dynamic shapes of the peaks and amplitudes of diagonal and cross peaks to obtain
structural information. It is important to note that such contributions do not
exclusively stem from the sample molecules alone. Rather, the dynamics also
originate from intermolecular interactions with its local environment, e.g. a solvent.
In other words, 2D IR spectroscopy not only measures the dynamics of some sample
molecules it also reports on the ultrafast dynamic fluctuations of the direct
environment. If the sample molecule is too rigid to structurally fluctuate by itself on
ultrafast time scales, then the 2D IR spectra can be interpreted as a snapshot of the
dynamic environment. In that way 2D IR spectroscopy directly accesses an
intermediate regime between purely homogeneous and purely heterogeneous
systems by allowing one to observe a ‘‘time-dependent’’ heterogeneity of a sample
down to the sub-picosecond timescale and thus gaining insight into line-broadening
mechanisms. This approach is in contrast to most NMR methods, which generally
probe time-averaged structure of molecules with an intrinsic temporal resolution of
milliseconds. It is noteworthy, however, that different approaches exist also for
NMR to go beyond that intrinsic temporal resolution, and to obtain information
from time scales about picoseconds [26–28]. These methods are rather specialized
and ultrafast laser spectroscopy constitutes the best and most direct way to access
that temporal range and the associated dynamics.
2D IR spectroscopy exhibits a couple of other advantages over 2D NMR
spectroscopy or X-ray crystallography regarding molecular structure determination.
NMR spectroscopy often requires sample concentrations in the mM regime to
obtain reasonable signals. However, many samples, and especially bio-molecules
such as proteins, tend to aggregate under these conditions, thus making structure
elucidation of the monomers challenging. Biomolecules are often also difficult to
crystallize, what makes X-ray structure determination difficult in some cases. In
addition, X-ray techniques do not resolve the molecular structure under fully
solvated conditions. 2D IR spectroscopy allows circumventing such problems by its
inherent ability to perform measurements at even very low concentrations (sub mM)
of bulk solution samples [29]. As another advantage, isotope-labelling is widely
known to provide rich structural information, especially in large molecules such as
proteins [30]. That approach has become popular in order to overcome a
comparatively low selectivity of IR spectroscopy (as compared to NMR) with
respect to a decisive functional group if many residues exhibit similar transition
frequencies, e.g. amide modes in proteins. Increasing the sensitivity even further
and adding the possibility of spatial resolution, 2D IR spectroscopy can be
combined with different forms of optical near field spectroscopy and micro- or even
nanoscopy. This is done by coupling the incident light to plasmonically active
nanostructures [31]. In a similar context, 2D IR spectroscopy can be applied to study
a vast range of different samples. Variants exist that measure 2D IR spectra from
small and large molecules in bulk solution, in solid-state samples such as
amorphous powders or crystals, in biological membranes or from molecular
monolayers at different types of interfaces.
Exploiting the properties of ultrashort pulses from different light sources, 2D IR
can also optimally be extended by different other frequency ranges from optical
spectroscopy. That includes the entire currently accessible range of laser light
Top Curr Chem (Z) (2017) 375:86
123
118
Reprinted from the journal
structural information. It is important to note that such contributions do not
exclusively stem from the sample molecules alone. Rather, the dynamics also
originate from intermolecular interactions with its local environment, e.g. a solvent.
In other words, 2D IR spectroscopy not only measures the dynamics of some sample
molecules it also reports on the ultrafast dynamic fluctuations of the direct
environment. If the sample molecule is too rigid to structurally fluctuate by itself on
ultrafast time scales, then the 2D IR spectra can be interpreted as a snapshot of the
dynamic environment. In that way 2D IR spectroscopy directly accesses an
intermediate regime between purely homogeneous and purely heterogeneous
systems by allowing one to observe a ‘‘time-dependent’’ heterogeneity of a sample
down to the sub-picosecond timescale and thus gaining insight into line-broadening
mechanisms. This approach is in contrast to most NMR methods, which generally
probe time-averaged structure of molecules with an intrinsic temporal resolution of
milliseconds. It is noteworthy, however, that different approaches exist also for
NMR to go beyond that intrinsic temporal resolution, and to obtain information
from time scales about picoseconds [26–28]. These methods are rather specialized
and ultrafast laser spectroscopy constitutes the best and most direct way to access
that temporal range and the associated dynamics.
2D IR spectroscopy exhibits a couple of other advantages over 2D NMR
spectroscopy or X-ray crystallography regarding molecular structure determination.
NMR spectroscopy often requires sample concentrations in the mM regime to
obtain reasonable signals. However, many samples, and especially bio-molecules
such as proteins, tend to aggregate under these conditions, thus making structure
elucidation of the monomers challenging. Biomolecules are often also difficult to
crystallize, what makes X-ray structure determination difficult in some cases. In
addition, X-ray techniques do not resolve the molecular structure under fully
solvated conditions. 2D IR spectroscopy allows circumventing such problems by its
inherent ability to perform measurements at even very low concentrations (sub mM)
of bulk solution samples [29]. As another advantage, isotope-labelling is widely
known to provide rich structural information, especially in large molecules such as
proteins [30]. That approach has become popular in order to overcome a
comparatively low selectivity of IR spectroscopy (as compared to NMR) with
respect to a decisive functional group if many residues exhibit similar transition
frequencies, e.g. amide modes in proteins. Increasing the sensitivity even further
and adding the possibility of spatial resolution, 2D IR spectroscopy can be
combined with different forms of optical near field spectroscopy and micro- or even
nanoscopy. This is done by coupling the incident light to plasmonically active
nanostructures [31]. In a similar context, 2D IR spectroscopy can be applied to study
a vast range of different samples. Variants exist that measure 2D IR spectra from
small and large molecules in bulk solution, in solid-state samples such as
amorphous powders or crystals, in biological membranes or from molecular
monolayers at different types of interfaces.
Exploiting the properties of ultrashort pulses from different light sources, 2D IR
can also optimally be extended by different other frequency ranges from optical
spectroscopy. That includes the entire currently accessible range of laser light
Top Curr Chem (Z) (2017) 375:86
123
118
Reprinted from the journal