Duanfu Xu

Orthogonal Sample Design Scheme for Two-Dimensional Synchronous Spectroscopy and Its Application in Probing Intermolecular Interactions

This paper introduces a new approach to probing intermolecular interactions based on a framework of two-dimensional (2D) synchronous spectroscopy. Mathematical analysis performed on 2D synchronous spectra using variable concentration as an external perturbation shows that the cross-peaks are composed of two parts. The first part reflects intermolecular interactions that manifest in the form of deviation from the Beer–Lambert law. The second part is related simply to the concentration variations of the solutes and is responsible for the generation of interfering cross-peaks not related to the intermolecular interactions in the system. It is the second part that prevents the reliable identification of intermolecular interactions. We propose a way of selecting the concentrations of solutes so that the resultant dynamic concentration vectors of different solutes become orthogonal to one another. Therefore, the contribution of the second part to the cross-peaks can be effectively removed by the dot product of orthogonal vectors. Our new approach has been tested on a simulated chemical system and a real chemical system. The results demonstrate that interfering cross-peaks can be successfully removed from a 2D synchronous spectrum so that the cross-peaks can be used as a reliable tool to characterize or probe intermolecular interactions.

Copper Binding to Bilirubin as Determined by FT-IR and EPR Spectroscopy

Copper is known to form complexes with bilirubin (H2BR). Such complexes have received increased attention because of their clinical significance as free-radical scavengers. The purpose of this study was to examine a series of Cu2+ BR complexes to ascertain the nature of the binding between Cu2+ and BR. Several physical measurements of the salts were made, such as Fourier transform infrared (FT-IR) and electron paramagnetic resonance (EPR). The complexes were prepared by dissolving protonated BR in NaOH and adding different ratios of aqueous CuCl2. At ratios of Cu2+/H2BR of 1:1 and 2:1, soluble complexes were formed. In solution, EPR spectra demonstrated nine hyperfine peaks, which, from the splitting, is indicative of Cu2+ coordinated to four nitrogen atoms coming from two molecules of BR. The solid obtained from the solutions demonstrated predominant infrared absorptions at 1574 and 1403 cm−1 (previously assigned as COO− vibrations, asymmetric and symmetric), whereas the 1710-cm−1 vibration appears only as a shoulder (previously assigned as the free COOH vibration), indicative that most of the COO− groups have reacted with sodium, thus accounting for the aqueous solubility. The NH stretching vibration in the pyrrole group of H2BR has disappeared and is replaced with the OH stretching vibration in H2O. At higher ratios of 3:1 and 5:1 (Cu2+/H2BR), black precipitates are formed, which produce no EPR signals. Furthermore, the NH vibration disappears as in the soluble solution complexes. It can be concluded that the insoluble salts (higher Cu2+/H2BR ratios) are mixed complexes containing the Cu–nitrogen chelate and Cu salts involving the COOH groups.

A spectroscopic investigation of the formation mechanism of pigment gallstones

Further spectroscopic studies of gallstones are reviewed with an emphasis on the formation of black pigment gallstones. This type of gallstone appears mainly in Western countries, with only 3% of the cholelithiasis patients in China having black gallstones. Fourier transform infrared absorption and electron paramagnetic resonance are used as spectroscopic probes of gallstones and their metal bilirubinate components. Nonlinear phenomena in gallstone formation were investigated through the appearance of ring structure in gallstones and fractal patterns in the formation in the precipitates of bile salt systems. Although a complete understanding of gallstone formation has not yet been achieved, interesting progress toward this goal has been made recently.

New Cryogenic Apparatus for FT-IR Spectroscopic Studies

Cryogenic Fourier transform infrared (FT-IR) spectroscopy, being an effective technique in improving the spectral resolution and signal-to-noise ratio, is utilized in our group. However, we have found the traditional cryogenic apparatus inconvenient in the following aspects: (1) The system does not work very well in preventing moisture/frost from condensing on the salt windows or samples. (2) The indirect way of cooling using a metal cold finger limits the efficiency of the system. We have designed a new cryogenic apparatus. The apparatus is composed of a cooling stage and a measuring stage. In the cooling stage, the sample is directly immersed in liquid nitrogen. In the measuring stage, the sample is lifted just above the surface of liquid nitrogen, and the FT-IR spectrum is recorded. The dry cold nitrogen gas evaporated from liquid nitrogen forms a protective layer surrounding the sample and prevents moisture from condensing on the sample. Thus, neither vacuum nor salt windows are needed in cryogenic spectroscopic experiments using this method. The experimental results show that our method is effective in cryogenic FT-IR spectroscopic studies.

Preparation and characterization of erbium-doped sol-gel silica glasses

Erbium-doped silica glasses were made by sol-gel process. Intensive photoluminescence (PL) spectra from the Er-doped silica glasses at room temperature were measured. A broadband peak at 1535 nm, corresponding to the 4I13/2-4I15/2 transition, its full width at half- maximum (FWHM) of 10 nm, and a shoulder at 1546 nm in the PL spectra were observed. At lower temperatures, main line of 1535 nm and another line of 1552 nm instead of 1546 nm appear. So two types of luminescence centers must exist in the samples at different temperature. The intensity of main line does not decrease obviously with increasing temperature. By varying the Er ion concentration in the range of 0.2 wt% - 5 wt%, the highest photoluminescence intensity was obtained at 0.2 wt% erbium doped concentration. Luminescence intensity decreases with increasing erbium concentration. Cooperative upconversion was used to explain the concentration quenching of luminescence from silica glass with high erbium concentration. Extended x-ray absorption fine structure measurements were carried out. It was found that the majority of the erbium impurities in the glasses have a local structure of eight first neighbor oxygen atoms at a mean distance of 0.255 nm, which is consistent with the typical coordination structure of rare earth ion.

Far Infrared Spectroscopic Characterization of Sugars and their Metal Complexes

This paper mainly reports the far IR spectra of mono- and disaccharides and their metal complexes. The results suggest that the far IR spectra may be used to identify different saccharides and provide definite information about metal-saccharide complex formation.

FT-IR Study on the Secondary Structure of Calmodulin-BCP

Calmodulin is a small, highly conserved protein which is present in all eukaryotic cells and plays a regulatory role in a diverse set of calcium- dependent cellular processes. To enhance the understanding of the nature of calmodulin in the biological process and its structure-function relationship, calmodulin has been widely studied by NMR, X-ray diffraction, CD and FTIR methods. Most investigations in this field are concentrated on the calmodulin from bovine brain, whereas the calmodulin from plants are rarely reported. In this paper, we are focus on the secondary structure of calmodulin from brassica campestris pollen(CaM-BCP) with FTIR and CD method.

Fourier transform infrared investigation on the interaction between CrCl3 and polyvinylpyrrolidone

Band shifting, splitting and widening of the amide I band of the CrCl3/poly(N-vinyl- pyrrolidone) system was studied by FTIR. Secondary derivative spectra show the band consists of several overlapping bands. It suggests Cr3+/amide group complexes of various coordinating and aggregating states co-exist in the system.

FT-IR Spectroscopic Characterization and EXAFS Studies of Carbohydrates and Their Metal Complexes

The study of carbohydrate has been of great importance due to its physiological function. The interaction between the carbohydrates and various metal ions have been examined[l], while little is reported concerning the interaction with lanthanide ions. The spectroscopic methods provide a powerful tool to study the interaction between metal ions and carbohydrates. Nevertheless, the reliable correlation between the structural information and the spectral properties for carbohydrate complex is still lacking. In this paper, FTIR(near, mid and far IR) and EXAFS were used to characterize the mono- and disaccharide and their metal complexes.