Determination of orientational order parameters in oriented lipid membrane systems by angle-resolved fluorescence depolarization experiments

Angle-resolved fluorescence depolarization experiments have been performed on some lipid membrane systems in order to test the validity of a recently developed theory describing this type of experiments. Good agreement between theory and experiment was found. The experiments yield the order parameters 〈P₂〉 and 〈P₄〉 and a rotational correlation time τ_c. On applying an information-theoretic form of the orientational distribution function. 〈P₂〉 and 〈P₄〉 were used to estimate the degree of order in the systems studied. Knowledge of 〈P₄〉 proved useful to get more detailed information on the orientational order. Another finding was that the angle between the absorption and emission moment of the used fluorescent probe, diphenyl-hexatriene, depends on the membrane system in which it is incorporated. Finally, the experimental results point to the fact that for some membrane systems the molecular motion is inadequately described by the simple strong-collision model.     

On the thermodynamic limit of photovoltaic energy conversion

An infinite stack ofp—n junctions with smoothly varying bandgap from ∞ to 0 is considered. AnI —V characteristic is derived, which is more correct than the classical exponential characteristic. It is shown that open-circuit operation is a reversible process and leads to the Carnot efficiency, if one defines the efficiency in the way that is usual in the theory of thermodynamic engines. If instead one uses the definition of efficiency usual in photovoltaics, open-circuit mode gives rise to zero efficiency. Then operation at maximum efficiency equals operation at maximum power and is not reversible.     

Penalty for impulse noise, derived from annoyance ratings for impulse and road-traffic sounds

In the laboratory, four groups of 16 subjects rated the annoyance caused by three types of impulse sounds (regular and irregular gunfire noise and metal-construction noise) and by road-traffic sounds, all presented in background noise. The subjects were presented with the sounds for 5-min periods. The annoyance ratings were related to the A-weighted equivalent level (Leq) of the sounds. From these annoyance ratings a correction term or penalty was derived, which, added to the Leq of the impulse sounds, gives the level of equally annoying traffic noise. The correction was determined for conditions in which (1) only the annoyance caused by specific sources, or (2) the annoyance caused by the total sound (specific source plus background) had to be rated. In addition, the indoor Leq of the constantly present background noise was 35 or 55 dB(A) by and large, the results showed that for lower levels of the sounds an impulse-noise correction of at least 10 dB was required, whereas for higher levels the derived correction was about equal to the ISO/R 1996 penalty of 5 dB. This conclusion, based on the relation between Leq and annoyance ratings, is consistent with the correction based on Leq and the percentage of subjects who reported to be "very much annoyed." For equivalent levels of the impulse sounds at which 33% of the subjects claimed to be very much annoyed, the correction was 10 dB for the conditions in which the indoor Leq of the background noise was 35 dB(A), and 5 dB when this Leq was 55 dB(A).(ABSTRACT TRUNCATED AT 250 WORDS)     

Trigonal pyramidal carbon geometry as model for electrophilic addition–substitution and elimination reactions and its significance in enzymatic processes

Various examples are given in which compounds are characterized as products or intermediates in a (distorted) trigonal pyramidal (TP) geometry. These observations have taken place mainly in the field of carbocation chemistry. Special attention is given to carbenium ions formed by halogen addition to 1,1‐diarylsubstituted ethylenes focused on the electronic effects of the C‐halogen bond as axial bond in a TP geometry with regard to the π‐distribution in the rest of the molecular system. The experimental verification is accompanied by quantum chemical calculations. We also used the TP structure as a reactive model for specific enzymatic reactions. The relevance of this geometry is shown for the dehalogenation reaction of the nucleophilic displacement in dichloroethane catalyzed by haloalkane dehalogenase and for the decarboxylation of L ‐ornithine with ornithine decarboxylase under loss of carbon dioxide. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

The influence of subinhibitory concentrations of ampicillin and vancomycin on physico-chemical surface characteristics of Enterococcus faecalis 1131

The effects of growth in the presence of sub-inhibitory concentrations of ampicillin and vancomycin on physico-chemical cell surface properties of Enterococcus faecalis 1131 have been determined. Growth in the presence of antibiotics yielded increased exposure of nitrogen and oxygen at the cell surface, proportional to the amount of antibiotic added to the growth medium, probably as a result of progressive removal of lipoteichoic acid (LTA) by ampicillin and vancomycin. Bacterial isoelectric points (IEP's), derived from particulate microelectrophoresis at different suspension pH, increased concurrently. Water contact angles on bacterial lawns only varied slightly in the presence of antibiotic during growth, whereas formamide contact angles after growth in the presence of vancomycin were significantly higher than after growth in the absence of this antibiotic, yielding a strongly electron-donating character of the cell surface. Surface thermodynamical analyses indicated favourable conditions for adhesion to hexadecane and chloroform, but more negative values of interaction free energies did not necessarily coincide with increased adhesion. Bacterial adhesion to hexadecane and chloroform increased when the pH of the bacterial suspension approached the isoelectric point of the organisms, because of the minimal electrostatic interactions.     

Characterization of leukemic and normal white blood cells by Curie-point pyrolysis—mass spectrometry: II. Biochemical interpretation of some of the differences in the pyrolysis patterns

Differences found in nucleotide and lipid related mass peaks in the Curie-point pyrolysis—mass spectra of leukemic and normal white blood cells were further investigated using several different pyrolysis-mass spectrometric (Py—MS) modes. Time-resolved pyrolysis profiles of mass peaks 114 and 116 in the white blood cell spectra were found to match the time profiles of the corresponding peaks in the pyrolysis—mass spectra of RNA and DNA respectively. Visual comparison of spectra from standard samples of RNA, DNA, lecithin and cholesterol with the white blood cell spectra further defined the mass peaks contributed by these compounds. Direct probe MS of low-volatile pyrolysis condensates from white blood cells and standard compounds demonstrated the presence of characteristic mass peaks representing nucleic acid components, e.g. bases and nucleoside fragments, as well as of phospholipids and cholesterol. The strong changes observed in the relative abundance of these components between leukemic and normal white blood cells paralleled changes seen in the Curie-point pyrolysis—mass spectra. Finally, the use of collision induced dissociation. MS (MS/MS) enabled confirmation of the identity of individual compounds, e.g. thymine, in white blood cells pyrolyzates. This finding appears to indicate the possibility of direct measurement of DNA content of whole cells by Py-MS/MS techniques.     

Low-spin state structure of [Fe(chloroethyltetrazole)6](BF4)2 obtained from synchrotron powder diffraction data

The complex [Fe(teec)6](BF4)2 (teec = chloroethyltetrazole) shows a two-step complete spin-crossover transition in the temperature range 300-90 K. Time-resolved synchrotron powder diffraction experiments have been carried out in this temperature range, and crystal structure models have been obtained from the powder patterns by using the parallel tempering technique. Of these models, the low-spin state structure at 90 K has been refined completely with Rietveld refinement. Its structural characteristics are discussed in relation to the high-spin state model and other spin-crossover compounds. The complex shows a remarkable anisotropic unit-cell parameter contraction that is dependent on the applied cooling rate. In addition, the possible important implications for the interpretation of spin-crossover behavior in terms of structural changes are discussed.     

The agonistic binding site at the histamine H2 receptor. I. Theoretical investigations of histamine binding to an oligopeptide mimicking a part of the fifth transmembrane α-helix

Summary Mutation studies on the histamine H₂ receptor were reported by Gantz et al. [J. Biol. Chem., 267 (1992) 20840], which indicate that both the mutation of the fifth transmembrane Asp¹⁸⁶ (to Ala¹⁸⁶) alone or in combination with Thr¹⁹⁰ (to Ala¹⁹⁰) maintained, albeit partially, the cAMP response to histamine. Recently, we have shown that histamine binds to the histamine H₂ receptor as a monocation in its proximal tautomeric form, and, moreover, we suggested that a proton is donated from the receptor towards the tele-position of the agonist, thereby triggering the biological effect [Nederkoorn et al., J. Mol. Graph., 12 (1994) 242; Eriks et al., Mol. Pharmacol., 44 (1993) 886]. These findings result in a close resemblance with the catalytic triad (consisting of Ser, His and Asp) found in serine proteases. Thr¹⁹⁰ resembles a triad's serine residue closely, and could also act as a proton donor. However, the mutation of Thr¹⁹⁰ to Ala¹⁹⁰ — the latter is unable to function as a proton donor — does not completely abolish the agonistic cAMP response. At the fifth transmembrane -helix of the histamine H₂ receptor near the extracellular surface, another amino acid is present, i.e. Tyr¹⁸², so an alternative couple of amino acids, Tyr¹⁸² and Asp¹⁸⁶, could constitute the histamine binding site at the fifth -helix instead of the (mutated) couple Asp¹⁸⁶ and Thr¹⁹⁰. In the first part of our present study, this hypothesis is investigated with the aid of an oligopeptide with an -helical backbone, which represents a part of the fifth transmembrane helix. Both molecular mechanics and ab initio data lead to the conclusion that the Tyr¹⁸²/Asp¹⁸⁶ couple is most likely to act as the binding site for the imidazole ring present in histamine.