Optical probes solvation dynamics

Time-resolved fluorescence spectroscopy of polar fluorescent probes that have a dipole moment that depends upon electronic state has recently been used extensively to study microscopic solvation dynamics of a broad range of solvents. Section II of this paper deals with the subject in detail. The basic concept is outlined in Figure 1, which shows the dependence of the nonequilibrium free energies (Fg and Fe) of solvated ground state and electronically excited probes, respecitvely, as a function of a generalized solvent coordinate. Optical excitation (vertical) of an equilibrated ground state probe produces a nonequilibrium configuration of the solvent about the excited state of the probe. Subsequent relaxation is accompanied by a time-dependent fluorescence spectral shift toward lower frequencies, which can be monitored and analyzed to quantify the dynamics of solvation via the empirical solvation dynamics function C(t), which is defined by Eq. (1). 

We recently developed a systematic method that uses the intrinsic tryptophan residue (Trp or W) as a local optical probe [49, 50]. Using site-directed mutagenesis, tryptophan can be mutated into different positions one at a time to scan protein surfaces. With femtosecond temporal and single-residue spatial resolution, the fluorescence Stokes shift of the local excited Trp can be followed in real time, and thus, the location, dynamics, and functional roles of protein-water interactions can be studied directly. With MD simulations, the solvation by water and protein (residues) is differentiated carefully to determine the hydration dynamics. Here, we focus our own work and review our recent systematic studies on hydration dynamics and protein-water fluctuations in a series of biological systems using the powerful intrinsic tryptophan as a local optical probe, and thus reveal the dynamic role of hydrating water molecules around proteins, which is a longstanding unresolved problem and a topic central to protein science. 

Because of their clear optical window, these RTlLs have extensively been used as a medimn for photophysical studies of various probe molecules and solvation dynamics in recent years. [30,39-41]... 

In Section 11.1.2 we argued that the solvation shell is not static. To probe the dynamics of the motion under the solvent-solute potential we need to displace the shell from equihbrium in a sudden manner. Ultrafast optical excitation of a solute from the ground state to an electronically excited state, particularly when the excited state has a very different dipole moment from the ground state, creates suitable initial conditions (see Figme 11.5). 

We conclude this article on a note of optimistic speculation. Clearly the above results on solvated electrons establish the potential of ultrashort laser pulses to probe the fundamental details of the dynamics of electron transfer reactions, which will be the cornerstone for the development of microscopic theories of electron dynamics in the condensed phase. Electrons are ubiquitous species, and the practical reflection of this appears in research areas such as photosynthesis, dielectric breakdown, fast optical... 

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