Slice Spectra Research Articles

Determination of relative rate of spectral events by novel modification of two-dimensional correlation spectroscopy

Sign of two-dimensional (2D) correlation peaks provides information on sequence of spectral events. This information is related to molecular mechanism of changes in a given system. Recently, few papers addressing the problems with interpretation of the sign of 2D correlation peaks have been published. To overcome these problems, a modification of the generalized 2D correlation method has been proposed. This method compares variations in the dynamic spectrum with a linear change at a reference point. The rates of spectral responses at individual wavenumbers are proportional to magnitudes of the peaks in the slice of asynchronous spectrum at the reference point. This way, analysis of complex 2D contour plots is replaced by a simple examination of one-dimensional (1D) slice spectrum. In spite of reduced ability of the resolution enhancement, in special cases the proposed method provides information not accessible from the classical 2D correlation analysis. At first, the principles of this method are shown with the synthetic data. Next, the influence of spectral separation, band width and position changes on the slice spectrum is evaluated. Finally, the proposed approach is applied to the experimental spectra of two hydrogen-bonded systems.

Moving-window 2D correlation spectroscopy in studies of fluphenazine–DPPC dehydrated film as a function of temperature

The effect of incorporating fluphenazine (FPh) into the dipalmitoylphosphatidylcholine (DPPC) multibilayers was studied by means of two-dimensional correlation spectroscopy (2DCOS) applied to attenuated total reflection (ATR) infrared spectra. DPPC is used as a model membrane that mimics the organization of lipids in biological membranes and their interaction with FPh. ATR-IR spectra for both DPPC dry film alone and the film doped with FPh were recorded as a function of temperature to provide information about the interaction between FPh molecules and DPPC lipid. The chain-melting phase-transition temperature changes are strictly correlated with the conformational order of the lipid hydrocarbon chains. To gain deeper insight into the accompanying spectral changes, we employed moving-window 2D correlation spectroscopy. Subdividing all the measurements from 10 to 90 °C into 20° subsets enables a detailed identification of spectral features induced by embedding FPh into DPPC multilayers. Moving-window analysis of the power spectra for the ν asym,symCH 2, δ sCH 2, and δ rCH 2 vibrations provides evidence that FPh is embedded in the region between the bilayers, penetrating their hydrophilic part, which destabilizes the interchain interaction. Above 60 °C the FPh–DPPC system reaches the liquid crystalline phase with the well-established location of FPh. A further temperature increase to 90 °C has little effect on the intrachain conformational order and the packing character of the FPh–DPPC system in the liquid crystalline phase. In addition, FPh hinders the formation of large domains. Comparison of the moving-window analysis done by using slice spectra for DPPC and FPh-doped DPPC dry film for ν asym,symCH 2, νC O, and ν sym PO 2 - shows that the interaction between the DPPC and FPh molecules is accompanied by very distinct spectral changes located in a both lower and narrower temperature range than those observed in pure DPPC film.

Use of Dendrograms of Slice Spectra as a New Graphical Tool for the Interpretation of Two-Dimensional Correlation Spectra

In this paper a procedure is established to identify spectral bands that show similar behavior in two-dimensional correlation spectra. Slice spectra of bands that are affected in a single spectral event are highly similar. It is shown that this can be used to simplify the interpretation of two-dimensional (2D) correlation diagrams. One functional group frequently gives rise to multiple spectral bands in a Raman spectrum. In such occasions, the sequential order rules assign a sequential order, which is invalid, to the changes in these bands. We therefore apply cluster analysis to the slice spectra of bands with intensity changes during the experiment. Results indicate that cluster analysis of selected slice spectra can help in the interpretation of 2D correlation spectra in an intuitive and graphical way. The procedure should be used together with the widely used sequential order rules to determine the order of spectral events. The procedure enabled us to reveal the reaction steps in two simulated datasets and a dataset that comprises Raman spectra of oxidating latex of Lactarius fluens (Basidiomycota, Fungi).

Equilibrium fluctuations of the Lennard-Jones cluster surface

Spectra of the cluster surface equilibrium fluctuations are treated by decomposition into the bulk and net capillary ones. The bulk fluctuations without capillary ones are simulated by the surface of a cluster truncated by a sphere. The bulk fluctuation spectrum is shown to be generated primarily by the discontinuity in the spatial distribution of cluster internal particles. The net capillary fluctuation slice spectrum is obtained in molecular dynamics simulation by subtraction of the bulk fluctuation spectrum from the total one. This net spectrum is in the best agreement with a theoretical estimation if we assume the intrinsic surface tension to be independent of the wave number. The wave number cutoff is brought in balance with the intrinsic surface tension and excess surface area induced by the capillary fluctuations. It is shown that the ratio of the ordinary surface tension to the intrinsic one can be considered as a universal constant independent of the temperature and cluster size.

Nano-TiO2 induced secondary structural transition of silk fibroin studied by two-dimensional Fourier-transform infrared correlation spectroscopy and Raman spectroscopy

By sol–gel processing, regenerated nano-TiO2/SF (silk fibroin) composite films were synthesized. The experimental results revealed that the nano-TiO2 particles were well dispersed in the regenerated silk fibroin. Using FT-IR and Raman spectroscopy, the secondary structures of these composite films with concentrations of 0, 0.2, 0.4, 0.8 and 1.6 wt% were characterized. Concentration-perturbed two-dimensional (2D) correlation spectra were calculated for the spectra in the 1800–1600 cm−1 region. To investigate nano-TiO2 particles induced changes in the secondary structure and hydration, the slice spectra were calculated from the synchronous and asynchronous spectra, respectively. The transmittance IR and Raman spectra measurement indicated that the secondary structure of the pure silk film was mostly random coil and α-helix, while the composite films were β-sheet. With increasing nano-TiO2 content, the secondary structure of composite films was changed from typical Silk I to typical Silk II. However, it was found that the transition of the SF's secondary structures would be restrained by excessive nano-TiO2 (over 0.8%) introduced into the composite SF films. Through the FT-IR absorbance and 2D correlation spectra, it was demonstrated that the formation of nano-TiO2 particles could induce the partial transformation of SF conformation from Silk I to Silk II.

Phase coupling analysis of gastric pressure activity via wavelet packet based diagonal slice spectra

We propose a new analysis method to detect quadratic phase coupling (QPC) behavior of human gastric interdigestive pressure activity that has been acquired by a telemetric capsule-like mini-robot. The method is referred to as diagonal slice spectra. They are the Fourier transforms of the diagonal slices of the triple correlations, and can actually detect the phase coupling and coupled components respectively by expanding the real process into the complex counterpart through Hilbert transform. In order to learn more about the QPC structure in a certain frequency band that we are mostly interested in and obtain a higher frequency resolution, the method, named the wavelet packet based diagonal slice spectrum, is introduced. It shows that the nonlinear QPC behavior occurs during gastric contractions (phase II), whereas no distinct phase coupling occurs during gastric motor quiescence (phase I). It is the nonlinear cell-to-cell coupling mechanisms, existence of fast and slow waves and their interactions that nonlinear QPC structure of the gastric pressure activity occurs.

Applications of moving window two-dimensional correlation spectroscopy to analysis of phase transitions and spectra classification.

Our recently proposed idea of moving window two-dimensional (2D) correlation spectroscopy, which partitions a data set into series of relatively small submatrices (windows) and calculates their covariance maps in succession, is tested for three convoluted data set. Phase-transition temperatures of oleic acid and poly-(N-isopropylacrylamide) in an aqueous solution are sought by analyzing covariances of their temperature-dependent near-infrared and infrared spectra, respectively, while Raman spectra of three kinds of polyethylene (PE) pellets are investigated to find the spectral differences among them and to classify randomly ordered spectra by a sample-sample (SS) covariance map. The criterion of mean of standard deviation of covariance matrices is used as an indicator of the crucial information present in these matrices so that only a few of them are discussed in details. The results are obtained quickly after very simple calculations and are studied at length. The baseline variation is not removed prior to the calculations but is found to be of use for the determination of the phase-transition temperatures. Randomly ordered Raman spectra of the PE pellets are classified by innovatively used and interpreted SS slice spectra, with the relation to principal component analysis discussed.

Potentials of variable–variable and sample–sample, generalized and statistical, two-dimensional correlation spectroscopies in investigations of chemical reactions

Generalized and statistical variable–variable (VV) and sample–sample (SS) two-dimensional (2D) correlation spectroscopies are used to investigate a three-band spectral model that represents a simple consecutive chemical reaction. Generalized and comprehensive linear algebra basis, which is essential for the analysis of three-component systems by 2D correlation, is provided. It is found from the analysis of slice spectra of synchronous and asynchronous VV correlation maps of the model that the spectral and concentration influences of an intermediate can be identified and related to the real characteristics of the intermediate. 2D correlation spectroscopy may be effectively employed to give a semiquantitative portrayal of a chemical reaction. The results of statistical 2D correlation spectroscopy are not directly related to the bands or concentration profiles, but are concerned with simple trends in intensity changes in the spectra. It seems that for all consecutive reactions where the mass of the system is entirely preserved, SS correlation maps may be the same, independent of spectroscopic techniques for monitoring reactions. If this is the case, the results of the present study offer a prospect for automatic determination of reaction constants in successive reactions by the means of 2D correlation spectroscopy.

Comparison of Principal Component Analysis and Generalized Two-Dimensional Correlation Spectroscopy: Spectral Analysis of Synthetic Model System and Near-Infrared Spectra of Milk

Principal component analysis (PCA) and generalized two-dimensional (2D) correlation spectroscopy have been applied to a two-component spectral model made up of highly overlapped bands and to near-infrared (NIR) spectra of milk. The analyses were made on mean-normalized and mean-centered data of the model. The dynamic loading of PCA is expanded into 2D contour by calculating a variance-covariance matrix. This 2D pattern is compared with a synchronous 2D spectrum calculated by generalized 2D correlation spectroscopy, and a high degree of similarity is discernible between the two kinds of 2D spectra. The one-dimensional (1D) features of the synchronous 2D map, slice spectra, are identical to the dynamic loading as well. We show that, for noncentered data of the model system, only in limited cases can the correlation between the first loadings and synchronous map be established. PCA of the milk spectra yields two factors. The 2D representation of the most important loading is significantly different from the synchronous spectrum calculated by the generalized 2D approach. The slice spectra from which the synchronous spectrum is made are found to be mutually different as one moves along the variable axis. A synchronous 2D map emphasizes only variables with the highest correlation of the spectral intensity changes. On the other hand, PCA is geometrically oriented; thus, as the complexity of the data increases, the differences between PCA and generalized 2D correlation spectroscopy become striking. The selectivity of the asynchronous 2D map is demonstrated for the bands due to milk fat.

Comparison between Conventional Spectral Analysis Methods, Chemometrics, and Two-Dimensional Correlation Spectroscopy in the Analysis of Near-Infrared Spectra of Protein

The present study has aimed at comparing three methods for analyzing near-infrared (NIR) spectra: conventional spectral analysis methods, chemometrics, and generalized two-dimensional (2D) correlation spectroscopy. In a comparison of these approaches, NIR spectra were measured for aqueous solutions of human serum albumin (HSA) with the concentration range of 0.5–5.0 wt %. Synchronous and asynchronous 2D correlation spectra were generated from the concentration-dependent NIR spectral variations of HSA in distilled water. The first and second loadings plots were calculated for principal component analysis (PCA) models based upon the above NIR spectral data. It was found that slice spectra calculated from the synchronous spectra are very close to the first loadings plots and that slice spectra from the asynchronous spectra have a close resemblance to the second loadings plots. The reasons why the synchronous and asynchronous spectra bear close resemblance to the first and second loadings plots for the PCA model, respectively, are discussed in the paper.

Two-Dimensional Near-Infrared Spectroscopy Study of Human Serum Albumin in Aqueous Solutions: Using Overtones and Combination Modes to Monitor Temperature-Dependent Changes in the Secondary Structure

FT-NIR spectra were measured for human serum albumin (HSA) in aqueous solutions with concentrations of 1.0, 2.0, 3.0, 4.0, and 5.0 wt % over a temperature range of 45−80 °C. Concentration-perturbed two-dimensional (2D) correlation spectra were calculated for the spectra in the 7500−5500 cm-1 and 4900−4200 cm-1 regions at different temperatures. To investigate temperature-induced changes in the secondary structure and hydration, power spectra and slice spectra were calculated from the synchronous and asynchronous spectra, respectively. In the power spectra, a band near 4600 cm-1 due to the combination mode of amide B and amide II (amide B/II) shows an abrupt shift by 5 cm-1 between 58 and 60 °C, indicating that the secondary structure of HSA changes suddenly near 60 °C. Both the power and slice spectra in the 7500−5500 cm-1 region provide explicit evidence that the hydration changes markedly near 60 °C. A comparison of the temperature-dependent frequency shifts between the band near 4600 cm-1 due to amide ...

Truncated sinc slice excitation for 31P spectroscopic imaging

The shortest possible delay ( Td) between slice selection and data acquisition is important for producing high quality 31P spectra. In single slice multivoxel spectroscopic imaging, conventional excitation using sinc-shaped rf pulses within typical gradient limitations can have values of Td that lead to significant spectral distortion and loss of signal. Truncated sinc excitation, which ends the excitation close to the center of the main rf lobe has been suggested for MR angiographic applications to produce short values of Td. In this work, the slice profiles, spectral signal-to-noise ratio (SNR) and spectral distortions are compared using the minimum delay achievable on a commercial MRI system for conventional ‘sinc’ rf excitation and truncated sinc excitation. Slice profiles are calculated using the Bloch equations and measured with a phantom. SNR and spectral distortions are evaluated from whole slice spectra on a human volunteer. On an MRI system with 1 G/cm gradients (0.5 msec risetime), for a 2.5-cm slice at 31P frequencies, conventional excitation can be adjusted to achieve Td = 2.5 msec while truncated sine excitation yields Td = 1.5 msec. The truncated sinc excitation's shorter value of Td leads to much smaller spectral distortions, but its slice profile has “dispersive tails” which increase as more of the rf is truncated. Slice profile corrected SNR for the β-ATP peak of 31P on a human volunteer is equivalent for both sequences while, qualitatively, in the PDE region the truncated sinc approach has improved SNR.

SLIM: spectral localization by imaging.

Nonspectroscopic magnetic resonance (MR) imaging often shows that a slice is composed of several compartments, each of which can be assumed to have a spatially homogeneous magnetic resonance spectrum, e.g., a limb composed of fat, muscle, bone marrow, and tumor. We show how to use structural information from such a nonspectroscopic image in order to increase the efficiency of subsequent localized spectroscopic measurements. Specifically, knowledge of the boundaries of N compartments makes it possible to reconstruct compartmental spectra from spectroscopic signals from an entire cross section with N or more different degrees of phase encoding. Experimental studies of a two-compartment phantom show that this method (SLIM) can be used to derive regional hydrogen spectra of a single slice from signals with as few as 2 phase-encoding steps, although Fourier transform chemical-shift imaging requires 64 steps to achieve a result of comparable accuracy. SLIM required only 16 phase-encoding steps to obtain accurate regional single slice spectra in a human limb with three compartments. Spectra of similar quality, obtained by Fourier transform chemical-shift imaging, required 256 to 1024 steps.