Determination of lysine pK values using [5-13C]lysine: application to the lyase domain of DNA Pol beta

Determination of the protonation state of titratable protein residues is of critical importance for the interpretation of active site chemistry, as well as for understanding the role of electrostatic interactions in protein folding and stability. However, protein titration studies are limited by the fact that, at extreme pH values, increasing fractions of unfolded or partially unfolded structures may be present. This problem is particularly acute for lysine residues which have high pK values. In the present study, we point out that the use of the 13C resonance of lysine C-5 as a reporter for titration of the epsilon-amino group is preferable to the use of C-6 due to the 5-fold greater titration shift, so that reasonable results can be obtained using a two parameter fit of data obtained over a more limited pH range. A new synthetic procedure for [5-13C]lysine is described, and the pK value for Lys72 in the lyase domain of DNA polymerase beta has been determined using the [5-13C]lysine-labeled enzyme. The results agree well with recent studies of the Pol lambda lyase domain, demonstrating that the pK value for this residue is not optimized for Schiff base chemistry (Gao et al., Biochemistry 2006, 45, 1785-1794). We also have re-evaluated data for the pK of Lys73 in the TEM-1 beta-lactamase.     

Photoinduced charge carrier generation in a poly(3-hexylthiophene) and methanofullerene bulk heterojunction investigated by time-resolved terahertz spectroscopy

We report on the ultrafast photoinduced charge separation processes in varying compositions of poly(3-hexylthiophene) (P3HT) blended with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Through the use of time-resolved terahertz spectroscopy, the time- and frequency-dependent complex photoconductivity is measured for samples with PCBM weight fractions (WPCBM) of 0, 0.2, 0.5, and 0.8. By analysis of the frequency-dependent complex conductivity, both the charge carrier yield and the average charge carrier mobility have been determined analytically and indicate a short (<0.2 nm) carrier mean free path and a suppressed long-range transport that is characteristic of carrier localization. Studies on pure films of P3HT demonstrate that charge carrier generation is an intrinsic feature of the polymer that occurs on the time scale of the excitation light, and this is attributed to the dissociation of bound polaron pairs that reside on adjacent polymer chains due to interchain charge transfer. Both interchain and interfacial charge transfer contribute to the measured photoconductivity from the blended samples; interfacial charge transfer increases as a function of increasing PCBM. The addition of PCBM to the polymer films surprisingly does not dramatically increase the production of charge carriers within the first 2 ps. However, charge carriers in the 0.2 and 0.5 blended films survive to much longer times than those in the P3HT and 0.8 films.     

Ultraviolet Spectral Energy Differences Affect the Ability of Sunscreen Lotions to Prevent Ultraviolet-Radiation-Induced Immunosuppression*

Acute exposure to UV radiation causes immunosuppression of contact hypersensitivity (CH) responses. Past studies conducted with unfiltered sunlamps emitting nonsolar spectrum UV power (wavelengths below 295 nm) or using excessive UV doses have suggested sunscreens may not prevent UV-induced immunosuppression in mice. This study was thus designed to evaluate critically the effects of different UV energy spectra on the immune protection capacity of sunscreen lotions. Minimum immune suppression doses (MISD), i.e. the lowest UV dose to cause~50% suppression of the CH response to dinitrofluorobenzene in C3H mice, were established for three artificial UV sources. The MISD for each UV source was 0.25 kJ/m² for unfiltered FS20 sunlamps (FS), 0.90 kJ/m² for Kodacel-filtered FS20 sunlamps (KFS), which do not emit UV power at wavelengths <290 nm, and 1.35 kJ/m² for a 1000 W filtered xenon arc lamp solar simulator. Using MISD as baseline, sunscreens with labeled sun protection factors (SPF) of 4, 8, 15 and 30 were tested with each UV source to establish their relative immune protection factors. The immune protection factor of each sunscreen exceeded its labeled SPF in tests conducted with the solar simulator, which has a UV power spectrum (295–400 nm) similar to that of sunlight. Conversely, sunscreen immune protection factors were significantly less than the labeled SPF in tests conducted with FS and KFS. Comparison of the immunosuppression effectiveness spectra showed that relatively small amounts of nonsolar spectrum UV energy, i.e. UVC (200–290 nm) and/or shorter wavelength UVB (between 290 and 295 nm), produced by FS and KFS contributes significantly to the induction of immunosuppression. For example, 36.3% and 3.5% of the total immunosuppressive UV energy from FS and KFS, respectively, lies below 295 nm. Sunscreen absorption spectra showed that transmission of immunosuppressive UV energy below 295 nm for FS was at least eight-fold higher than that for KFS. Compared to the solar simulator UV spectrum the transmission of nonsolar immunosuppressive UV energy through sunscreens was >15-fold higher for FS and ≥1.5-fold higher for KFS. These data demonstrate that relevant evaluations of sunscreen immune protection can only be obtained when tests are conducted with UV sources that produce UV power spectra similar to that of sunlight and UV doses are employed that are based on established MISD.     

Solar cells from colloidal nanocrystals: Fundamentals,materials, devices,and economics

Recent advances in colloidal science are having a dramatic impact on the development of next generation low-cost and/or high-efficiency solar cells. Simple and safe solution phase syntheses that yield monodisperse, passivated, non-aggregated semiconductor nanocrystals of high optoelectronic quality have opened the door to several routes to new photovoltaic devices which are currently being explored. In one route, colloidal semiconductor nanocrystal “inks” are used primarily to lower the fabrication cost of the photoabsorbing layer of the solar cell. Nanocrystals are cast onto a substrate to form either an electronically coupled nanocrystal array or are sintered to form a bulk semiconductor layer such that the bandgap of either is optimized for the solar spectrum (1.0–1.6 eV if the photon to carrier quantum yields less than 100%). The sintered devices (and without special efforts, the nanocrystal array devices as well) are limited to power conversion efficiencies less than the Shockley–Queisser limit of 33.7% but may possibly be produced at a fraction of the manufacturing cost of an equivalent process that uses vacuum-based deposition for the absorber layer. However, some quantum confined nanocrystals display an electron-hole pair generation phenomena with greater than 100% quantum yield, called “multiple exciton generation” (MEG) or “carrier multiplication” (CM). These quantum dots are being used to develop solar cells that theoretically may exceed the Shockley–Queisser limit. The optimum bandgap for such photoabsorbers shifts to smaller energy (0.6–1.1 eV), and thus colloidal quantum dots of low bandgap materials such as PbS and PbSe have been the focus of research efforts, although multiple exciton generation has also been observed in several other systems including InAs and Si. This review focuses on the fundamental physics and chemistry of nanocrystal solar cells and on the device development efforts to utilize colloidal nanocrystals as the key component of the absorber layer in next generation solar cells. Development efforts are put into context on a quantitative and up-to-date map of solar cell cost and efficiency to clarify efforts and identify potential opportunities in light of technical limitations and recent advances in existing technology. Key nanocrystal/material selection issues are discussed, and finally, we present four grand challenges that must be addressed along the path to developing low-cost high-efficiency nanocrystal based solar cells.     

Debenzylation using catalytic hydrogenolysis in trifluoroethanol, and the total synthesis of (−)-raumacline

N-Debenzylations using catalytic hydrogenolysis often fail to take place unless alcoholic solvents are used, but this can lead to N-alkylation as a side reaction; using trifluoroethanol as the solvent overcomes this problem, and leads to highly reliable hydrogenolyses for a wide range of substrates, including the final deprotection step in our total synthesis of (−)-raumacline.     

Explosivstoffe

Analytical and Bioanalytical Chemistry -     

Harze

Ohne Zusammenfassung     

Two-nucleon momentum distributions measured in 3He(e,e'pp)n

We have measured the 3He(e,e'pp)n reaction at 2.2 GeV over a wide kinematic range. The kinetic energy distribution for "fast" nucleons (p>250 MeV/c) peaks where two nucleons each have 20% or less, and the third nucleon has most of the transferred energy. These fast pp and pn pairs are back to back with little momentum along the three-momentum transfer, indicating that they are spectators. Calculations by Sargsian and by Laget also indicate that we have measured distorted two-nucleon momentum distributions by striking one nucleon and detecting the spectator correlated pair.     

Observation of an exotic S = +1 baryon in exclusive photoproduction from the deuteron

In an exclusive measurement of the reaction gammad-->K(+)K(-)pn, a narrow peak that can be attributed to an exotic baryon with strangeness S=+1 is seen in the K(+)n invariant mass spectrum. The peak is at 1.542+/-0.005 GeV/c(2) with a measured width of 0.021 GeV/c(2) FWHM, which is largely determined by experimental mass resolution. The statistical significance of the peak is (5.2+/-0.6)sigma. The mass and width of the observed peak are consistent with recent reports of a narrow S=+1 baryon by other experimental groups.