PubMed Health⌕ Search

Biomedical subjects

Robert J Meier

Publications and source records attributed to Robert J Meier.

9 recordsLinked to original sources
PubMed2006

Detecting the effect of very low amounts of penetrants in lipid bilayers using Raman spectroscopy.

We show that, by the influence on the gauche content in the acyl chains, the effect of very low concentrations (order 10(-2) %) of penetrants (here 2,4-dichlorophenol) in lipid bilayers (here dipalmitoyl-glycerophosphatidylcholine) can be experimentally seen in the Raman spectra. The observed effects suggest increased order and are in accordance with previous X-ray diffraction and calorimetric data. These findings show great promise for studying in situ the effect of low amounts of various small molecules on cell membranes using Raman spectroscopy.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2006

The phase transition behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) model membrane influenced by 2,4-dichlorophenol--an FT-Raman spectroscopy study.

The effect of 2,4-dichlorophenol (DCP) on the structures and phase transitions of fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes was studied using FT-Raman spectroscopy. Whereas the Raman frequency shifts of the most frequently investigated bands of C-C and C-H stretching regions only indicate the main phase transition (P(beta')-L(alpha)) of the pure DPPC/water system, the Raman shift of C-H scissoring vibration at 1440 cm(-1) was found to be able to reveal the pretransition (L(beta')-P(beta')) as well. Analyzing the spectral parameters of the trans band at 1128 cm(-1), which does not overlap with DCP vibrational modes, a continuous decrease of trans conformations was found with increasing DCP concentration at 26 degrees C accompanying the phase transitions L(beta')-P(beta') and P(beta')-L(alpha). The intensity ratio of the symmetrical and asymmetrical methylene stretching bands (at 2850 cm(-1) and 2880 cm(-1)), defined as the disorder parameter by Levin [Levin, I.W., 1985. Two types of hydrocarbon chain interdigitation in sphingomielin bilayers. Biochemistry 24, 6282-6286], indicated that in the interdigitated phase (L(I)) the order is markedly high and comparable with that of L(beta). Both the phase transition P(beta')-L(alpha) in the DCP/DPPC molar ratio range of 10/100-50/100 and the phase transition L(I)-L(alpha) led to a significant increase of disordered chains and the presence of DCP molecules induced a more disordered chain region than that observed in the L(alpha) phase of DPPC. Nevertheless, it was found that the L(alpha) phase with DCP contains approximately the same amount of trans conformers than that without DCP.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2005

Vibrational spectroscopy: a 'vanishing' discipline?

The aim of this tutorial review is to convince a broad readership that vibrational spectroscopy, although according to some vibrational spectroscopists seemingly less in focus nowadays than in days past, is far from 'dead'. It may seem to some that infrared and Raman spectroscopy are less in focus than in times past, despite the unique analytical capabilities. Vibrational spectroscopy is particularly powerful for non-destructive characterisation of substances, including living material. But compared to the past, a shift in applications has taken place, bringing new opportunities. This is partly due to the introduction of new features, including imaging and 2D correlation spectroscopy. Another factor is the recognition that vibrational spectroscopy can play a role in new rather than only in the traditional fields of application, e.g. new applications in the life-science field (living cells, cancer research), the characterisation of soil. But also the traditional application in catalysis sees new development within the context of Operando spectroscopy.

Animals↗
PubMed2003

EPR and modelling studies of hydrogen-abstraction reactions relevant to polyolefin cross-linking and grafting chemistry.

EPR spectroscopy has been employed to study directly the selectivity of hydrogen-atom abstraction by some alkoxyl radicals from a variety of linear and branched alkanes, as well as linear alkenes, chosen as models for low molecular-weight polyolefin cross-linking systems. In situ thermal and photolytic approaches, as well as spin-trapping, have been employed to provide information relating to an accessible temperature range of 233-453 K. in part to mimic conditions relevant to melt processing of polyolefins. Rate constants (in the range 3 x 10(3)-3.7 x 10(5) dm3 mol-1 s-1 per hydrogen) have been determined for C-H abstraction at room temperature. Radical selectivity is largely governed by enthalpic effects (modelled via bond dissociation energy calculations and kinetic analysis). Direct evidence has been obtained for lack of reactivity, as a result of unfavourable steric interactions, for the secondary and tertiary C-H bonds in 2,4-dimethylpentane and 2,4,6-trimethylheptane, models for polypropylene. This has been rationalized via free-energy calculations using DFT.

Journal Article↗
PubMed2003

A recent development in computational chemistry: chemical reactions from first principles molecular dynamics simulations.

First-principles molecular dynamics simulations have recently been found an effective tool to study a large variety of chemical problems. Finite temperature simulations reveal unique information, including explicit dynamical effects and the evaluation of proper free energy differences. Moreover, dynamics simulations reveal information on the flexibility of molecular systems, and elucidate, often otherwise inaccessible, mechanistic details of chemical reactions. In addition this methodology allows the study of larger, periodic, systems, revealing computationally unique information which may be directly compared to experiments on realistic chemical systems. A variety of examples will be given, although most focus on the important field of catalysis.

Biochemistry↗
PubMed2003

Are current DFT methods sufficiently reliable for real-world molecular systems?

Whereas density functional theory (DFT) based computations are the main tools in contemporary computational chemistry, there is an issue of chemical accuracy and reliability which, one the one hand, is recognised but, on the other hand, is not always sufficiently dealt with when it comes to application. Examples are given where DFT apparently fails. It is suggested that an effort needs to be undertaken to thoroughly investigate the limitations of current DFT calculations.

Journal Article↗
PubMed2003

Surface-sensitive molecular information on polymers from UV-resonance Raman spectroscopy.

Ultraviolet resonance Raman spectroscopy (UV-RRS) has been applied to a series of samples of poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) and poly(ethylene terephthalate) (PET) of varying thickness. The spectra demonstrate that under resonance conditions, when absorption is very strong, only a very thin top layer of the sample is probed (hundreds of nanometers range). This allows probing molecular vibrational spectra of the top layer of the sample, with a surface-resolution at least an order of magnitude better than in the case of normal non-resonance Raman spectroscopy and using a microscope.

Crystallography↗
PubMed2003

Human skull from the Taza locality, Jijel, Algeria.

Archaeological excavations were carried out in 1990 in prehistorically inhabited caves sites in the Taza locality along the eastern Algerian coast. Recovered materials included stone and bone artifacts, faunal remains and a single human skull (Taza 1). A radiocarbon date for the upper cave level from which the skull was found was 16,100 B.P. While the skull, which is likely female, is very small overall, its shape compared closely with other North African cranial series. The obtained date and archaeological materials are characteristic of the Iberomaurusian industry of the Late Paleolithic period of North Africa.

Adult↗