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Vibrational Probes: From Small Molecule Solvatochromism Theory and Experiments to Applications in Complex Systems
journal contribution
posted on 2017-03-27, 16:03 authored by Bartosz Błasiak, Casey H. Londergan, Lauren J. Webb, Minhaeng ChoConspectusThe vibrational frequency of a chosen normal
mode is one of the
most accurately measurable spectroscopic properties of molecules in
condensed phases. Accordingly, infrared absorption and Raman scattering
spectroscopy have provided valuable information on both distributions
and ensemble-average values of molecular vibrational frequencies,
and these frequencies are now routinely used to investigate structure,
conformation, and even absolute configuration of chemical and biological
molecules of interest. Recent advancements in coherent time-domain
nonlinear vibrational spectroscopy have allowed the study of heterogeneous
distributions of local structures and thermally driven ultrafast fluctuations
of vibrational frequencies. To fully utilize IR probe functional groups
for quantitative bioassays, a variety of biological and chemical techniques
have been developed to site-specifically introduce vibrational probe
groups into proteins and nucleic acids. These IR-probe-labeled biomolecules
and chemically reactive systems are subject to linear and nonlinear
vibrational spectroscopic investigations and provide information on
the local electric field, conformational changes, site–site
protein contacts, and/or function-defining features of biomolecules.A rapidly expanding library of data from such experiments requires
an interpretive method with atom-level chemical accuracy. However,
despite prolonged efforts to develop an all-encompassing theory for
describing vibrational solvatochromism and electrochromism as well
as dynamic fluctuations of instantaneous vibrational frequencies,
purely empirical and highly approximate theoretical models have often
been used to interpret experimental results. They are, in many cases,
based on the simple assumption that the vibrational frequency of an
IR reporter is solely dictated by electric potential or field distribution
around the vibrational chromophore. Such simplified description of
vibrational solvatochromism generally referred to as vibrational Stark
effect theory has been considered to be quite appealing and, even in
some cases, e.g., carbonyl stretch modes in amide, ester, ketone,
and carbonate compounds or proteins, it works quantitatively well,
which makes it highly useful in determining the strength of local
electric field around the IR chromophore. However, noting that the
vibrational frequency shift results from changes of solute–solvent
intermolecular interaction potential along its normal coordinate,
Pauli exclusion repulsion, polarization, charge transfer, and dispersion
interactions, in addition to the electrostatic interaction between
distributed charges of both vibrational chromophore and solvent molecules,
are to be properly included in the theoretical description of vibrational
solvatochromism. Since the electrostatic and nonelectrostatic intermolecular
interaction components have distinctively different distance and orientation
dependences, they affect the solvatochromic vibrational properties
in a completely different manner.Over the past few years, we
have developed a systematic approach
to simulating vibrational solvatochromic data based on the effective
fragment potential approach, one of the most accurate and rigorous
theories on intermolecular interactions. We have further elucidated
the interplay of local electric field with the general vibrational
solvatochromism of small IR probes in either solvents or complicated
biological systems, with emphasis on contributions from non-Coulombic
intermolecular interactions to vibrational frequency shifts and fluctuations.
With its rigorous foundation and close relation to quantitative interpretation
of experimental data, this and related theoretical approaches and
experiments will be of use in studying and quantifying the structure
and dynamics of biomolecules with unprecedented time and spatial resolution
when combined with time-resolved vibrational spectroscopy and chemically
sensitive vibrational imaging techniques.
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Keywords
vibrational imaging techniquestime-domain nonlinear vibrational spectroscopyPauli exclusion repulsionComplex Systems ConspectusThe vibrational frequencyvibrational Stark effect theorytime-resolved vibrational spectroscopysimulating vibrational solvatochromic datavibrational frequency shift resultsSmall Molecule Solvatochromism Theoryvibrational solvatochromismsolvatochromic vibrational propertiesatom-level chemical accuracyvibrational frequenciesvibrational chromophorevibrational probe groupsvibrational frequency shiftsnonlinear vibrational spectroscopic investigationsIR