Spectrophotometric determination of chromium(VI) as dichromate after extraction with the pentamethylene-bis(triphenylphosphonium) cation

Chromium(VI) (0–100 μg) can be determined spectrophotometrically at 365 nm after its extraction as pentamethylene-bis(triphenylphosphonium) dichromate into 1,2-dichloroethane. The system is applied to the determination of chromium in a range of steels.     

Temperature-responsive protein pores

We describe temperature-responsive protein pores containing single elastin-like polypeptide (ELP) loops. The ELP loops were placed within the cavity of the lumen of the alpha-hemolysin (alphaHL) pore, a heptamer of known crystal structure. The cavity is roughly spherical with a molecular surface volume of about 39,500 A3. In an applied potential, the wild-type alphaHL pore remained open for long periods. In contrast, the ELP loop-containing alphaHL pores exhibited transient current blockades, the nature of which depended on the length and sequence of the inserted loop. Together with similar results obtained with poly(ethylene glycols) covalently attached within the cavity, the data suggest that the transient current blockades are caused by excursions of ELP into the transmembrane beta-barrel domain of the pore. Below its transition temperature, the ELP loop is fully expanded and blocks the pore completely, but reversibly. Above its transition temperature, the ELP is dehydrated and the structure collapses, enabling a substantial flow of ions. Potential applications of temperature-responsive protein pores in medical biotechnology are discussed.     

Photoisomerization of an individual azobenzene molecule in water: an on-off switch triggered by light at a fixed wavelength

The alpha-hemolysin (alphaHL) pore was used as a nanoreactor for the direct observation of the reversible photoisomerization of individual tethered azobenzene molecules in an aqueous environment. alphaHL pores, PAZO, were used that had been derivatized within the lumen at a single cysteine residue with 4-((4-(2-chloroethanoamido)phenyl)diazenyl)benzenesulfonate. Trans-cis isomerizations were monitored at the single-molecule level by observing the modulation of the current passing through PAZO by electrical recording in planar bilayers. When PAZO was irradiated at 330 nm, continuous interconversion between the trans and cis states was observed. Either the trans or the cis state was maintained in the dark, depending upon which was present when the light source was shuttered. The cis state of PAZO was surprisingly stable in the dark, and no cis --> trans transitions were seen over a total observation period of more than 8 h. Therefore, based on our findings, it might be possible to make fast digital nanoscale switches operated by light of a fixed wavelength.     

The CF bond as a tool in the conformational control of amides

The conformation of 2-fluoro-N-(2-fluoroethyl)-propionamide 4 in the solid state indicates the influence of both a β-fluorine-amide gauche effect and an α-fluoroamide effect. The structure reveals the influence of two recently observed stereoelectronic effects associated with the CF bond, which has resulted in the successful prediction of the solid state conformation of amide 4. A gauche relationship (−69.9°) was observed for atoms N(1)C(4)C(5)F and a syn planar (2.0°) relationship was observed for N(1)C(3)C(2)F. The paper demonstrates the predictive power of using the CF bond as a tool in influencing the conformation of amides and peptides.     

ALKALI-LABILE LESION and URACIL-DNA-GLYCOSYLASE-SENSITIVE SITE REMOVAL AFTER BrdUrd and UVB TREATMENT OF CHINESE HAMSTER CELLS*

Abstract— A marked cell-cycle dependency for the recovery of cells after BrdUrd/UVB treatment has led us to look for similar characteristics in the molecular events associated with DNA repair. Such characteristics are reported here for the repair of alkali-labile lesions. When asynchronously growing cells were uniformly substituted with BrdUrd (a condition that results in the greatest cell killing), the repair kinetics followed a simple exponential response with a half-time of approximately 17min. However, when lesions were restricted to3–5% of the genome and the repair observed in the mid-S phase (a condition associated with sublethal damage repair), the DNA repair kinetics were complex. The rejoining of the DNA was biphasic with greater than 90% of the lesion with a half-time of less than6–7 min. Uracil removal followed similar kinetics. Caffeine, a potent inhibitor of cell survival after BrdUrd/UVB treatment, had no measurable effect on either uracil removal or alkali-labile lesion repair.     

Photoelectron spectroscopic and thermodynamic studies of the free surface of benzyl alcohol

He(I) photoelectron spectra of the liquid surface and the vapour of benzyl alcohol (C₆H₅CH₂OH) are reported. The gas-phase spectrum resembles that of benzene except that between the 1e_1g(π) and the 3e_2g benzenoid bands there is an additional peak in benzyl alcohol at 10.69 eV. In the spectrum of the liquid the additional peak is either absent or of much reduced intensity and this, together with consideration of the spectra of related molecules, leads to its assignment as the ionization of a “lone pair” orbital. In the condensed phase the lone pair electrons become involved in hydrogen bonding. Considerations of the excess energy and entropy of the surface of liquid benzyl alcohol point to a surface in which the hydrogen bonds are not broken, in agreement with the spectroscopic result.