Negative Cross-peaks Research Articles

Two-trace two-dimensional (2T2D) correlation infrared spectral analysis of Spirulina platensis and its commercial food products coupled with chemometric analysis

Spirulina plays an important role in the food industry due to its rich protein content and other nutritional values. 2D-COS spectral analysis is widely used nowadays in understanding the change in the molecular mechanism of biological samples. Two trace 2D-COS (2T2D-COS) is a technique that has gained much attention due to improved accuracy and higher spectral resolution. The selected band area measurement (Lipid/Amide I, Lipid/Carbohydrates, Amide I/ Amide II) shows that the samples S2 and S3 have higher quality among other samples. A positive correlation of increased lipid oxidation with lipid changes exists among all the samples studied. Synchronous spectra show auto peaks at 2870 cm−1, 1624 cm−1, 1034 cm−1 for samples S2 and S3 which helps in assessing food quality. Asynchronous negative cross-peaks (2870, 2815) indicate a higher presence of polysaccharides in the samples S1 and S4. The negative cross-peaks (1206, 1036) show deformation in the pyranose ring of polysaccharides in the S4 sample due to food processing. The negative PCA loading at 997 cm−1 shows the polymerization of carbohydrates mechanism occurring at glucan bands. Hierarchical cluster analysis shows clustering of dissimilar products occurring at a higher value. ROC curve depicts the higher reliability of our findings using 2T2D-COS.

Preparation and characterization of the dialdehyde hydroxypropyl methylcellulose/collagen (DHPMC/COL) solutions

Dialdehyde hydroxypropyl methylcellulose (DHPMC)/Collagen (DHPMC/COL) solutions were prepared with different DHPMC contents (0–70 wt%). According to the viscosimetric method, all samples were compatible. The compatibility was related to Schiff’s base, which formed between the amino groups of collagen and the dialdehyde groups of DHPMC. When DHPMC content increased, the cross-linking degree increased, and the synchronous negative cross-peaks between the C-O(H) of DHPMC at 1031 cm−1 and amide bands of collagen at 1623 cm−1, 1523 cm−1, and 1238 cm−1 indicated that hydrogen bonds mainly formed between DHPMC and collagen. As a result, the thermal stability improved. The morphology showed more sheet-like structures than collagen. When the DHPMC content was 50 wt%, the thermal denaturation temperature (Td) and residual weight% were 8.5 °C and 10.22%, respectively, higher than other samples. However, the cross-linking degree decreased as the DHPMC content increased further. The intensity of negative cross-peak was decreased in synchronous correlation spectra, while the intensity of cross-peak at 1031 cm−1 was enhanced, indicating that hydrogen bonds formed between the C–O and C–O(H) of the DHPMC were enhanced. Consequently, the thermal stability decreased and more porous structures were observed compared to those when DHPMC content was 50 wt%.

Analysis of three-site T2-T2 exchange NMR

T2-T2 exchange NMR is a unique method to investigate the pore space and fluid dynamics in porous media. While two-site relaxation exchange is well understood, three-site exchange is not. We analyze the solutions for three-site T2-T2 exchange NMR analytically and by computer simulation. Three main results are obtained. First, the exchange map can be asymmetric in the case of microscale vortex motion in violation of the principle of detailed balance. Second, the apparent longitudinal relaxation times and/or apparent transverse relaxation times can be complex valued. In the case of complex apparent transverse relaxation times, the three-site exchange map coalesces to a two-site exchange map with characteristic oscillations in the time domain. As a result of the oscillations, the shorter relaxation time is less than expected. Third, there can be negative cross-peaks in the exchange map for certain combinations of longitudinal and transverse relaxation times or if the mixing period is shorter than the evolution and detection periods. In view of these results experimental exchange maps may need to be reevaluated.

The origin of negative cross-peaks in proton-spin diffusion spectrum of fully protonated solids at fast MAS: Coherent or incoherent effect?

Spin-diffusion (SD) is amongst the first methods proposed to spatially transfer polarization between dipolar-coupled nuclear spins. Lab-frame SD has proved particularly useful in structural characterization of a large variety of molecules. During SD, the rate of magnetization transfer between the two nuclei depends on the square of the dipolar coupling and the zero-quantum lineshape of the two spins. The relative sign of the diagonal and cross-peaks is determined by the spin part of the dipolar Hamiltonian. Practically, SD experiments are used in two ways: (a) SD transfer amongst only the protons (known as proton spin-diffusion or PSD) and b) SD amongst rare nuclei, coupled to a strong proton bath, known as proton driven spin-diffusion (PDSD). It is well established that the diagonal and cross-peaks have the same sign during SD based polarization transfer. 2D PSD experiments recorded on Histidine.HCl.H2O sample at fast magic angle spinning (MAS) show that some of the cross-peaks in the 2D spectrum are negative with respect to the diagonal peaks. Cross-relaxation due to stochastic motion is generally believed to give rise to such negative peaks. Herein, we use theoretical calculations, numerical simulations and experiments to show that the origin of the negative cross-peaks in PSD spectrum is due to coherent interactions. The origin of negative peaks can be specifically ascribed to a four spin, double-flip-double flop term, in the third-order Hamiltonian. These terms become the dominant terms at fast spinning when additional — conditions are satisfied.

Correlating nuclear frequencies by two-dimensional ELDOR-detected NMR spectroscopy

ELDOR (Electron Double Resonance)-detected NMR (EDNMR) is a pulse EPR experiment that is used to measure the transition frequencies of nuclear spins coupled to electron spins. These frequencies are further used to determine hyperfine and quadrupolar couplings, which are signatures of the electronic and spatial structures of paramagnetic centers. In recent years, EDNMR has been shown to be particularly useful at high fields/high frequencies, such as W-band (∼95GHz, ∼3.5T), for low γ quadrupolar nuclei. Although at high fields the nuclear Larmor frequencies are usually well resolved, the limited resolution of EDNMR still remains a major concern. In this work we introduce a two dimensional, triple resonance, correlation experiment based on the EDNMR pulse sequence, which we term 2D-EDNMR. This experiment allows circumventing the resolution limitation by spreading the signals in two dimensions and the observed correlations help in the assignment of the signals.First we demonstrate the utility of the 2D-EDNMR experiment on a nitroxide spin label, where we observe correlations between 14N nuclear frequencies. Negative cross-peaks appear between lines belonging to different MS electron spin manifolds. We resolved two independent correlation patterns for nuclear frequencies arising from the EPR transitions corresponding to the 14N mI=0 and mI=−1 nuclear spin states, which severely overlap in the one dimensional EDNMR spectrum. The observed correlations could be accounted for by considering changes in the populations of energy levels that S=1/2, I=1 spin systems undergo during the pulse sequence. In addition to these negative cross-peaks, positive cross-peaks appear as well. We present a theoretical model based on the Liouville equation and use it to calculate the time evolution of populations of the various energy levels during the 2D-EDNMR experiment and generated simulated 2D-EDMR spectra. These calculations show that the positive cross-peaks appear due to off resonance effects and/or nuclear relaxation effects. These results suggest that the 2D-EDNMR experiment can be also useful for relaxation pathway studies. Finally we present preliminary results demonstrating that 2D-EDNMR can resolve overlapping 33S and 14N signals of type 1 Cu(II) center in 33S enriched Azurin.

Differentiation of Asian ginseng, American ginseng and Notoginseng by Fourier transform infrared spectroscopy combined with two-dimensional correlation infrared spectroscopy

The herbal materials of Asian ginseng (the root of Panax ginseng), American ginseng (the root of Panax quinquefolius) and Notoginseng (the root of Panax notoginseng) were differentiated by conventional Fourier transform infrared spectroscopy (1D-FTIR) and two-dimensional (2D) correlation FTIR applying a thermal perturbation. Altogether 30 samples were collected and analyzed. Their entire 1D-FTIR spectra in the range of 4000–400 cm −1 and 2D-FTIR spectra in the region of 850–1530 cm −1 were generally similar based on the peaks position and intensities. This indicated the chemical constituents in these species of herbs were not distinctively different. However, variation in peak intensity were observed at about 1640 cm −1, 1416 cm −1, 1372 cm −1 and 1048 cm −1 in the 1D-FTIR spectra among these species for their ease differentiation. Clustering analysis of 1D-FTIR showed that these species located in different clusters. Much difference in their second derivative FTIR pattern among the three species also provided information for easy differentiation. These species of herbs were further identified based on the positions and intensities of relatively strong auto-peaks, positive or negative cross-peaks in their 2D-FTIR spectra. The findings provide a rapid and new operational procedure for the differentiation of these notable herbs. The visual and colorful 2D-FTIR spectra can provide dynamic structural information of chemical components in analyte and demonstrated as a powerful and useful approach for herbs identification.

Correlated hydrogen bonding fluctuations and vibrational cross peaks in N-methyl acetamide: Simulation based on a complete electrostatic density functional theory map

The coherent nonlinear response of the entire amide line shapes of N-methyl acetamide to three infrared pulses is simulated using an electrostatic density functional theory map. Positive and negative cross peaks contain signatures of correlations between the fundamentals and the combination state. The amide I-A and I-III cross-peak line shapes indicate positive correlation and anticorrelation of frequency fluctuations, respectively. These can be ascribed to correlated hydrogen bonding at C[double bond]O and N-H sites. The amide I frequency is negatively correlated with the hydrogen bond on carbonyl C[double bond]O, whereas the amide A and III are negatively and positively correlated, respectively, with the hydrogen bond on amide N-H.

Characterization of a chiral stationary phase by HR/MAS NMR spectroscopy and investigation of enantioselective interaction with chiral ligates by transferred NOE.

The surface chemistry of a chiral stationary phase (CSP) with a (tert-butyl carbamoyl) quinine selector immobilized on thiol-modified silica has been characterized by (1)H HR/MAS NMR and (29)Si CP/MAS NMR spectroscopy. The mostly well-resolved (1)H signals could be assigned to stem from the surface-bound selector and the latter suggested a bi- and trifunctional silane linkage. Suspended-state NMR spectroscopy thus proved a well-characterized surface chemistry as proposed. To study chiral recognition phenomena in the presence of the CSP, (1)H HR/MAS 2D transfer NOESY investigations in methanol-d(4) have been undertaken with various solutes including N-3,5-dinitrobenzoyl derivatives of leucine (DNB-Leu) and N-acetyl phenylalanine (Ac-Phe). Both (R)- and (S)-enantiomers of DNB-Leu and Ac-Phe interacted with the tBuCQN-CSP as indicated by negative cross-peaks in the trNOESY spectra, while the 2D NOESY of the dissolved solutes in absence of the chiral stationary phase showed positive cross-peaks. The intensities of the trNOE cross-peaks were much stronger for the (S)-enantiomers. This stereoselectivity paralleled the experimental chromatographic behavior, where the (S)-enantiomers revealed stronger binding and retention on the tBuCQN-CSP as well. Hence, we were able to correlate the retention behavior to the trNOE NMR spectroscopic data in a qualitative manner.

Elucidation of cross relaxation in liquids by two-dimensional N.M.R. spectroscopy

Two-dimensional N.M.R. spectroscopy is applied to the elucidation of cross relaxation pathways in liquids. The theory underlying two dimensional studies of cross relaxation and of transient nuclear Overhauser effects is developed. The influence of the correlation time of the molecular random process is investigated. It is found that in the limit of short correlation times (extreme narrowing limit) weak negative cross-peaks are observed. However, for long correlation times (spin diffusion limit) strong positive cross-peaks can be obtained. The technique appears particularly promising for the study of cross relaxation in macromolecules. Examples of intra- and intermolecular cross relaxation in the extreme narrowing limit are presented.

High-Order Spin Diffusion Mechanisms in 19F 2-D NMR of Oxyfluorides

Negative cross-peaks have been observed in the 19F 2-D magnetization-exchange MAS NMR spectra of Ba2MoO3F4 under fast-spinning conditions. The polarization transfer dynamics are studied as a function of the spinning frequency and the frequency separation of the resonances. The results are consistent with a novel mechanism, in which four spins simultaneously exchange Zeeman magnetization with each other, in an energy-conserving process.

Distance Measurements by Dipolar Recoupling Two-Dimensional Solid-State NMR

We present a two-dimensional NMR technique for the measurement of dipolar couplings in polycrystalline solids. This experiment is fully transverse and uses a windowless dipolar recoupling pulse sequence (DRAWS, described in Gregory, D. M.; et al. Chem. Phys. Lett. 1995, 246, 654−663) to effect coherence transfer. Direct, internuclear coherence transfer produces negative cross-peaks in the 2D spectrum. Cross-peak development and experimental requirements for obtaining distances from the two-dimensional solid-state NMR spectra of two- and three-spin systems are discussed, and demonstrations are shown for thymidine-2,4-13C2 and l-alanine-13C3. Internuclear distances are derived by comparison of experimental cross-peak buildup curves with numerical simulations. In the three-spin system, indirect coherence-transfer mechanisms prohibit the interpretation of buildup curves as due to isolated spin pair interactions and limit the accuracy of some distance measurements. This 2D technique can also be used for spectr...

Two-Dimensional Near-Infrared Correlation Spectroscopy Study of Premelting Behavior of Nylon 12

The premelting behavior of Nylon 12 has been investigated by two-dimensional (2D) Fourier-transform (FT) near-infrared (NIR) correlation spectroscopy. FT-NIR spectra of Nylon 12 in a cast film have been measured over a temperature range of 30−150 °C. The 2D NIR correlation analysis reveals that there are at least eight bands in the 6800−6100 cm-1 region where the first overtone of an NH stretching mode of Nylon 12 is expected to appear. They may be ascribed to free- and hydrogen-bonded NH groups in various environments. The asynchronous 2D NIR correlation spectrum in the above region indicates that the amide group with a free carbonyl oxygen (structure C) appears first and then unassociated free amide (structure A) and amide with free NH (structure B) follow as the temperature is increased. In the 6000−5500 cm-1 region of the synchronous spectrum, four dominant autopeaks corresponding to the first overtones of the CH2 stretching modes are observed at 5840, 5770, 5680, and 5640 cm-1, and negative cross peaks are found between the 5770 cm-1 band and the rest. This observation, together with the careful inspection of the dynamic NIR spectra, suggests that the band at 5770 cm-1 is due to the contribution of the CH stretching vibration of an ordered or highly associated form of Nylon 12, which decreases with temperature, while other NIR bands correspond to more disordered forms. The corresponding asynchronous spectrum indicates that the intensity variation of the bands at 5840, 5680, and 5640 cm-1 actually occurs at a lower temperature compared to the onset of the intensity decrease of the 5770 cm-1 band. A substantial amount of disordered or dissociated components corresponding to the above three bands start appearing before the disappearance of more ordered components represented by the 5700 cm-1 band. Probably, they appear as the premelting precursors (or even possibly as the indirect cause) to the precipitous decrease of the ordered components associated with the melting of Nylon 12 occurring at a much higher temperature.

Membrane hydration and structure on a subnanometer scale as seen by high resolution solid state nuclear magnetic resonance: POPC and POPC/C12EO4 model membranes

The position on a subnanometer scale and the dynamics of structurally important water in model membranes was determined using a combination of proton magic-angle spinning NMR (MAS) with two-dimensional NOESY NMR techniques. Here, we report studies on phosphocholine lipid bilayers that were then modified by the addition of a nonionic surfactant that is shown to dehydrate the lipid. These studies are supplemented by 13C magic-angle spinning NMR investigations to get information on the dynamics of segmental motions of the membrane molecules. It can be shown that the hydrophilic chain of the surfactant is positioned at least partially within the hydrophobic core of the lipid bilayer. With the above NMR approach, we are able to establish molecular contacts between water and the lipid headgroup as well as with certain groups of the hydrocarbon chains and the glycerol backbone. This is possible because high resolution proton and 13C-NMR spectra of multilamellar bilayer membranes are obtained using MAS. A phase-sensitive NOESY must also be applied to distinguish positive and negative cross-peaks in the two-dimensional plot. These studies have high potential to investigate membrane proteins hydration and structural organization in a natural lipid bilayer surrounding.

Elucidation of cross relaxation in liquids by two-dimensional N.M.R. spectroscopy

Two-dimensional N.M.R. spectroscopy is applied to the elucidation of cross relaxation pathways in liquids. The theory underlying two dimensional studies of cross relaxation and of transient nuclear Overhauser effects is developed. The influence of the correlation time of the molecular random process is investigated. It is found that in the limit of short correlation times (extreme narrowing limit) weak negative cross-peaks are observed. However, for long correlation times (spin diffusion limit) strong positive cross-peaks can be obtained. The technique appears particularly promising for the study of cross relaxation in macromolecules. Examples of intra- and intermolecular cross relaxation in the extreme narrowing limit are presented.