O.

Phosphate vibrations serve as local probes of hydrogen bonding and structural fluctuations of hydration shells around ions. Interactions of H2PO4(-) ions and their aqueous environment are studied combining femtosecond 2D infrared spectroscopy, ab-initio calculations, and hybrid quantum-classical molecular dynamics (MD) simulations. Two-dimensional infrared spectra of the symmetric (νS(PO2(-))) and asymmetric (νAS(PO2(-))) PO2(-) stretching vibrations display nearly homogeneous lineshapes and pronounced anharmonic couplings between the two modes and with the δ(P-(OH)2) bending modes. The frequency-time correlation function derived from the 2D spectra consists of a predominant 50 fs decay and a weak constant component accounting for a residual inhomogeneous broadening. MD simulations show that the fluctuating electric field of the aqueous environment induces strong fluctuations of the νS(PO2(-)) and νAS(PO2(-)) transition frequencies with larger frequency excursions for νAS(PO2(-)). The calculated frequency-time correlation function is in good agreement with the experiment. The ν(PO2(-)) frequencies are mainly determined by polarization contributions induced by electrostatic phosphate-water interactions. H2PO4(-)/H2O cluster calculations reveal substantial frequency shifts and mode mixing with increasing hydration. Predicted phosphate-water hydrogen bond (HB) lifetimes have values on the order of 10 ps, substantially longer than water-water HB lifetimes. The ultrafast phosphate-water interactions observed here are in marked contrast to hydration dynamics of phospholipids where a quasi-static inhomogeneous broadening of phosphate vibrations suggests minor structural fluctuations of interfacial water.

Hydrogen bonding induced enhancement of Fermi resonances: ultrafast vibrational energy flow dynamics in aniline-d5.

With hydrogen bonding of the amino group of aniline-d5 we can identify the roles of Fermi enhanced combination and overtone states in intramolecular vibrational re-distribution (IVR) pathways for N-H stretching excitations. Using linear Fourier transform infrared (FT-IR) spectroscopy, ultrafast one- and two-color IR-pump-IR-probe spectroscopy, and femtosecond two-dimensional IR spectroscopy, we can identify the primary accepting modes for N-H stretching excitations. In particular, a key role is played by the δ(NH2) bending degree of freedom, either via its δ = 2 overtone state or via a combination state with the ν(C═C) ring stretching mode. No significant transient population in these Fermi enhanced combination/overtone states can be observed, a consequence of similar decay rates of these Fermi enhanced combination/overtone states and of the N-H stretching states. A similar magnitude of the transient response of the two fingerprint modes regardless of direct excitation of the Fermi enhanced combination/overtone levels or of the N-H stretching states suggests an underlying coupling mechanism facilitating common IVR pathways. This mechanism is expected to be of general importance for other organic compounds with hydrogen-bonded amino groups, including DNA bases.

Ultrafast vibrational dynamics of BH4(-) ions in liquid and crystalline environments.

Ultrafast vibrational dynamics of BH4(-) ions, the key units in boron hydride materials for hydrogen storage, are studied in diluted polar liquid solution and in NaBH4 crystallites by femtosecond infrared spectroscopy. Two-color pump-probe experiments reveal v = 1 lifetimes of 3 ps for the asymmetric BH4(-) stretching mode ν3 and of 3.6 ps for the asymmetric bending mode ν4 in the solvent isopropylamine. We provide direct evidence for the BH4(-) stretching relaxation pathway via the asymmetric bending mode ν4 by probing the latter after femtosecond excitation of ν3. Pump-probe traces measured in the crystalline phase show signatures of radiative coupling between the densely packed BH4(-) oscillators, most clearly manifested in an accelerated subpicosecond depopulation of the v = 1 state of the ν4 mode. The radiative decay is followed by incoherent vibrational relaxation similar to the liquid phase. The excess energy released in the relaxation processes of the BH4(-) intramolecular modes is transferred into the environment with thermal pump-probe signals being much more pronounced in the dense solid than in the diluted solution.

Structural Dynamics of Hydrated Phospholipid Surfaces Probed by Ultrafast 2D Spectroscopy of Phosphate Vibrations.

The properties of biomembranes depend in a decisive way on interactions of phospholipids with hydrating water molecules. To map structural dynamics of a phospholipid-water interface on the length and time scale of molecular motions, we introduce the phospholipid symmetric and asymmetric phosphate stretch vibrations as probes of interfacial hydrogen bonds and electrostatic interactions. The first two-dimensional infrared spectra of such modes and a line shape analysis by density matrix theory reveal two distinct structural dynamics components; the first 300 fs contribution is related to spatial fluctuations of charged phospholipid head groups with additional water contributions at high hydration levels; the second accounts for water-phosphate hydrogen bonds persisting longer than 10 ps. Our results reveal a relatively rigid hydration shell around phosphate groups, a behavior relevant for numerous biomolecular systems.

Femtosecond OH bending dynamics of water nanopools confined in reverse micelles.

The relaxation of OH bend excitations of H2O confined in reverse micelles is studied by femtosecond broadband pump-probe spectroscopy for the two ionic systems dioctyl sodium sulfosuccinate (AOT) and dioleoylphosphatidylcholine (DOPC) and for the nonionic tetraethylene glycol dodecyl ether (Brij-30). In the ionic AOT/DOPC reverse micelles, the OH bending lifetime T1 decreases from T1 > 615 fs for a 3:1 ratio of water and AOT/DOPC molecules (w0 = 3) to T1 = 345 fs for a 16:1 ratio (w0 = 16). In contrast, H2O in Brij-30 reverse micelles shows a much shorter T1 = 400 fs at w0 = 2 which decreases to T1 = 250 fs at w0 = 8. OH bend relaxation proceeds mainly via librational overtones of the bend-excited water molecules with a rate correlating with the energy mismatch between the v = 1 OH bend state and the librational overtone. In the ionic systems, the lower librational frequencies at small w0 result in a larger mismatch and longer T1 times. In the nonionic case, the w0-independent librational frequencies with a small energy mismatch lead to shorter T1 times. For w0 ≥ 8, the energy flow into the first hydration shell of the bend-excited molecules makes an additional contribution to the relaxation rate in all systems.

N-H stretching excitations in adenosine-thymidine base pairs in solution: pair geometries, infrared line shapes, and ultrafast vibrational dynamics.

We explore the N-H stretching vibrations of adenosine-thymidine base pairs in chloroform solution with linear and nonlinear infrared spectroscopy. Based on estimates from NMR measurements and ab initio calculations, we conclude that adenosine and thymidine form hydrogen bonded base pairs in Watson-Crick, reverse Watson-Crick, Hoogsteen, and reverse Hoogsteen configurations with similar probability. Steady-state concentration and temperature dependent linear FT-IR studies, including H/D exchange experiments, reveal that these hydrogen-bonded base pairs have complex N-H/N-D stretching spectra with a multitude of spectral components. Nonlinear 2D-IR spectroscopic results, together with IR-pump-IR-probe measurements, as also corroborated by ab initio calculations, reveal that the number of N-H stretching transitions is larger than the total number of N-H stretching modes. This is explained by couplings to other modes, such as an underdamped low-frequency hydrogen-bond mode, and a Fermi resonance with NH(2) bending overtone levels of the adenosine amino-group. Our results demonstrate that modeling based on local N-H stretching vibrations only is not sufficient and call for further refinement of the description of the N-H stretching manifolds of nucleic acid base pairs of adenosine and thymidine, incorporating a multitude of couplings with fingerprint and low-frequency modes.

N-H stretching modes of adenosine monomer in solution studied by ultrafast nonlinear infrared spectroscopy and ab initio calculations.

The N-H stretching vibrations of adenine, one of the building blocks of DNA, are studied by combining infrared absorption and nonlinear two-dimensional infrared spectroscopy with ab initio calculations. We determine diagonal and off-diagonal anharmonicities of N-H stretching vibrations in chemically modified adenosine monomer dissolved in chloroform. For the single-quantum excitation manifold, the normal mode picture with symmetric and asymmetric NH(2) stretching vibrations is fully appropriate. For the two-quantum excitation manifold, however, the interplay between intermode coupling and frequency shifts due to a large diagonal anharmonicity leads to a situation where strong mixing does not occur. We compare our findings with previously reported values obtained on overtone spectroscopy of coupled hydrogen stretching oscillators.

Ultrafast vibrational dynamics of water confined in phospholipid reverse micelles.

Ultrafast dynamics of OH stretching excitations of H2O confined in dioleoylphosphatidylcholine (DOPC) reverse micelles, a phospholipid model system, are studied in femtosecond pump-probe experiments. Measurements in a wide range of hydration show that spectral diffusion within the OH stretching band accelerates substantially with increasing water content. Concomitantly, the OH stretching lifetime decreases from approximately 500 fs at a 1:1 ratio of water and DOPC molecules (w0 = 1) to 300 fs for large water pools (ratio 16:1, w0 = 16). Two-color pump-probe studies mapping the ultrafast OH bending response after OH stretch excitation demonstrate that the relaxation pathway of the OH stretch vibration involves the OH bending mode. After OH stretch relaxation at high hydration levels, vibrational excess energy is randomized within the water pool and then transferred to the surrounding solvent.

Ultrafast Energy Redistribution in Local Hydration Shells of Phospholipids: A Two-Dimensional Infrared Study.

Structural and functional properties of phospholipids are strongly influenced by dynamics of their hydration shells. Here, we show that local water pools as small as three water molecules around the polar headgroups in phospholipid reverse micelles (dioleoylphosphatidylcholine, DOPC) serve as efficient sinks of excess energy released during vibrational relaxation. Transient two-dimensional (2D) infrared spectra of OH stretching excitations of H2O shells demonstrate a subpicosecond buildup of a hot water ground state, in which excess energy is randomized in low-frequency modes. An analysis of center line slopes of the 2D spectra reveals kinetics of energy dissipation that are significantly faster than structural fluctuations of the water pool and remain unchanged at intermediate hydration levels between three and eight water molecules per polar headgroup. Our results suggest that confined small water pools in biomolecular systems are sufficient to dissipate excess energy originating from the decay of electronic or vibrational excitations.

Ultrafast energy migration pathways in self-assembled phospholipids interacting with confined water.

Phospholipids self-assembled into reverse micelles in benzene are introduced as a new model system to study elementary processes relevant for energy transport in hydrated biological membranes. Femtosecond vibrational spectroscopy gives insight into the dynamics of the antisymmetric phosphate stretching vibration ν(AS)(PO(2))(-), a sensitive probe of local phosphate-water interactions and energy transport. The decay of the ν(AS)(PO(2))(-) mode with a 300-fs lifetime transfers excess energy to a subgroup of phospholipid low-frequency modes, followed by redistribution among phospholipid vibrations within a few picoseconds. The latter relaxation is accelerated by adding a confined water pool, an efficient heat sink in which the excess energy induces weakening or breaking of water-water and water-phospholipid hydrogen bonds. In parallel to vibrational relaxation, resonant energy transfer between ν(AS)(PO(2))(-) oscillators delocalizes the initial excitation.