广义二维相关光谱学进展

介绍了一种重要的广义二维相关光谱方法,对其基本原理、获取谱图的方法、数学计算过程、谱图的性质及其解释规则等作了详细的介绍,并介绍了其在化学、物理、生物、医药等领域中的应用。     

Two-dimensional infrared spectroscopy study on phase transition and structural variations of a hydrogen-bonded liquid crystal

Infrared (IR) spectra have been measured for a liquid crystal (LC) consisting of one trans-butene diacid (BD) molecule as a proton donor and two 4-(2,3,4-tridecyloxybenzoyloxy)-4'-stilbazoles (DBS) molecules as a proton acceptor (DBS:BD:DBS) linked together with each other by inter-molecular hydrogen bonds over a temperature range from 20 to 120 degrees C to explore its phase transition and heat-induced structural variations. The temperature-dependent IR spectra have shown that the inter-molecular hydrogen bonds are stable in the liquid crystalline phase but become slightly decoupled with temperature increasing. Two kinds of two-dimensional (2D) correlation spectroscopy, variable-variable (VV) and sample-sample (SS) 2D spectroscopy, have been employed to analyze the observed temperature-dependent spectral variations more efficiently. The SS 2D correlation analysis in the spectral range of 2700-1800 cm(-1) has demonstrated that a change in hydrogen bonds in the LC starts from 40 degrees C, which is not clarified by differential scanning calorimetry (DSC) and conventional IR and Raman spectroscopic analyses. On the other hand, the phase transition of LC revealed by SS 2D spectroscopy in the specific spectral regions of 1750-1650 and 3000-2700 cm(-1) is in a good agreement with that revealed by DSC for the heating process. The VV 2D correlation spectroscopy analysis has provided information about the structural variations of inter-molecular hydrogen bonds. The different species of hydrogen-bonded and free -COOH and -COO- groups in the LC have been clarified by the VV 2D correlation analysis. It has also elucidated the specific order of the temperature-induced structural changes in the intra- and inter-molecular hydrogen bonds concerning with the -COOH and/or -COO- groups in the LC.     

二维相关分析中参考谱的选择对分离重叠峰的影响

二维相关光谱是一项将光谱强度看作两个独立的光谱变量的函数的技术, 它是由动态光谱经过数学转化后得到的. 在扰动过程中, 动态光谱等于实际测得的光谱减去参考谱, 参考谱的选择是任意的, 甚至可以为0, 但是在实际应用时, 人们逐渐发现参考谱的选择会对二维相关光谱产生一定的影响. 本篇文章采用模拟的方法, 建立光谱模型, 光谱强度按e指数形式单调变化, 比较以平均谱为参考谱和不设参考谱得到的二维相关光谱图, 分析它们的区别, 在不同参考谱条件下, 利用二维相关光谱分离重叠峰, 得到的结果也不相同, 将两种条件综合利用可以得到更多更正确的信息.     

二维相关分析光谱技术

给出推导二维相关光谱和推广至普遍意义上的数学过程,其物理含义和二维相关光谱的性质及其解释规则。以应用实例给出了该光谱技术在分析结构、相互作用等相关性方面的独特性质。还给出紫外可见二维相关光谱。     

Application of two‐dimensional near‐infrared (2D‐NIR) correlation spectroscopy to the discrimination of three species of Dendrobium

The objective of this paper was to apply two‐dimensional (2D) near‐infrared (NIR) correlation spectroscopy to the discrimination of three species of Dendrobium. Generalized 2D‐NIR correlation spectroscopy was able to enhance spectral resolution, simplify the spectrum with overlapped bands and provide information about temperature‐induced spectral intensity variations that was hard to obtain from one‐dimensional NIR spectroscopy. The FT‐NIR spectra were measured over a temperature range of 30–140°C. The 2D synchronous and asynchronous spectra showed remarkable differences within the range of 5600–4750 cm⁻¹ between different species of Dendrobium. Copyright © 2009 John Wiley & Sons, Ltd.     

Dynamic two-dimensional fluorescence correlation spectroscopy. Generalized correlation and experimental factors

Dynamic two-dimensional fluorescence correlation spectroscopy (2D FCS) is presented in the general form. Dynamic 2D FCS evaluates the time correlation function between two wavelength axes when an external perturbation is applied to the sample. It displays the vibronic features with similar time response functions in the synchronous correlation spectrum and the features with different time responses in the asynchronous correlation spectrum. The correlation analysis allows detailed assignments of the vibronic spectra of multicomponent samples. The emission-emission 2D FCS has proven to be able to resolve spectra with substantial overlaps, of species in equilibrium with each other, and of reacting species whose kinetic constants are linked and multiexponential. Similarly, the correlation analysis between excitation wavelengths allows the assignment of the excitation bands to fluorescent components. When a sinusoidal light source is used to excite the sample, the excitation-emission correlation requires the collection of only four spectra, two in-phase and two quadrature. The two-dimensional excitation-emission correlation analysis uncovers the association between the excitation and the emission vibronic features, enabling the complete assignment of the component spectra. The band associations and spectral assignments are facilitated by the two-dimensional phase map that is constructed from the synchronous and asynchronous correlation spectra. Spectral resolution can be optimized by varying the frequency of excitation and is not influenced by the detector phase angle used to collect the spectra. The resolution power of the 2D FCS is demonstrated with the retrieval of the anthracene emission spectrum from a pyrene-anthracene mixture when it contributes only 4% to the total fluorescence intensity.     

Two-dimensional correlation analysis useful for spectroscopy, chromatography, and other analytical measurements.

Application of generalized two-dimensional (2D) correlation in various analytical fields is explored. 2D correlation is a powerful and versatile technique applicable to spectroscopy, chromatography, and other measurements. Construction of 2D spectra is relatively straightforward, requiring only a series of systematically varying analytical signals, like spectra or chromatograms, induced by an external perturbation applied to the system of interest. Perturbation can take many different forms, like change in temperature, pressure or concentration, chemical reactions, electrical or mechanical stimuli, and so on. A set of analytical signals collected under a perturbation are then converted to 2D correlation spectra, which provide rich and useful information about the presence of coordinated or independent changes among signals, as well as relative directions and sequential order of signal intensity variations. The signal resolution is also enhanced by spreading overlapped bands along the second dimension. Illustrative examples of 2D correlation are given for spectroscopic and chromatographic applications.     

Chemical‐Shift‐Resolved 19F NMR Spectroscopy between 13.5 and 135 MHz: Overhauser–DNP‐Enhanced Diagonal Suppressed Correlation Spectroscopy

Overhauser–DNP‐enhanced homonuclear 2D ¹⁹F correlation spectroscopy with diagonal suppression is presented for small molecules in the solution state at moderate fields. Multi‐frequency, multi‐radical studies demonstrate that these relatively low‐field experiments may be operated with sensitivity rivalling that of standard 200–1000 MHz NMR spectroscopy. Structural information is accessible without a sensitivity penalty, and diagonal suppressed 2D NMR correlations emerge despite the general lack of multiplet resolution in the 1D ODNP spectra. This powerful general approach avoids the rather stiff excitation, detection, and other special requirements of high‐field ¹⁹F NMR spectroscopy.     

Rheo-optical near-infrared (NIR) spectroscopy study of low-density polyethylene (LDPE) in conjunction with projection two-dimensional (2D) correlation analysis

A rheo-optical characterization technique based on near-infrared (NIR) spectroscopy is developed specifically to probe the submolecular-level deformation caused during a mechanical test. An illustrative example of the mechanical deformation of low-density polyethylene (LDPE) is provided to show how it can be utilized. A set of NIR spectra of the polymer sample were collected by using an acousto-optic tunable filter (AOTF) NIR spectrometer coupled with a tensile testing machine as an excitation device. While the substantial level of variation of spectral intensity was readily captured during the mechanical deformation of the LDPE, main feature of the NIR spectra was overwhelmed by the contribution from the baseline change. Projection 2D correlation analysis was then applied to selectively extract the signal contribution from the baseline fluctuation. The 2D correlation spectra revealed the predominant extension of amorphous tie chains followed by the rotation of crystalline lamellae, which induce elastic and plastic deformation of the LDPE, respectively.     

二维相关振动光谱技术

从发展历史、计算方程、性质规则等方面系统地介绍了近年来发展起来的二维相关光谱技术.结合各种常见的一维振动光谱, 如红外、拉曼、荧光、近红外-红外等光谱举例阐述了二维振动光谱的优势及其普适性.介绍了在广义二维相关光谱理论上最新延伸发展起来的二维样品-样品相关技术和二维杂化相关技术的基本理论, 并将之与传统的二维变量-变量相关技术(广义二维相关光谱)进行了比较.     

Understanding the Structural Differences between Spherical and Rod‐Shaped Human Insulin Nanoparticles Produced by Supercritical Fluids Precipitation

In this study, the thermal denaturation mechanism and secondary structures of two types of human insulin nanoparticles produced by a process of solution‐enhanced dispersion by supercritical fluids using dimethyl sulfoxide (DMSO) and ethanol (EtOH) solutions of insulin are investigated using spectroscopic approaches and molecular dynamics calculations. First, the temperature‐dependent IR spectra of spherical and rod‐shaped insulin nanoparticles prepared from DMSO and EtOH solution, respectively, are analyzed using principal component analysis (PCA) and 2D correlation spectroscopy to obtain a deeper understanding of the molecular structures and thermal behavior of the two insulin particle shapes. All‐atom molecular dynamics (AAMD) calculations are performed to investigate the influence of the solvent molecules on the production of the insulin nanoparticles and to elucidate the geometric differences between the two types of nanoparticles. The results of the PCA, the 2D correlation spectroscopic analysis, and the AAMD calculations clearly reveal that the thermal denaturation mechanisms and the degrees of hydrogen bonding in the spherical and rod‐shaped insulin nanoparticles are different. The polarity of the solvent might not alter the structure or function of the insulin produced, but the solvent polarity does influence the synthesis of different shapes of insulin nanoparticles.     

Application of two-dimensional near-infrared correlation spectroscopy to the discrimination of Chinese herbal medicine of different geographic regions

Fructus Lycii is a traditional Chinese medicinal herb. The objective of this paper was to apply two-dimensional (2D) near-infrared (NIR) correlation spectroscopy to the discrimination of Fructus Lycii of four different geographic regions. Generalized 2D-NIR correlation spectroscopy was able to enhance spectral resolution, simplify the spectrum with overlapped bands, and provide information about temperature-induced spectral intensity variations that was hard to obtain from one-dimensional NIR spectroscopy. The 2D synchronous and asynchronous spectra showed remarkable differences within the range of 4950-5700cm(-1) between samples of different geographic regions. Using NIR instead of IR made the 2D approach more convenient and fast, and it can be applied to more area like process control. This approach can also be applied analogously to the discrimination of other Chinese herbal medicine of different geographic regions.     

Two-dimensional infrared correlation spectroscopic studies of polymer blends--specific interactions in polyethylene adipate/cholesteryl palmitate blends

Generalized two-dimensional infrared (2D IR) correlation spectroscopy has been applied to the study of the conformational changes and specific interactions in blends of polyethylene adipate (PEA) and cholesteryl palmitate (CP). IR spectra of CP, PEA, and their blends of different compositions: 10/90, 16/84, 32/68, 64/36, and 80/20 have been recorded. In order to apply 2D IR correlation analysis, the samples are divided into two sets: set A with high PEA content 0/100, 10/90, 16/84, 32/68 CP/PEA and set B with high CP content 64/36, 80/20 and 100/0 CP/PEA. The 2D IR synchronous correlation analysis separates the bands of PEA from those of CP. The cross-peaks in 2D IR asynchronous correlation analysis are indicative of the specific interaction or the conformational change in the blends. The bands of CO, OH and C-O vibrations of PEA, and CH(3) and C-O vibrations of CP that are very sensitive to the intermolecular hydrogen bonding effect and consequently the partial miscibility of components, have been assigned by 2D correlation analysis.     

Noise perturbation in functional principal component analysis filtering for two-dimensional correlation spectroscopy: its theory and application to infrared spectra of a poly(3-hydroxybutyrate) thin film

A method based on noise perturbation in functional principal component analysis (NPFPCA) is for the first time introduced to overcome the noise interference problem in two-dimensional correlation spectroscopy (2D-COS). By the systematic addition of synthetic noise to the dynamic multivariate spectral data, the functional principal component analysis (FPCA) described in this report is able to accurately determine which eigenvectors are representing significant signals instead of noise in the original data. This feature is especially useful for the data reconstruction and noise filtering. Reconstructed data resulted from the smooth eigenvectors can produce much more reliable 2D correlation spectra by removing the correlation artifacts from noise, which in turn enable more accurate interpretation of the spectral variations. The usefulness of this method is demonstrated with a theoretical framework and applications to the 2D correlation analyses of both simulated data and temperature-dependent reflection-absorption infrared spectra of a poly(3-hydroxybutyrate) (PHB) thin film.     

Probing beer aging chemistry by nuclear magnetic resonance and multivariate analysis

This paper describes the use of nuclear magnetic resonance (NMR) spectroscopy, in tandem with multivariate analysis (MVA), for monitoring the chemical changes occurring in a lager beer exposed to forced aging (at 45 °C for up to 18 days). To evaluate the resulting compositional variations, both principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) were applied to the NMR spectra of beer recorded as a function of aging and a clear aging trend was observed. Inspection of PLS-DA loadings and peak integration enabled the changing compounds to be identified, revealing the importance of well known markers such as 5-hydroxymethylfurfural (5-HMF) as well as a range of other relevant compounds: amino acids, higher alcohols, organic acids, dextrins and some still unassigned spin systems. In addition, the multivariate analysis method of 2D correlation analysis was applied to the NMR data enabling the relevant compound variations to be confirmed and inter-compound correlations to be assessed, some reflecting common metabolic/chemical pathways and, therefore, offering improved insight into the chemical aspects of beer aging.     

Comparative evaluation of bioactivity of crystalline trypsin for drying by Fourier-transformed infrared spectroscopy

The purpose of this study was to evaluate the enzymatic stability of colloidal trypsin powder during heating in a solid-state by using Fourier transform infrared (FT-IR) spectra with chemoinformatics and generalized two-dimensional (2D) correlation spectroscopy. Colloidal crystalline trypsin powders were heated using differential scanning calorimetry. The enzymatic activity of trypsin was assayed by the kinetic degradation method. Spectra of 10 calibration sample sets were recorded three times with a FT-IR spectrometer. The maximum intensity at 1634 cm⁻¹ of FT-IR spectra and enzymatic activity of trypsin decreased as the temperature increased. The FT-IR spectra of trypsin samples were analyzed by a principal component regression analysis (PCR). A plot of the calibration data obtained was made between the actual and predicted trypsin activity based on a two-component model with γ² = 0.962. On the other hand, a 2D method was applied to FT-IR spectra of heat-treated trypsin. The result was consistent with that of the chemoinformetrical method. The results for deactivation of colloidal trypsin powder by heat-treatment indicated that nano-structure of crystalline trypsin changed by heating reflecting that the β-sheet was mainly transformed, since the peak at 1634 cm⁻¹ decreased with dehydration. The FT-IR chemoinformetrical method allows for a solid-state quantitative analysis of the bioactivity of the bulk powder of trypsin during drying.     

Characterization of fungal degraded lime wood by FT-IR and 2D IR correlation spectroscopy

The action of soft-rot fungus Chaetomium globosum has been studied. The decayed lime wood samples were observed for different periods of exposure. The degree of decay was determined by weight loss which was of 50.4% after 133 days. The samples were analyzed by FT-IR and 2D IR correlation spectroscopy.The intensity bands assigned to different vibrations from cellulose and hemicelluloses show a decrease, while the intensities of the bands assigned to C–O vibrations due to the formation of oxidized structures increase. At the same time, the intensity of the band assigned to C–O in metoxyl groups from lignin shows a decrease with increasing exposure time. The differences between reference and decayed wood spectra were examined in detail using 2D correlation spectroscopy and the second derivative analysis for two exposure time periods — of 0–70 days and 70–133 days. The formation of reactive species due to oxidation reactions induced by enzymes and the demethoxylation of the lignin structure was evidenced.     

Parallel factor (PARAFAC) kernel analysis of temperature- and composition-dependent NMR spectra of poly(lactic acid) nanocomposites

The parallel factor (PARAFAC) kernel matrix to analyze a sample system stimulated by more than one type of perturbation is described. PARAFAC kernel is a quantitative representation of the synchronicity and asynchronicity observed within the PARAFAC score matrices generated by carrying out two-dimensional (2D) correlation analyses. Thus, kernel matrix representation provides more intuitively understandable interpretation to the conventional PARAFAC trilinear model. In this study, the utility of PARAFAC kernel is demonstrated by the study of poly(lactic acid)-nanocomposite undergoing a structural change depending on the temperature as well as the clay content in the sample. Seemingly complicated variation of nuclear magnetic resonance (NMR) spectra induced by the change in the temperature and clay content are readily analyzed by the multiple-perturbation 2D correlation spectroscopy and PARAFAC kernel. PARAFAC kernel revealed that crystalline and amorphous structures of the PLA substantially undergo thermal deformation, and these variations are also influenced by the presence of the clay.     

Absolute Minimal Sampling of Homonuclear 2D NMR TOCSY Spectra for High-Throughput Applications of Complex Mixtures

Modern applications of 2D NMR spectroscopy to diagnostic screening, metabolomics, quality control, and other high-throughput applications are often limited by the time-consuming sampling requirements along the indirect time domain t₁. 2D total correlation spectroscopy (TOCSY) provides unique spin connectivity information for the analysis of a large number of compounds in complex mixtures, but standard methods typically require >100 t₁ increments for an accurate spectral reconstruction, rendering these experiments ineffective for high-throughput applications. For a complex metabolite mixture it is demonstrated that absolute minimal sampling (AMS), based on direct fitting of resonance frequencies and amplitudes in the time domain, yields an accurate spectral reconstruction of TOCSY spectra using as few as 16 t₁ points. This permits the rapid collection of homonuclear 2D NMR experiments at high resolution with measurement times that previously were only the realm of 1D experiments.     

Improvements in depth‐profiling of thick samples by laser‐induced breakdown spectroscopy using linear correlation

In this work, the capability of linear correlation for depth profiling by laser‐induced breakdown spectroscopy (LIBS) is studied for the first time. A software was specially developed for the calculus of the linear correlation coefficients and its representation in the format of depth profiles. Thick layered samples (layers with thickness of tens to hundreds of micrometers) of different nature, archaeological ceramics and polymer coatings on steel, were characterized by LIBS using the conventional approach based on intensity profiles, and the correlation method. The results revealed that, without using any normalization, the comparison of LIB spectra through the linear correlation coefficient gave an improvement of the depth profile quality and the interface localization by minimizing the influence of fluctuations and decay of the signals in the global intensity of spectra, caused by sources other than concentration variations. Copyright © 2006 John Wiley & Sons, Ltd.