Aufklärung der Solvatationsdynamik eines einzelnen Wassermoleküls durch Infrarotspektroskopie: Theorie und Experiment

Abstract

Die Dynamik und Energetik von Wasser an Grenzflächen oder in biologischen Systemen spielt eine grundlegende Rolle bei allen Solvatations-und biologischen Phänomenen in wässrigen Lçsungen. Insbesondere ist die Migration von Wassermolekülen der erste Schritt, der den Gesamtprozess in zeitlicher Hinsicht steuert. Es ist jedoch fast unmçglich, im Experiment die Dynamik individueller Wassermoleküle in der Lçsung zu verfolgen, da sich die Signale der Moleküle in heterogenen Umgebungen überlagern. Auch wenn für Simulationen der Molekulardynamik diese Einschränkung nicht gilt, fehlt es an experimentellen Daten, um die berechnete Dynamik zu validieren. Im Folgenden zeigen wir eine neue Strategie auf, mithilfe derer die berechnete Dynamik durch die Messung zeitaufgelçster Infrarotspektren verifiziert wird. Die Koexistenz von schneller und langsamer Wassermolekül-Migration um eine CONH-Peptid-Bindung wird anhand eines Modellsystems gezeigt, das einen Teil eines hydratisierten Peptids repräsentiert.

Real-time observation of photoionization-induced water migration dynamics in 4-methylformanilide–water by picosecond time-resolved infrared spectroscopy and ab initio molecular dynamics simulations

“…The latter are simulated according to the procedure that has been presented previously (for details see Ref. 21,24,36,37 ). Briefly, the ps-TRIR spectrum is expressed as a trajectory average with the envelope of a FWHM of 3 ps were employed.…”

“…19 To circumvent the problem of inhomogeneous water molecules, we have been working on hydration dynamics at a single solvent molecular level by probing solvated clusters in the gas phase using time-resolved infrared (TRIR) spectroscopy and advanced ab initio molecular dynamics (MD) simulations. [20][21][22][23][24][25] This approach offers size-and isomer-selectivity as well as full control over solvation structure and energy in the cluster.…”

“…The dynamics is typically initiated by UV laser excitation / photoionization of the clusters, and migration of the water molecule is monitored by changes in the TRIR spectra at the picosecond (ps) time scale. This technique of UV-UV ( ′ ) -IR ps time-resolved IR (ps-TRIR) spectroscopy combined with mass spectrometry has been successfully applied to a variety of monohydrated clusters, including the single water CO → NH migration around the peptide linkage in trans-acetanilide (tAA) 20,21 and trans-formanilide (tFA), 24 the CN → NH migration around 4-aminobenzonitrile, 22 and the NH → OH migration in 5-hydroxyindole. 25 Surprisingly, the lifetimes of these single water migrations vary over four orders of magnitude, ranging from 4 ps to 53 ns.…”

“…For example, the ab initio "on-the-fly" MD simulations reproduce the time-evolutions (TEs) obtained by the ps-TRIR spectra on monohydrated clusters of tAA and tFA (tAA-W and tFA-W), and visualize the migration trajectories of water as shown in Figure 1. 21,24 The water migration in tFA + -W(CO) (hereafter, W(CO) and W(NH) mean the water molecule bound to the CO and NH sites, respectively) triggered by photoionization of the aromatic chromophore proceeds in the molecular plane. The water molecule released from the initial CO binding site migrates toward the final NH site, but overshoots and moves like a largeamplitude damped pendulum around the NH destination site.…”

“…24 The migration dynamics in the related tAA + -W(CO) cluster has been also simulated, and the twice faster migration than in tFA + -W is well reproduced (Figure 1). 21 tAA is a methyl-substituted derivative of tFA and thus the faster migration can be rationalized by the effect of the CH 3 group. The CH 3 group provides low-energy vibrational levels due to its (hindered) internal rotation, which accelerates the IVR.…”