The enantiospecific synthesis of (4S,5R)-and (4S,5S)-4,5-dihydroxydecanoic acid γ-lactones,proposed autoregulators from Streptomyces griseus

Both the (4S,5R)- and (4S,5S)-isomers of 4,5-dihydroxydecanoic acid γ-lactone, natural products and proposed autoregulators of Streptomyces Griseus, have been synthesised directly and in high yield from D-ribose via a common intermediate.     

A model of head-related transfer functions based on principal components analysis and minimum-phase reconstruction.

Free-field to eardrum transfer functions (HRTFs) were measured from both ears of 10 subjects with sound sources at 265 different positions. A principal components analysis of the resulting 5300 HRTF magnitude functions revealed that the HRTFs can be modeled as a linear combination of five basic spectral shapes (basis functions), and that this representation accounts for approximately 90% of the variance in the original HRTF magnitude functions. HRTF phase was modeled by assuming that HRTFs are minimum-phase functions and that interaural phase differences can be approximated by a simple time delay. Subjects' judgments of the apparent directions of headphone-presented sounds that had been synthesized from the modeled HRTFs were nearly identical to their judgments of sounds synthesized from measured HRTFs. With fewer than five basis functions used in the model, a less faithful reconstruction of the HRTF was produced, and the frequency of large localization errors increased dramatically.     

Linear and angular annelation effects on the 13C NMR spectra of tricyclic aromatic ketones. 1—annelated indanones

Carbon-13 NMR chemical shifts are reported for nine tricyclic aromatic ketones formally derived from indanone. The influences of remote ring size, as well as linear, angular exo and angular endo ketone orientation are examined. Results are compared with available ¹H NMR data. For indanone itself, based on selective ¹H decoupling experiments, a recently reported CIDNP derived ¹³C signal assignment is shown to be in error.     

Resolution of front-back ambiguity in spatial hearing by listener and source movement.

Normally, the apparent position of a sound source corresponds closely to its actual position. However, in some experimental situations listeners make large errors, such as indicating that a source in the frontal hemifield appears to be in the rear hemifield, or vice versa. These front-back confusions are thought to be a result of the inherent ambiguity of the primary interaural difference cues, interaural time difference (ITD) in particular. A given ITD could have been produced by a sound source anywhere on the so-called "cone of confusion." More than 50 years ago Wallach [J. Exp. Psychol. 27, 339-368 (1940)] argued that small head movements could provide the information necessary to resolve the ambiguity. The direction of the change in ITD that accompanies a head rotation is an unambiguous indicator of the proper hemifield. The experiments reported here are a modern test of Wallach's hypothesis. Listeners indicated the apparent positions of real and virtual sound sources in conditions in which head movements were either restricted or encouraged. The front-back confusions made in the restricted condition nearly disappeared in the condition in which head movements were encouraged. In a second experiment head movements were restricted, but the sound source was moved, either by the experimenter or by the listener. Only when the listener moved the sound source did front-back confusions disappear. The results clearly support Wallach's hypothesis and suggest further that head movements are not required to produce the dynamic cues needed to resolve front-back ambiguity.     

Moisture effects in lithium-doped polyimide films

The interaction of lithium-doped polyimide films with water has been investigated. Contact angle and water absorption/desorption measurements have been performed to obtain an idea of the hygroscopic nature of these lithium-doped films. Contact angles were found to decrease as the amount of surface lithium increased. Film water content was also found to increase as the weight percent of total lithium content in the film increased. This phenomenon has been used to rationalize the electrical resistivity–temperature profile of certain lithium films which display an increase in electrical resistivity upon drying.     

氨基葡聚糖对水溶液中铜离子的吸附与脱附

自虾、蟹壳等水产加工废料中提取的甲壳质,经脱乙酰基反应,可得氨基葡聚糖,单体结构可表示为左图。该碱性多糖无毒,不溶于水及碱性溶液,在pH～4.5的稀酸中会溶涨,酸性更强时可溶解并成盐。若要求该聚合物以稳定的固态存在于水中,介质的酸性只允许在很小的范围内变化。聚糖中的氨基与过渡金属离子有良好的螯合作用,可作为固体吸附剂吸附水中微量的有害重金属离子。据文献报导,这类吸附大多呈Langmuir型,但Pb(Ⅱ)与Cr(Ⅲ)是例外,它们的吸附等温线表现出单层吸附饱和后,又呈现多层吸附的特征。扫描电镜的照片表明聚糖吸附Pb(Ⅱ)、Cr(Ⅲ)后,表面有瘤状小结节生成。溶液pH升高有利于重     

C-Nucleoside studies-15: Synthesis of 3-β-D-arabinofuranosylpyrazoles and the D-arabinofuranosyl analogue of formycin

A synthesis of 7-amino-3-(β -D-arabinofuranosyl)pyrazolo[4 3-d]pyrimidine (13), the D-arabino analogue of formycin (1), from D-mannose is described. 1-(2,4-Dinitrophenyl)-3-(penta-O-acetyl-D-manno-pentahydroxypentyl)pyrazole was converted into the 4-nitro-derivative 18 in 90% yield using trifluoroacetyl nitrate. In four steps 18 was transformed into the 3-β-D-arabinofuranosylpyrazole 12 which gave the N-nitro derivative 24 in two further stages. Cine-substitution of the N-nitro group in 24 by cyanide ion was followed by elaboration of the pyrimidine ring to give 13. All the reactions proceeded with complete regio and stereoselectivity.     

Obtention selective de synthons nucleosidiques utilisables en synthese oligoribonucleotidique : differenciation 2′,3′ en serie nucleosidique

A practical regio- and stereoselective synthesis of asymmetrically substituted ribonucleosides is described in which glycosylation of ribofuranoid 2-O-esters (alkyl or silyl) with either pyrimidines or purines is the key step.     

Analysis of chemical dynamics in microenvironments

A variety of techniques exist that provide chemical information in the form of a spatially resolved image. For example, magnetic resonance imaging has significant advantages for biological applications because it allows measurements in intact tissues. However, it is restricted at the present time to species of high concentration. The most common approach for chemically selective imaging is to use optical microscopy coupled with fluorescence measurements. A variety of fluorophores with high selectivity for specific ions are commercially available and are readily used in such applications. While imaging techniques provide an overall view of the microenvironment, probes that give a high fidelity view of a specific species and its concentration at a single location are also useful. Thus, there is particular interest in developing local sampling techniques. Electrochemical techniques, both potentiometric and amperometric, are particularly useful in this regard. For example, neurotransmitters, compounds secreted from nerve terminals to relay information from one neuron to another, can readily be examined with electrodes of micrometer dimensions. In this way, the mechanisms that control the secretion and the regulation of these compounds in the extracellular space can be evaluated. These tools can also be used at the level of single cells, and when placed at different locations, can be used to provide a chemical image. This approach essentially represents a marriage of electrochemical techniques with those of the scanning microscopies, an area that will see considerable growth in the new millennium.     

Photoionization of Al IV (neon-type) from its ground 2s 22p 6 1 S e and excited 2s 22p 53p 3,1 P 0 3 S e states using the R-matrix method

The R-matrix method is used to calculate the total photoionization cross-sections from the ground 2s ²2p ^{6 1} S ^e and the first three excited 2s ²2p ⁵3p ^3,1 S ^e states of Al IV, for photon energies ranging from the first ionization threshold to just above the second threshold of the residual ion Al V. The two lowest LS terms of Al V − 2s ²2p ^{5 2} P ⁰, 2s2p ^{6 2} S ^e, 2s2p ^{6 2} S ^e, represented by sophisticated configuration interaction wavefunctions, are included in the R-matrix calculation. The resulting cross-sections are affected by Rydberg series of resonances converging to the 2s2p ^{6 2} S ^e excited threshold.