Odour sampling. 2. Comparison of physical and aerodynamic characteristics of sampling devices: a review

Sampling devices differing greatly in shape, size and operating condition have been used to collect air samples to determine rates of emission of volatile substances, including odour. However, physical chemistry principles, in particular the partitioning of volatile substances between two phases as explained by Henrys Law and the relationship between wind velocity and emission rate, suggests that different devices cannot be expected to provide equivalent emission rate estimates. Thus several problems are associated with the use of static and dynamic emission chambers, but the more turbulent devices such as wind tunnels do not appear to be subject to these problems. In general, the ability to relate emission rate estimates obtained from wind tunnel measurements to those derived from device-independent techniques supports the use of wind tunnels to determine emission rates that can be used as input data for dispersion models.     

Physical chemistry of polyglutamine: intriguing tales of a monotonous sequence

Polyglutamine (polyQ) sequences of unknown normal function are present in a significant number of proteins, and their repeat expansion is associated with a number of genetic neurodegenerative diseases. PolyQ solution structure and properties are important not only because of the normal and abnormal biology associated with these sequences but also because they represent an interesting case of a biologically relevant homopolymer. As the common thread in expanded polyQ repeat diseases, it is important to understand the structure and properties of simple polyQ sequences. At the same time, experience has shown that sequences attached to polyQ, whether in artificial constructs or in disease proteins, can influence structure and properties. The two major contenders for the molecular source of the neurotoxicity implicit in polyQ expansion within disease proteins are a populated toxic conformation in the monomer ensemble and a toxic aggregated species. This review summarizes experimental and computational studies on the solution structure and aggregation properties of both simple and complex polyQ sequences, and their repeat-length dependence. As a representative of complex polyQ proteins, the behavior of huntingtin N-terminal fragments, such as exon-1, receives special attention.     

Statistical and biological consequences of preferential sampling in phytosociology: Theoretical considerations and a case study

Due to the long tradition of the Braun-Blanquet approach, many relevés using this approach have been made. Recent developments in vegetation-plot databases provide an opportunity to effectively use these relevés to study ecological problems as well. Opinions differ, however, concerning the applicability of these datasets, often with their use being restricted to exploration and hypothesis generation only. We assert that preferential sampling, which is characteristic of the Braun-Blanquet approach, means using a special definition of statistical population rather than non-random sampling. We present a case study, where consequences of using a preferential and non-preferential definition of statistical population are studied. Although the traits of stands that are preferred or avoided by the phytosociologist during preferential sampling can be identified, there are no general rules that could predict the difference between the preferential and non-preferential datasets obtained for the same object.     

Physical chemistry and evolution of salt tolerance in halobacteria

The cellular constituents of extremely halophilic bacteria not only tolerate high salt concentration, but in many cases require it for optical functioning. The characteristics affected by salt include enzyme activity, stability, allosteric regulation, conformation and subunit association. The salt effects are of two major kinds: electrostatic shielding of negative charges by cations at low salt concentration, and hydrophobic stabilization by salting-out type salts at high salt concentration. The composition of halobacterial proteins shows an excess of acidic amino acids and a deficiency of nonpolar amino acids, which accounts for these effects. Since the cohesive forces are weaker and the repulsing forces are stronger in these proteins, preventing aggregation in salt, these structures are no longer suited for functioning in the absence of high salt concentrations. Unlike these nonspecific effects, ribosomes in halobacteria show marked preference for potassium over sodium ions. To ensure the proper intracellular ionic composition, powerful ion transport systems have evolved in the halobacteria, resulting in the extrusion of sodium ions and their replacement by potassium. It is likely that such membrane transport system for ionic movements is a necessary requisite for salt tolerance.Proceedings of the Fourth College Park Colloquium on Chemical Evolution:Limits of Life, University of Maryland, College Park, 18–20 October 1978.     

Algal Polyunsaturated Fatty Acids: Food,Pheromones and Foul Odour

Algal polyunsaturated fatty acids (PUFAs) are known to be high‐energy, often‐essential resources to freshwater aquatic food webs. On the other hand, high PUFA cell content in some algal taxa has been linked with the production of derivatives that may act as pheromones, allelogens or toxins. It has been known for some time that these compounds function in marine chemical ecology, but recent evidence indicates that they may play similar roles in freshwater ecosystems. This paper presents field and laboratory studies of planktonic and periphytic communities from freshwater systems, and shows that these communities are rich sources of certain PUFAs and derivatives which may function in both positive and negative foodweb interactions. This also has important implications for surface water quality, as these compounds are potent sources of rancid fishy odours.     

Odour perception in honeybees: coding information in glomerular patterns

Major advances have been made during the past two years in understanding how honeybees process olfactory input at the level of their first brain structure dealing with odours, the antennal lobe (the insect analogue of the mammalian olfactory bulb). It is now possible to map physiological responses to morphologically identified olfactory glomeruli, allowing for the creation of a functional atlas of the antennal lobe. Furthermore, the measurement of odour-evoked activity patterns has now been combined with studies of appetitive odour learning. The results show that both genetically determined components and learning-related plasticity shape olfactory processing in the antennal lobe.     

Odour discrimination by frog olfactory receptors: a second study

Single unit activity of olfactory neuroreceptors was recordedin frogs. Stimulations with 20 pure chemicals delivered at knownconcentrations elicited excitatory and/or inhibitory responsesin 60 of the 76 recorded units. The responses exhibited varioustime patterns, partly depending on stimulus intensity. Longlasting after-effects were observed. Out of a total of 1520odour trials, 317 excited and 33 inhibited the cells, leadingto a receptor overall responsiveness of 23%. Various degreesof individual selectivity were encountered in the receptors;the greatest number responded to seven of the 20 odorants. Amarked tendency to stimulate the same receptors was observedfor several odorants. Three groups could be evidenced: benzene,anisole, dichlorobenzene and bromobenzene; camphor and cineole;tert-butyl alcohol, cyclohexanone and cyclohexanol. Fatty acidstended to be grouped. Sulphurous compounds elicited few responses,except tiophenol. Most of the neuroreceptors responded to odorantsbelonging to more than one odour group.     

Some considerations on the proper use of computational tools in transition metal chemistry

The current understanding of transition metal chemistry is reviewed placing the attention at the applications and applicability of computational quantum chemistry to the calculation and prediction of spectroscopic properties of transition metal complexes and molecular magnets.