Probing Solvation Dynamics around Aromatic and Biological Molecules at the Single-Molecular Level

Abstract

Solvation processes play a crucial role in chemical reactions and biomolecular recognition phenomena. Although solvation dynamics of interfacial or biological water has been studied extensively in aqueous solution, the results are generally averaged over several solvation layers and the motion of individual solvent molecules is difficult to capture. This review describes the development and application of a new experimental approach, namely, picosecond time-resolved pump-probe infrared spectroscopy of size- and isomer-selected aromatic clusters, in which for the first time the dynamics of a single individual solvent molecule can be followed in real time. The intermolecular isomerization reaction is triggered by resonant photoionization (pump), and infrared photodissociation (probe) at variable delay generates the spectroscopic signature of salient properties of the reaction, including rates, yields, pathways, branching ratios of competing reactions, existence of reaction intermediates, occurrence of back reactions, and time scales of energy relaxation processes. It is shown that this relevant information can reliably be decoded from the experimental spectra by sophisticated molecular dynamics simulations. This review covers a description of the experimental strategies and spectroscopic methods along with all applications to date, which range from aromatic clusters with nonpolar solvent molecules to aromatic monohydrated biomolecules.

Photoionization-induced π ↔ H site switching dynamics in phenol+–Rg (Rg = Ar, Kr) dimers probed by picosecond time-resolved infrared spectroscopy

“…In addition, the lowest frequency intramolecular vibration of PhOH + is 177 cm À1 , 46 i.e. This behavior is different for (acidic) aromatic dimers with polyatomic solvent molecules (S = H 2 O, CH 4 ), 1,2,45,[47][48][49][50][51] because then even in the dimers (n = 1) bath modes are available for IVR, which quench the back reaction. Thus, the excess energy is distributed predominantly among the intermolecular modes.…”

“…[1][2][3] The dynamics of this intermolecular p -H (hydrophobichydrophilic) site switching reaction triggered by ionization has been measured in real time by applying picosecond time-resolved vibrational (infrared) spectroscopy (ps-TRIR) to molecular clusters isolated in the gas phase. [1][2][3] The dynamics of this intermolecular p -H (hydrophobichydrophilic) site switching reaction triggered by ionization has been measured in real time by applying picosecond time-resolved vibrational (infrared) spectroscopy (ps-TRIR) to molecular clusters isolated in the gas phase.…”

“…1,2,[4][5][6][7] This p -H switching reaction enables us to investigate fundamental aspects of solvation dynamics induced by photoexcitation in a well-defined molecular system. [17][18][19] In this context, PhOH-S n clusters with nonpolar solvent atoms or molecules (S = Rg, CH 4 ) serve as a benchmark system not only for experimental works [1][2][3][4][5][6][7] but also for accurate quantum chemical calculations. [8][9][10][11][12][13][14][15][16] They significantly affect the energetics and dynamics of chemical reactions and structural rearrangements, for example protein folding and the selfassembly of supramolecules and nanostructures.…”

“…40,[42][43][44] When multiple Rg atoms solvate the p-cloud, the way of condensation of the Rg atoms in PhOH-Rg n (n Z 2) is determined by the relative strengths of the Rg-Rg and p-Rg interactions. 1,2,[4][5][6][7] In the larger PhOH + -Rg n clusters (n Z 2), the reaction proceeds in a one-way, single step p -H forward reaction with 100% yield. 7,40 Deposition of a positive charge on PhOH switches the dominant intermolecular interaction site from the p electron system to the OH group, due to the additional charge-induced polarization forces, which substantially enhance the strength of hydrogen bonding.…”

Infrared spectroscopy of O˙− and OH− in water clusters: evidence for fast interconversion between O˙− and OH˙OH−

“…2,3 To approach the structure of the primary hydration shell of ions, infrared multiple photon dissociation (IRMPD) spectroscopy has been successfully applied. [4][5][6][7][8][9][10][11] The hydroxide ion (OH À ) is an essential ionic species in aqueous chemistry and its microscopic hydration has been studied in numerous reactivity 12,13 and spectroscopy 7,[14][15][16][17][18][19][20] experiments, mainly elucidating the structure of clusters. Infrared (IR) spectra exhibit broad absorption features that were assigned to rapid fluctuation of isomeric structures.…”