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B. Czarnik-Matusewicz and Y.M. Jung
are performed in two independent time dimensions to derive two-dimensional (2d)
spectra in the frequency domain. the extent of the vibrational spectrum over the two
frequency axes reports on how the excitation of a vibration with a given frequency
influences all the other vibrations within a detection window after a waiting period.
Such a spectrum gives insight into the couplings between the different excitations
of the system under study and the time evolution of these couplings. therefore, the
2d IR measurement must be performed within a picosecond or faster, i.e., on a time
scale that is fast compared to most protein dynamics. If the waiting time is varied,
the correlation between the excitation and detection frequencies is studied and is the
subject of correlation-2d IR spectroscopy. In contrast to the phrases “2d IR spectroscopy” and “correlation-2d IR spectroscopy,” which refer to nonlinear optical
2d spectroscopic techniques, the phrase “2d correlation spectroscopy (2dCoS)”
refers to a simple mathematical procedure used to generate 2d correlation maps
from spectra that are collected during the interaction of different electromagnetic
probes with an externally perturbed system composed of absorbing molecules.
8.2 Protein Research and 2DCoS
In 1985, Yang et al. [3] have shown that when processed by Fourier self-deconvolution, the resolution-enhanced infrared spectra of a protein substantially improved
quantitative estimates of the proportion of each secondary conformation in protein,
which was measured both as a powder and in an aqueous solution. these results
initiated a “golden period” for infrared spectroscopy in protein research; this period
has been punctuated by results showing that the application of resolution enhancement procedures such as Fourier self-deconvolution, curve fitting, and second derivative spectroscopy could provide varying degrees of success [4].
In parallel to studies of proteins by infrared spectroscopy, the two-dimensional
correlation method has been developed. one year after Yang’s paper [3], in 1986,
Noda presented the basic concept for perturbation-based two-dimensional spectroscopy applicable to infrared data [5]. In 1993, to analyze a set of dynamic spectra obtained under an external perturbation, Noda published a generalized 2d correlation
spectroscopy [6] that attracted the attention of researchers working in different subjects. As a model-free approach, 2dCoS has been an alternative to the model-based
Fourier self-deconvolution, curve fitting, and second derivative methods. In an unbiased manner, 2dCoS allows the detection of the location of highly overlapped bands
and the separation of these bands into components that could be assigned to individual absorbers, i.e., the α-helix, β-sheet, PPII, β-turn, unordered elements, and side
chains. Because of synchronous and asynchronous correlation among the intensities
of the sub-bands, these sub-bands could be very reliably assigned to different functional groups, structure elements, and components of the studied protein system. As
the data subjected to 2dCoS are composed from a series of spectra of the protein
system measured as a function of a perturbation, 2dCoS opens a new possibility for
monitoring the evolution of spectral changes that arise from forms that are active
B. Czarnik-Matusewicz and Y.M. Jung
are performed in two independent time dimensions to derive two-dimensional (2d)
spectra in the frequency domain. the extent of the vibrational spectrum over the two
frequency axes reports on how the excitation of a vibration with a given frequency
influences all the other vibrations within a detection window after a waiting period.
Such a spectrum gives insight into the couplings between the different excitations
of the system under study and the time evolution of these couplings. therefore, the
2d IR measurement must be performed within a picosecond or faster, i.e., on a time
scale that is fast compared to most protein dynamics. If the waiting time is varied,
the correlation between the excitation and detection frequencies is studied and is the
subject of correlation-2d IR spectroscopy. In contrast to the phrases “2d IR spectroscopy” and “correlation-2d IR spectroscopy,” which refer to nonlinear optical
2d spectroscopic techniques, the phrase “2d correlation spectroscopy (2dCoS)”
refers to a simple mathematical procedure used to generate 2d correlation maps
from spectra that are collected during the interaction of different electromagnetic
probes with an externally perturbed system composed of absorbing molecules.
8.2 Protein Research and 2DCoS
In 1985, Yang et al. [3] have shown that when processed by Fourier self-deconvolution, the resolution-enhanced infrared spectra of a protein substantially improved
quantitative estimates of the proportion of each secondary conformation in protein,
which was measured both as a powder and in an aqueous solution. these results
initiated a “golden period” for infrared spectroscopy in protein research; this period
has been punctuated by results showing that the application of resolution enhancement procedures such as Fourier self-deconvolution, curve fitting, and second derivative spectroscopy could provide varying degrees of success [4].
In parallel to studies of proteins by infrared spectroscopy, the two-dimensional
correlation method has been developed. one year after Yang’s paper [3], in 1986,
Noda presented the basic concept for perturbation-based two-dimensional spectroscopy applicable to infrared data [5]. In 1993, to analyze a set of dynamic spectra obtained under an external perturbation, Noda published a generalized 2d correlation
spectroscopy [6] that attracted the attention of researchers working in different subjects. As a model-free approach, 2dCoS has been an alternative to the model-based
Fourier self-deconvolution, curve fitting, and second derivative methods. In an unbiased manner, 2dCoS allows the detection of the location of highly overlapped bands
and the separation of these bands into components that could be assigned to individual absorbers, i.e., the α-helix, β-sheet, PPII, β-turn, unordered elements, and side
chains. Because of synchronous and asynchronous correlation among the intensities
of the sub-bands, these sub-bands could be very reliably assigned to different functional groups, structure elements, and components of the studied protein system. As
the data subjected to 2dCoS are composed from a series of spectra of the protein
system measured as a function of a perturbation, 2dCoS opens a new possibility for
monitoring the evolution of spectral changes that arise from forms that are active