Photoactivated h/d exchange in tyrosine: involvement of a radical anion intermediate

The aromatic hydrogen nuclei of tyrosine are photochemically labile and exchange with deuterons in neutral D(2)O solution. The site meta to the ring hydroxyl substituent is preferentially deuterated, exhibiting a meta/ortho deuteration rate of approximately 4:1. In contrast with acid-catalyzed H/D exchange and with nearly all of the reported photoactivated H/D exchange studies, the UV-induced H/D exchange of tyrosine is optimal at pH 9 and is effectively quenched at acid pH. Photochemical H/D exchange is strongly stimulated by the alpha-amino group (the aromatic hydrogens of p-cresol are far less subject to exchange) and by imidazole or phosphate buffers. On the basis of the results obtained here and on the previously identified cyclohexadienyl radical (Bussandri, A.; van Willigen, H. J. Phys. Chem. A 2002, 106, 1524-1532), we conclude that the exchange reaction involves a radical intermediate and results from two distinct roles of tyrosine: (1) as a phototransducer of light energy into solvated electrons (e(aq)(-)), and (2) as an acceptor of an electron to create a radical anion intermediate which is rapidly protonated, yielding a neutral cyclohexadienyl radical. Regeneration of the tyrosine can occur via a bimolecular redox reaction of the cyclohexadienyl and phenoxyl radicals to yield a carbocation/phenoxide pair, followed by deprotonation of the carbocation. The oxidation step is pH dependent, requiring the deprotonated form of the cyclohexadienyl radical. The H/D exchange thus results from a cyclic one-electron (Birch) reduction/protonation/reoxidation (by phenoxyl radical)/deprotonation cycle. Consistent with these mechanistic conclusions, the aromatic hydrogens of tyrosine O-methyl ether are photochemically inert, but become labile in the presence of tyrosine at high pH. The deuteration rate of O-methyl tyrosine is lower than that of tyrosine and shows a preference for the ortho positions. This difference is proposed to result from a variation in the oxidation step, characterized by a preferential oxidation of a cyclohexadienyl resonance structure with the unpaired electron localized on the oxygen substituent.     

Constraining the phase of Bs-Bs mixing

New physics contributions to Bs-Bs mixing can be parametrized by the size (rs2) and the phase (2thetas) of the total mixing amplitude relative to the standard model amplitude. The phase has so far been unconstrained. We first use the D0 measurement of the semileptonic CP asymmetry ASL to obtain the first constraint on the semileptonic CP asymmetry in Bs decays, ASLs=-0.008+/-0.011. Then we combine recent measurements by the CDF and D0 Collaborations--the mass difference (DeltaMs), the width difference (DeltaGammas), and ASL;s--to constrain 2thetas. The errors on DeltaGammas and ASL;s should still be reduced to have a sensitive probe of the phase, yet the central values are such that the regions around 2thetas approximately 3pi/2 and, in particular, 2thetas approximately pi/2, are disfavored.     

Long-range attraction between charge-mosaic surfaces across water

We have measured directly the forces across water between hydrophilic surfaces covered with a random mosaic of positive and negative charged domains. We find a strong, long-ranged attraction between them at a surface separation comparable with the charge domain size (many tens of nanometers). This attraction persists at higher salt concentration, but its range then becomes comparable to the Debye screening length. We attribute the attraction to correlation between negative and positive regions on opposing surfaces, facilitated by the lateral mobility of the charge patches on the surfaces.     

Stallings’ folds for cube complexes

We describe a higher dimensional analogue of Stallings’ folding sequences for group actions on CAT(0) cube complexes. We use it to give a characterization of quasiconvex subgroups of hyperbolic groups that act properly and cocompactly on CAT(0) cube complexes via finiteness properties of their hyperplane stabilizers.     

Photocycles of Channelrhodopsin-2

Recent developments have used light-activated channels or transporters to modulate neuronal activity. One such genetically-encoded modulator of activity, channelrhodopsin-2 (ChR2), depolarizes neurons in response to blue light. In this work, we first conducted electrophysiological studies of the photokinetics of hippocampal cells expressing ChR2, for various light stimulations. These and other experimental results were then used for systematic investigation of the previously proposed three-state and four-state models of the ChR2 photocycle. We show the limitations of the previously suggested three-state models and identify a four-state model that accurately follows the ChR2 photocurrents. We find that ChR2 currents decay biexponentially, a fact that can be explained by the four-state model. The model is composed of two closed (C1 and C2) and two open (O1 and O2) states, and our simulation results suggest that they might represent the dark-adapted (C1-O1) and light-adapted (C2-O2) branches. The crucial insight provided by the analysis of the new model is that it reveals an adaptation mechanism of the ChR2 molecule. Hence very simple organisms expressing ChR2 can use this form of light adaptation.     

Adsorption isotherms and Sips models of nitrogen,methane, ethane,and propane on commercial activated carbons and polyvinylidene chloride

In this paper we present the measured isotherms of nitrogen, methane, ethane, and propane on three carbons: Norit RB2, Chemviron AP 4-60, and highly activated Saran. The measurements are taken at temperatures between 300 and 400 K, in 20 K steps. The measured data is fitted to the Sips adsorption model, where the Sips parameters are determined by a linearization method. The Sips parameters are further adjusted to realize a logic dependence on temperature and the parameter characteristics are discussed. Subsequently, the Sips model is modified to incorporate the temperature dependence. Including the temperature dependence results in a slightly higher error relative to the experimental results (typically 10 % as compared to 6 %). The immediate research product is a convenient expression for every adsorbate-adsorbent system which is discussed in this paper, for calculating the adsorption concentration as a function of temperature and pressure. A more general research product is a better understanding of the Sips parameter characteristics that should help in developing future adsorbents on demand.     

Elucidating the role of many-body forces in liquid water. I. Simulations of water clusters on the VRT(ASP-W) potential surfaces

We test two new potentials for water, fit to vibration-rotation tunneling (VRT) data by employing diffusion quantum Monte Carlo simulations to calculate the vibrational ground-state properties of water clusters. These potentials, VRT(ASP-W)II and VRT(ASP-W)III, are fits of the highly detailed ASP-W (anisotropic site potential with Woermer dispersion) ab initio potential to (D(2)O)(2) microwave and far-infrared data, and along with the SAPT5s (five-site symmetry adapted perturbation theory) potentials, are the most accurate water dimer potential surfaces in the literature. The results from VRT(ASP-W)II and III are compared to those from the original ASP-W potential, the SAPT5s family of potentials, and several bulk water potentials. Only VRT(ASP-W)III and the spectroscopically "tuned" SAPT5st (with N-body induction included) accurately reproduce the vibrational ground-state structures of water clusters up to the hexamer. Finally, the importance of many-body induction and three-body dispersion are examined, and it is shown that the latter can have significant effects on water cluster properties despite its small magnitude.     

Perturbation theories and wave functions for calculation of electronic polarizabilities application to DNA bases

Three different forms of perturbation theories, variational perturbation, finite perturbation and second-order, are evaluated regarding their value for calculation of electronic polarizabilities of small and intermediate size molecules. It is concluded that with the practical constraint of a small basis set the variational perturbation method is the most promising alternative for calculation of polarizabilities. For several small molecules, our calculated polarizabilities indicate that both IEHT and ab initio wave functions give values in close agreement with each other. Variational perturbation calculations of polarizabilities with IEHT wave functions also include the DNA bases.