Conformational analysis of dipeptide‐derived polyisocyanides

The conformational properties of polymers derived from isocyanodipeptides have been investigated with a combination of model calculations, X‐ray diffraction, and circular dichroism spectroscopy. Depending on the configuration of the side chains, defined arrays of hydrogen bonds along the polymeric backbone are formed. This leads to a well‐defined conformation as, for example, expressed in the formation of lyotropic liquid‐crystalline phases and increased helical stability. Upon the disruption of the hydrogen bonds by a strong acid, a less well‐defined macromolecular conformation is observed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1725–1736, 2003     

Evaluation of piezoelectric PVDF polymers for use in space environments. III. Comparison of the effects of vacuum UV and gamma radiation

Films of piezoelectric PVDF and P(VDF‐TrFE) were exposed to vacuum UV (115–300 nm VUV) and γ‐radiation to investigate how these two forms of radiation affect the chemical, morphological, and piezoelectric properties of the polymers. The extent of crosslinking was almost identical in both polymers after γ‐irradiation, but surprisingly, was significantly higher for the TrFE copolymer after VUV‐irradiation. Changes in the melting behavior were also more significant in the TrFE copolymer after VUV‐irradiation due to both surface and bulk crosslinking, compared with only surface crosslinking for the PVDF films. The piezoelectric properties (measured using d₃₃ piezoelectric coefficients and D‐E hysteresis loops) were unchanged in the PVDF homopolymer, while the TrFE copolymer exhibited more narrow D‐E loops after exposure to either γ‐ or VUV‐radiation. The more severe damage to the TrFE copolymer in comparison with the PVDF homopolymer after VUV‐irradiation is explained by different energy deposition characteristics. The short wavelength, highly energetic photons are undoubtedly absorbed in the surface layers of both polymers, and we propose that while the longer wavelength components of the VUV‐radiation are absorbed by the bulk of the TrFE copolymer causing crosslinking, they are transmitted harmlessly in the PVDF homopolymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3253–3264, 2006     

Chloryl nitrate: a novel product of the OClO + NO3 + M recombination

The products of the reaction of OClO with NO3 were investigated between 220 and 298 K using a flow reactor and infrared, visible, and ultraviolet analysis. At temperatures below 250 K new infrared and ultraviolet absorption features were observed and assigned to the novel compound chloryl nitrate (O2ClONO2). Additionally, ClO and NO2 were observed as reaction products, indicating the existence of a second reaction channel. O2ClONO2 formation predominates at temperatures below 230 K. The reaction rate constant at 220 K is estimated to be on the order of 10(-14) cm3 molecule-1 s-1 in 1-5 Torr of helium. These observations suggest that O2ClONO2 may exist in the terrestrial stratosphere.     

Dimerization of formic acid--an example of a "noncovalent" reaction mechanism

The pulse deposition technique allows selectively the isolation of monomeric or dimeric formic acid in argon matrices at 7 K. Warming of matrices containing the monomer M from 7 K to 40 K results in the decrease of M and formation of a dimer B. This dimer is also labile, and further warming finally produces a second dimer A. By comparison with density functional theory (DFT) calculations and gas phase IR spectra taken from the literature, the latter dimer A was identified as the C2h-symmetrical cyclic dimer. The unstable dimer B was identified as the acyclic Cs-symmetrical dimer. An activation energy of 2.3 kcal mol(-1) was calculated for the B --> A rearrangement at the B3LYP/ 6-311 ++ G(d,p) level of theory, which is in qualitative agreement with the experimental finding of a slow thermal reaction under the conditions of matrix isolation.     

Interspike interval embedding of chaotic signals

According to a theorem of Takens [Lecture Notes in Mathematics (Springer-Verlag, Berlin, 1981), Vol. 898], dynamical state information can be reproduced from a time series of amplitude measurements. In this paper we investigate whether the same information can be reproduced from interspike interval (ISI) measurements. Assuming an integrate-and-fire model coupling the dynamical system to the spike train, there is a one-to-one correspondence between the system states and interspike interval vectors of sufficiently large dimension. The correspondence implies in particular that a data series of interspike intervals, formed in this manner, can be forecast from past history. This capability is demonstrated using a nonlinear prediction algorithm, and is found to be robust to noise. A set of interspike intervals measured from a simple neuronal circuit is studied for deterministic structure using a prediction error statistic. (c) 1995 American Institute of Physics.     

Studies on polymers and composites from lignin and fiber derived from sugar cane

Sugarcane fiber (i.e. bagasse) lignin has a larger fraction of aromatics unsubstitution in the ortho position than hardwood or softwood lignin and hence has the greater ability to be derivatized. Furthermore, organosolv lignin has a higher purity than sulfonated and kraft lignins. This work examines the purification of organosolv lignin derived from bagasse and the physico‐chemical properties of the lignin and lignin‐phenol formaldehyde (PF) resin coatings, and composites. The wetability tests have shown that lignin and lignin‐PF resin films are effective water barrier coatings, though the contact angles of lignin‐PF resin films were considerably less than the wax films. The overall mechanical properties (i.e. peak stress, peak strain and modulus) of the bagasse fiber composites were lower than the values obtained with the composites without the inclusion of bagasse fiber. Copyright © 2007 John Wiley & Sons, Ltd.     

An altered mechanism of hydrolysis for a metal-complexed phosphate diester

Isotope effects in the nucleophile and in the leaving group were measured to gain information about the mechanism and transition state of the hydrolysis of methyl p-nitrophenyl phosphate complexed to a dinuclear cobalt complex. The complexed diester undergoes hydrolysis about 1011 times faster than the corresponding uncomplexed diester. The kinetic isotope effects indicate that this rate acceleration is accompanied by a change in mechanism. A large inverse 18O isotope effect in the bridging hydroxide nucleophile (0.937 +/- 0.002) suggests that nucleophilic attack occurs before the rate-determining step. Large isotope effects in the nitrophenyl leaving group (18Olg = 1.029 +/- 0.002, 15N = 1.0026 +/- 0.0002) indicate significant fission of the P-O ester bond in the transition state of the rate-determining step. The data indicate that in contrast to uncomplexed diesters, which undergo hydrolysis by a concerted mechanism, the reaction of the complexed diester likely proceeds via an addition-elimination mechanism. The rate-limiting step is expulsion of the p-nitrophenyl leaving group from the intermediate, which proceeds by a late transition state with extensive bond fission to the leaving group. This represents a substantial change in mechanism from the hydrolysis of uncomplexed aryl phosphate diesters.     

Combining the lattice-sum and reaction-field approaches for evaluating long-range electrostatic interactions in molecular simulations

A new scheme, the lattice-sum-emulated reaction-field (LSERF) method, is presented that combines the lattice-sum (LS) and reaction-field (RF) approaches for evaluating electrostatic interactions in molecular simulations. More precisely, the LSERF scheme emulates a RF calculation (based on an atomic cutoff) via the LS machinery. This is achieved by changing the form of the electrostatic interactions in a standard LS calculation (Coulombic) to the form corresponding to RF electrostatics (Coulombic plus quadratic reaction-field correction term, truncated at the cutoff distance). It is shown (both analytically and numerically) that in the limit of infinite reciprocal-space accuracy, (i) the LSERF scheme with a finite reaction-field cutoff and a given reaction-field permittivity is identical to the RF scheme with the same parameters (and an atomic cutoff), and (ii) the LSERF scheme is identical to the LS scheme in the limit of an infinite reaction-field cutoff, irrespective of the reaction-field permittivity. This new scheme offers two key advantages: (i) from a conceptual point of view, it shows that there is a continuity between the RF and LS schemes and unifies them into a common framework; (ii) from a practical point of view, it allows us to perform RF calculations with arbitrarily large reaction-field cutoff distances for the same computational costs as a corresponding LS calculation. The optimal choice for the cutoff will be the one that achieves the best compromise between artifacts arising from the dielectric heterogeneity of the system (short cutoff) and its artificial periodicity (long cutoff). The implementation of the LSERF method is extremely easy, requiring only very limited modifications of any standard LS code. For practical applications to biomolecular systems, the use of the LSERF scheme with large reaction-field cutoff distances is expected to represent a significant improvement over the current RF simulations involving comparatively much shorter cutoffs.     

Biological Risk Assessment of Three Dental Composite Materials following Gas Plasma Exposure

Gas plasma is an approved technology that generates a plethora of reactive oxygen species, which are actively applied for chronic wound healing. Its particular antimicrobial action has spurred interest in other medical fields, such as periodontitis in dentistry. Recent work has indicated the possibility of performing gas plasma-mediated biofilm removal on teeth. Teeth frequently contain restoration materials for filling cavities, e.g., resin-based composites. However, it is unknown if such materials are altered upon gas plasma exposure. To this end, we generated a new in-house workflow for three commonly used resin-based composites following gas plasma treatment and incubated the material with human HaCaT keratinocytes in vitro. Cytotoxicity was investigated by metabolic activity analysis, flow cytometry, and quantitative high-content fluorescence imaging. The inflammatory consequences were assessed using quantitative analysis of 13 different chemokines and cytokines in the culture supernatants. Hydrogen peroxide served as the control condition. A modest but significant cytotoxic effect was observed in the metabolic activity and viability after plasma treatment for all three composites. This was only partially treatment time-dependent and the composites alone affected the cells to some extent, as evident by differential secretion profiles of VEGF, for example. Gas plasma composite modification markedly elevated the secretion of IL6, IL8, IL18, and CCL2, with the latter showing the highest correlation with treatment time (Pearson’s r > 0.95). Cell culture media incubated with gas plasma-treated composite chips and added to cells thereafter could not replicate the effects, pointing to the potential that surface modifications elicited the findings. In conclusion, our data suggest that gas plasma treatment modifies composite material surfaces to a certain extent, leading to measurable but overall modest biological effects.