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Objective: A formaldehyde detection method was established for the accurate analysis of the migration of formaldehyde in water and acid simulants from the polylactic acid straws. Methods: The preferred chromatographic conditions were: mobile phase water-acetonitrile (Vwater∶Vacetonitrile of 55∶45), column temperature of 35 ℃, detection wavelength of 355 nm. The optimized 2,4-dinitrophenylhydrazine derivative conditions were: the acetic acid in acidic 2,4-dinitrophenylhydrazine solution of 1.5 mL/100 mL (pH 3.07), the molar ratio of 2,4- dinitrophenylhydrazine of quadruple formaldehyde, the reaction temperature of 45 ℃, and the reaction time of 40 min. Results: The linear equation of formaldehyde-2,4-dinitrobenzene was y=55.247 0x-3.849 4 within 1.008 to 70.560 μg/mL, with the correlation coefficient of 0.999 5. The detection limit of the corresponding formaldehyde was 0.066 mg/kg, and the quantification limit of which was 0.22 mg/kg. The formaldehyde-DNPH was stable within 13.5 hours. The average recovery of simulants spiking with formaldehyde in 3 levels was from 92.6% to 104.1%, with SD (n=6) of 0.4% to 3.3%. The maximum migration of formaldehyde from disposable polylactic acid straws to three food simulants (water, 3% acetic acid solution, 4% acetic acid solution) was 1.839 mg/kg, less than the specific migration requirement of 15 mg/kg stipulated by China and the European Union. Conclusion: This method is accurate and efficient enough to apply for the analysis of formaldehyde migration from polylactic acid straws.

Two-dimensional electron–electron double resonance (2D-ELDOR) provides extensive insight into molecular motions. Recent developments permitting experiments at higher frequencies (95 GHz) provide molecular orientational resolution, enabling a clearer description of the nature of the motions. In this work, simulations are provided for the example of domain motions within proteins that are themselves slowly tumbling in solution. These show the nature of the exchange cross-peaks that are predicted to develop in real time from such domain motions. However, we find that the existing theoretical methods for computing 2D-ELDOR experiments over a wide motional range begin to fail seriously when applied to very slow motions characteristic of proteins in solution. One reason is the failure to obtain accurate eigenvectors and eigenvalues of the complex symmetric stochastic Liouville matrices describing the experiment when computed by the efficient Lanczos algorithm in the range of very slow motion. Another, perhaps more serious, issue is that these matrices are "non-normal," such that for the very slow motional range even rigorous diagonalization algorithms do not yield the correct eigenvalues and eigenvectors. We have employed algorithms that overcome both these issues and lead to valid 2D-ELDOR predictions even for motions approaching the rigid limit. They are utilized to describe the development of cross-peaks in 2D-ELDOR at 95 GHz for a particular case of domain motion. [ABSTRACT FROM AUTHOR]

The way in which multifrequency electron spin resonance (ESR) could be useful tool for studying dynamics of proteins is demonstrated. The analysis of 250 and 9 GHz ESR spectra using the slowly relaxing local structure (SRLS) model enables to separate the slower global motion of T4 lysozyme from the faster internal motions by employing the SRLS model, which distinguishes these modes.

Nuclear spin relaxation properties in terms of microdynamic parameters are studied. A new data analysis tool is devised for back-calculation in order to identify inappropriate elements of the dynamic model.

Research has been conducted on the adenylate kinase from Escherichia coli which consists of a polypeptide chain folded into several domains. The backbone dynamics of the ligand-free and the inhibitor-bound E. coli adenylate kinase have been investigated via the use of the slowly relaxing local structure (super)15 N spin relaxation and the resulrs are discussed.

In this paper the slow motion electron spin resonance (ESR) line shape theory is extended to hexagonal mesophases. The stochastic Liouville equation is applied to the dynamic description of both local molecular reorientation and azimuthal surface translational diffusion of the lipid around the cylinder axis. The established ESR line shape models allow a separation of contributions to the electron spin relaxation from the two motions, if macroscopically aligned hexagonal spectra are simulated. Such simulations can yield, not only the dynamic parameters such as local ordering parameter, local motional rate, and azimuthal surface diffusion coefficient, but also a disorder parameter accounting for residual cylinder disorder and the disorder effects due to finite cylinder length and curved surface along the cylinder axis. To test these models, they are applied to the analyses of X-band ESR spectra of the hexagonal phase of the sodium dodecyl sulphate (SDS)/decanol/water ternary system. The obtained surface diffusion coefficient and disorder parameter are in good agreement with previously reported values in a deuterium nuclear magnetic resonance (NMR) study. The local dynamic properties, which are not readily available by NMR method, are compared with those obtained for the micellar phase of the SDS/water binary system. Such a comparison reveals that while the local orderings are similar, the local dynamics is much slower in the hexagonal phase than in the micellar phase. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The experimental electron-spin resonance (ESR) line shapes of the micellar (L1) and lamellar (Lα) phases of the binary system 1-oleoyl-sn-glycero-3-phosphocholine (OlLPC)/water have been interpreted quantitatively using a two-dynamic (TD) model. This dynamic model comprises the local dynamics of Freed’s model [J. Chem. Phys. 55, 5270 (1971); 58, 3185 (1973); 77, 3915 (1982)] and a second Brownian motion referring to the combined effect of the micellar reorientation and the translational motion of the amphiphile molecule along the micellar surface. With this model, we focus our attention on a comparison of the local dynamics between the L1 and Lα phases. It is found from the TD model that the local ordering (λ) and dynamics (τ⊥) of the L1 phase change compared with those of the Lα phase according to λ(L1)>=λ(Lα) and τ⊥(L1)<τ⊥(Lα). The observed lower local ordering in the lamellar phase compared with that in the micellar phase may be interpreted in terms of the different packing constraints of the OlLPC molecule in the two phases. [ABSTRACT FROM AUTHOR]

The restricted diffusion-in-a-cone (DC) model is formulated within the context of a slow-motional electron-spin resonance (ESR) line shape to describe the local reorientation of amphiphile molecules in lipid bilayers. A special program is designed to reduce the computation amount, and the computational time for the DC model is comparable to that for the familiar Maier–Saupe (MS) potential model. Special emphasis is placed on comparing the two potential models in ESR line shape calculations. Illustrative analyses of ESR spectra are presented for some selected bilayers. It is found that the DC model is particularly suitable for describing the local dynamic states of single chain systems. The simulation results show that while the two models give similar values for the reorientational rates, the DC model predicts lower local molecular orders. [ABSTRACT FROM AUTHOR]

Deuteriumline shape analysis of CD3groups has emergedas a particularly useful tool for studying microsecond–millisecondprotein motions in the solid state. The models devised so far consistof several independently conceived simple jump-type motions. Theyare comprised of physical quantities encoded in their simplest form;improvements are only possible by adding yet another simple motion,thereby changing the model. The various treatments developed are case-specific;hence comparison among the different systems is not possible. Herewe develop a new methodology for 2H NMR line shape analysisfree of these limitations. It is based on the microscopic-order-macroscopic-disorder(MOMD) approach. In MOMD motions are described by diffusion tensors,spatial restrictions by potentials/ordering tensors, and geometricfeatures by relative tensor orientations. Jump-type motions are recoveredin the limit of large orientational potentials. Model improvementis accomplished by monitoring the magnitude, symmetry, and orientationof the various tensors. The generality of MOMD makes possible comparisonamong different scenarios. CD3line shapes from the ChickenVillin Headpiece Subdomain and the Streptomyces Subtilisin Inhibitorare used as experimental examples. All of these spectra are reproducedby using rhombic local potentials constrained for simplicity to begiven by the L= 2 spherical harmonics, and by axialdiffusion tensors. Potential strength and rhombicity are found tobe ca. 2–3 kBT. The diffusion tensor is tilted at 120° from the C–CD3axis. The perpendicular (parallel) correlation times forlocal motion are 0.1–1.0 ms (3.3–30 μs). Activationenergies in the 1.1–8.0 kcal/mol range are estimated. Futureprospects include extension to the 2H relaxation limit,application to the 15N and 13C NMR nuclei, andaccounting for collective motions and anisotropic media. [ABSTRACT FROM AUTHOR]

The ESR spectra from DNA oligomers was analyzed.

Research was conducted to prove that ESR at high frequencies will be more sensitive to the faster dynamics that may well be averaged out at lower frequencies using spectral simulations of nitroxide spin label spectra at low and high frequencies. By using the slowly relaxing local structure model as reference, it was demonstrated under what conditions the microscopic-order macroscopic-disorder model and the fast internal motion model were valid.

Discusses a structural mode-coupling approach to [sup 15]N NMR relaxation in proteins. Local ordering associated with the coupling potential; Model-fitting and model-selection schemes; Relaxation data from isotropically tumbling proteins.

We have developed the stochastic microscopic-order-macroscopic-disorder (MOMD) approach for elucidating dynamic structures in the solid-state from 2 H NMR lineshapes. In MOMD, the probe experiences an effective/collective motional mode. The latter is described by a potential, u , which represents the local spatial-restrictions, a local-motional diffusion tensor, R , and key features of local geometry. Previously we applied MOMD to the well-structured core domain of the 3-fold-symmetric twisted polymorph of the Aβ 40 -amyloid fibril. Here, we apply it to the N-terminal domain of this fibril. We find that the dynamic structures of the two domains are largely similar but differ in the magnitude and complexity of the key physical parameters. This interpretation differs from previous multisimple-mode (MSM) interpretations of the same experimental data. MSM used for the two domains different combinations of simple motional modes taken to be independent. For the core domain, MOMD and MSM disagree on the character of the dynamic structure. For the N-terminal domain, they even disagree on whether this chain segment is structurally ordered (MOMD finds that it is), and whether it undergoes a phase transition at 260 K where bulklike water located in the fibril matrix freezes (MOMD finds that it does not). These are major differences associated with an important system. While the MOMD description is a physically sound one, there are drawbacks in the MSM descriptions. The results obtained in this study promote our understanding of the dynamic structure of protein aggregates. Thus, they contribute to the effort to pharmacologically control neurodegenerative disorders believed to be caused by such aggregates.