357 research outputs found
Doctor of Philosophy
dissertationDespite the tremendous potential of short interfering RNA (siRNA) as a novel biopharmaceutical, its therapeutic utility has not been maximized mainly due to lack of proper in vivo delivery vehicle, off-target effects and several off-pathway protein interactions instigating immunostimulation. Judicious chemical modification of different parts of the siRNA was foreseen as a potential solution to the off-target gene silencing and the off-pathway protein binding. In this study, 8-alkoxyadenosines were explored as a nucleobase modification in the context of the siRNA-based RNA interference (RNAi). These nucleosides are unusual in that they have the potential to exist as an equilibrium mixture of the syn and anti conformers. When placed opposite to U in the complementary strand, 8- alkoxyadenosines can exist in normal anti conformation and form canonical Watson- Crick hydrogen bonding; interestingly, with G as the base-pairing partner, these unusual nucleosides can potentially flip into the syn conformation and form unorthodox Hoogsteen base-pairing. 8-Alkoxyadenosine phosphoramidites were synthesized and incorporated into the guide strand of caspase 2 siRNA at four different positions - two in the seed region, one at the cleavage junction and another nearer to the 3ÂŽ-end of the guide strands. Thermal stabilities of the corresponding siRNA duplexes showed that U is still preferred over G as the base-pairing partner in the complementary strand. When compared to the unmodified iv positive control siRNAs, singly modified siRNAs have knocked down caspase 2 insert mRNA (generated from a recombinant plasmid) efficiently and with little or no loss of efficacy. Doubly modified siRNAs were found to be less effective and lose their efficacy at low nanomolar concentrations. Persistent placement of steric blockade in the minor groove affected the RNAi efficacy significantly; this observation supports the hypothesis and indicates the necessity of âswitching' the bulky alkyloxy groups in the major groove, when modified siRNAs interact with the RISC assembly. SiRNAs modified at positions 6 and 10 of the guide strand were found to be effective against preventing interaction with the RNA-dependent protein kinase (PKR). In summary, 8-alkoxyadenosine-containing siRNAs prevented undesired off-pathway protein binding, without compromising the RNAi efficacy significantly
Etude du lien entre la fréquence et les puissances actives pour le dimensionnement d'un microréseau alternatif ßloté avec sources d'énergie renouvelables
La qualitĂ© dâun rĂ©seau Ă©lectrique se mesure notamment par sa rĂ©silience, Ă savoir sa capacitĂ© Ă continuer de fournir, de façon stable, une Ă©lectricitĂ© de qualitĂ© malgrĂ© les incidents. Cette continuitĂ© dâalimentation est notamment assurĂ©e par le rĂ©glage de la frĂ©quence, ce dernier faisant office de moyen de coopĂ©ration entre les diffĂ©rentes sources du rĂ©seau. Les travaux de cette thĂšse Ă©tudient en profondeur les diffĂ©rentes façons dâassurer ce rĂ©glage de la frĂ©quence sur des microrĂ©seaux alternatifs ĂźlotĂ©s au regard des problĂ©matiques nouvelles : intĂ©gration croissante de sources dâĂ©nergie renouvelables intermittentes et baisse (voire disparition) de lâinertie inhĂ©rente aux groupes synchrones tournants. Lâobjectif principal est de proposer un ensemble de modĂšles analytiques permettant dâapprĂ©hender le comportement dynamique de la frĂ©quence et des puissances actives transitant sur un microrĂ©seau, quelle que soit la topologie de ce dernier, afin de constituer une aide au dimensionnement. Ă cet objectif global sâajoute un certain nombre de sous-objectifs qui structurent et guident lâensemble des travaux :- Un objectif pĂ©dagogique : les couplages entre lâĂ©quilibre des puissances actives et la frĂ©quence sur un rĂ©seau alternatif sont clairement explicitĂ©s, que ces couplages relĂšvent de la structure et des paramĂštres physiques du microrĂ©seau ou quâils soient liĂ©s aux lois et paramĂštres de commande. La lecture des modĂšles doit ainsi permettre de comprendre analytiquement les diffĂ©rents liens de cause Ă effet entre les paramĂštres du systĂšme, la frĂ©quence et les puissances,et les spĂ©cificitĂ©s propres Ă chaque topologie de microrĂ©seau.- Un objectif de simplicitĂ© et de modularitĂ© : les modĂšles proposĂ©s doivent pouvoir ĂȘtre adaptĂ©s selon les cas particuliers de topologies, afin que tout lecteur puisse les rĂ©utiliser facilement en dehors du cadre de cette Ă©tude. Pour ce faire, ces travaux utilisent des modĂšles mathĂ©matiques sous forme de schĂ©mas blocs, intĂ©grables sous Matlab.- Enfin, un objectif de modĂ©lisation « grand signal » : pour trancher avec lâapproche petit signal de beaucoup de modĂ©lisations de microrĂ©seaux classiques, et pour permettre dâapprĂ©hender le comportement du microrĂ©seau sur une large plage de dĂ©sĂ©quilibre de puissanc
Time-dependent density functional theory calculation of van der Waals coefficient of sodium clusters
In this paper we employ all-electron \textit{ab-initio} time-dependent
density functional theory based method to calculate the long range
dipole-dipole dispersion coefficient (van der Waals coefficient) of
sodium atom clusters containing even number of atoms ranging from 2 to 20
atoms. The dispersion coefficients are obtained via Casimir-Polder relation.
The calculations are carried out with two different exchange-correlation
potentials: (i) the asymptotically correct statistical average of orbital
potential (SAOP) and (ii) Vosko-Wilk-Nusair representation of
exchange-correlation potential within local density approximation. A comparison
with the other theoretical results has been performed. We also present the
results for the static polarizabilities of sodium clusters and also compare
them with other theoretical and experimental results. These comparisons reveal
that the SAOP results for C_{6} and static polarizability are quite accurate
and very close to the experimental results. We examine the relationship between
volume of the cluster and van der Waals coefficient and find that to a very
high degree of correlation C_{6} scales as square of the volume. We also
present the results for van der Waals coefficient corresponding to cluster-Ar
atom and cluster-N_{2} molecule interactions.Comment: 22 pages including 6 figures. To be published in Journal of Chemical
Physic
Ab initio calculations of the hydrogen bond
Recent x-ray Compton scattering experiments in ice have provided useful
information about the quantum nature of the interaction between HO
monomers. The hydrogen bond is characterized by a certain amount of charge
transfer which could be determined in a Compton experiment. We use ab-initio
simulations to investigate the hydrogen bond in HO structures by
calculating the Compton profile and related quantities in three different
systems, namely the water dimer, a cluster containing 12 water molecules and
the ice crystal. We show how to extract estimates of the charge transfer from
the Compton profiles.Comment: 16 pages, 7 figures, to appear in Phys. Rev.
Calculation of valence electron momentum densities using the projector augmented-wave method
We present valence electron Compton profiles calculated within the
density-functional theory using the all-electron full-potential projector
augmented-wave method (PAW). Our results for covalent (Si), metallic (Li, Al)
and hydrogen-bonded ((H_2O)_2) systems agree well with experiments and
computational results obtained with other band-structure and basis set schemes.
The PAW basis set describes the high-momentum Fourier components of the valence
wave functions accurately when compared with other basis set schemes and
previous all-electron calculations.Comment: Submitted to Journal of Physics and Chemistry of Solids on September
17 2004. Revised version submitted on December 13 200
Network equilibration and first-principles liquid water.
Motivated by the very low diffusivity recently found in ab initio simulations of liquid water, we have studied its dependence with temperature, system size, and duration of the simulations. We use ab initio molecular dynamics (AIMD), following the Born-Oppenheimer forces obtained from density-functional theory (DFT). The linear-scaling capability of our method allows the consideration of larger system sizes (up to 128 molecules in this study), even if the main emphasis of this work is in the time scale. We obtain diffusivities that are substantially lower than the experimental values, in agreement with recent findings using similar methods. A fairly good agreement with D(T) experiments is obtained if the simulation temperature is scaled down by approximately 20%. It is still an open question whether the deviation is due to the limited accuracy of present density functionals or to quantum fluctuations, but neither technical approximations (basis set, localization for linear scaling) nor the system size (down to 32 molecules) deteriorate the DFT description in an appreciable way. We find that the need for long equilibration times is consequence of the slow process of rearranging the H-bond network (at least 20 ps at AIMDs room temperature). The diffusivity is observed to be very directly linked to network imperfection. This link does not appear an artifact of the simulations, but a genuine property of liquid water
SCN1A variants from bench to bedside-improved clinical prediction from functional characterization
Variants in the SCN1A gene are associated with a wide range of disorders including genetic epilepsy with febrile seizures plus (GEFS+), familial hemiplegic migraine (FHM), and the severe childhood epilepsy Dravet syndrome (DS). Predicting disease outcomes based on variant type remains challenging. Despite thousands of SCN1A variants being reported, only a minority has been functionally assessed.
We review the functional SCN1A work performed to date, critically appraise electrophysiological measurements, compare this to in silico predictions, and relate our findings to the clinical phenotype.
Our results show, regardless of the underlying phenotype, that conventional in silico software correctly predicted benign from pathogenic variants in nearly 90%, however was unable to differentiate within the disease spectrum (DS vs. GEFS+ vs. FHM). In contrast, patchâclamp data from mammalian expression systems revealed functional differences among missense variants allowing discrimination between disease severities. Those presenting with milder phenotypes retained a degree of channel function measured as residual wholeâcell current, whereas those without any wholeâcell current were often associated with DS (pâ=â.024).
These findings demonstrate that electrophysiological data from mammalian expression systems can serve as useful disease biomarker when evaluating SCN1A variants, particularly in view of new and emerging treatment options in DS
A new approach to local hardness
The applicability of the local hardness as defined by the derivative of the
chemical potential with respect to the electron density is undermined by an
essential ambiguity arising from this definition. Further, the local quantity
defined in this way does not integrate to the (global) hardness - in contrast
with the local softness, which integrates to the softness. It has also been
shown recently that with the conventional formulae, the largest values of local
hardness do not necessarily correspond to the hardest regions of a molecule.
Here, in an attempt to fix these drawbacks, we propose a new approach to define
and evaluate the local hardness. We define a local chemical potential,
utilizing the fact that the chemical potential emerges as the additive constant
term in the number-conserving functional derivative of the energy density
functional. Then, differentiation of this local chemical potential with respect
to the number of electrons leads to a local hardness that integrates to the
hardness, and possesses a favourable property; namely, within any given
electron system, it is in a local inverse relation with the Fukui function,
which is known to be a proper indicator of local softness in the case of soft
systems. Numerical tests for a few selected molecules and a detailed analysis,
comparing the new definition of local hardness with the previous ones, show
promising results.Comment: 30 pages (including 6 figures, 1 table
Work functions, ionization potentials, and in-between: Scaling relations based on the image charge model
We revisit a model in which the ionization energy of a metal particle is
associated with the work done by the image charge force in moving the electron
from infinity to a small cut-off distance just outside the surface. We show
that this model can be compactly, and productively, employed to study the size
dependence of electron removal energies over the range encompassing bulk
surfaces, finite clusters, and individual atoms. It accounts in a
straightforward manner for the empirically known correlation between the atomic
ionization potential (IP) and the metal work function (WF), IP/WF2. We
formulate simple expressions for the model parameters, requiring only a single
property (the atomic polarizability or the nearest neighbor distance) as input.
Without any additional adjustable parameters, the model yields both the IP and
the WF within 10% for all metallic elements, as well as matches the size
evolution of the ionization potentials of finite metal clusters for a large
fraction of the experimental data. The parametrization takes advantage of a
remarkably constant numerical correlation between the nearest-neighbor distance
in a crystal, the cube root of the atomic polarizability, and the image force
cutoff length. The paper also includes an analytical derivation of the relation
of the outer radius of a cluster of close-packed spheres to its geometric
structure.Comment: Original submission: 8 pages with 7 figures incorporated in the text.
Revised submission (added one more paragraph about alloy work functions): 18
double spaced pages + 8 separate figures. Accepted for publication in PR
Proton transfer or hemibonding? The structure and stability of radical cation clusters
The basin hopping search algorithm in conjunction with second-order Moller-Plesset perturbation theory is used to determine the lowest energy structures of the radical cation clusters (NH_3)_n^+, (H_2O)_n^+, (HF)_n^+, (PH_3)_n^+, (H_2S)_n^+ and (HCl)_n^+, where n=2-4. The energies of the most stable structures are subsequently evaluated using coupled cluster theory in conjunction with the aug-cc-pVTZ basis set. These cationic clusters can adopt two distinct structural types, with some clusters showing an unusual type of bonding, often referred to as hemibonding, while other clusters undergo proton transfer to give an ion and radical. It is found that proton transfer based structures are preferred by the (NH_3)_n+, (H_2O)_n^+, and (HF)_n^+ clusters while hemibonded structures are favoured by (PH_3)_n^+, (H_2S)_n^+ and (HCl)_n^+. These trends can be attributed to the relative strengths of the molecules and molecular cations as BrĂžnsted bases and acids, respectively, and the strength of the interaction between the ion and radical in the ion-radical clusters
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