202 research outputs found
Entropy-Enthalpy Compensation May Be a Useful Interpretation Tool for Complex Systems Like Protein-DNA Complexes: An Appeal to Experimentalists
In various chemical systems enthalpy-entropy compensation (EEC) is a
well-known rule of behavior, although the physical roots of it are still not
completely understood. It has been frequently questioned whether EEC is a truly
physical phenomenon or a coincidence due to trivial mathematical connections
between statistical-mechanical parameters - or even simpler: A phantom effect
resulting from the misinterpretation of experimental data. Here, we review EEC
from a new standpoint using the notion of correlation which is essential for
the method of factor analysis, but is not conventional in physics and
chemistry. We conclude that the EEC may be rationalized in terms of hidden (not
directly measurable with the help of the current experimental set-up) but
physically real factors, implying a Carnot-cycle model in which a micro-phase
transition (MPT) plays a crucial role. Examples of such MPTs underlying
physically valid EEC should be typically cooperative processes in
supramolecular aggregates, like changes of structured water at hydrophobic
surfaces, conformational transitions upon ligand-biopolymer binding, and so on,
so forth. The MPT notion could help rationalize the occurrence of EEC in
connection with hydration and folding of proteins,enzymatic reactions,
functioning of molecular motors, DNA de- and rehybridization, as well as
similar phenomena.Comment: 8 pages, 2 Figures, Submitted for publicatio
Charge transport in poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA polymers
We investigate the charge transport in synthetic DNA polymers built up from
single types of base pairs. In the context of a polaron-like model, for which
an electronic tight-binding system and bond vibrations of the double helix are
coupled, we present estimates for the electron-vibration coupling strengths
utilizing a quantum-chemical procedure. Subsequent studies concerning the
mobility of polaron solutions, representing the state of a localized charge in
unison with its associated helix deformation, show that the system for
poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA polymers, respectively possess
quantitatively distinct transport properties. While the former supports
unidirectionally moving electron breathers attributed to highly efficient
long-range conductivity the breather mobility in the latter case is
comparatively restrained inhibiting charge transport. Our results are in
agreement with recent experimental results demonstrating that poly(dG)-poly(dC)
DNA molecules acts as a semiconducting nanowire and exhibits better conductance
than poly(dA)-poly(dT) ones.Comment: 11 pages, 5 figure
Localization Properties of Electronic States in Polaron Model of poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA polymers
We numerically investigate localization properties of electronic states in a
static model of poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA polymers with
realistic parameters obtained by quantum-chemical calculation. The randomness
in the on-site energies caused by the electron-phonon coupling are completely
correlated to the off-diagonal parts. In the single electron model, the effect
of the hydrogen-bond stretchings, the twist angles between the base pairs and
the finite system size effects on the energy dependence of the localization
length and on the Lyapunov exponent are given. The localization length is
reduced by the influence of the fluctuations in the hydrogen bond stretchings.
It is also shown that the helical twist angle affects the localization length
in the poly(dG)-poly(dC) DNA polymer more strongly than in the
poly(dA)-poly(dT) one. Furthermore, we show resonance structures in the energy
dependence of the localization length when the system size is relatively small.Comment: 6 pages, 6 figure
Pre-selectable integer quantum conductance of electrochemically fabricated silver point contacts
The controlled fabrication of well-ordered atomic-scale metallic contacts is
of great interest: it is expected that the experimentally observed high
percentage of point contacts with a conductance at non-integer multiples of the
conductance quantum G_0 = 2e^2/h in simple metals is correlated to defects
resulting from the fabrication process. Here we demonstrate a combined
electrochemical deposition and annealing method which allows the controlled
fabrication of point contacts with pre-selectable integer quantum conductance.
The resulting conductance measurements on silver point contacts are compared
with tight-binding-like conductance calculations of modeled idealized junction
geometries between two silver crystals with a predefined number of contact
atoms
Ensuring resilience and agility of complex organizational-technical systems
Modern organizational and technical systems have been developing in an environment that is marked by capriciousness, uncertainty, risk, variability, and evolution (CURVE factors). As organizational-technical systems grow bigger, their internal complexity increases, too, both structurally and dynamically. The article substantiates the appropriateness of employing the principles of systems engineering for managing such systems. The authors analyzed various theoretical concepts of and practice-based approaches to the development of systems engineering in the context of ensuring the resilience and agility of complex organizational-technical systems. Using the case of power engineering and hi-tech industries, the authors show that for organizations that operate critical infrastructure facilities it is essential to make sure that the system stays functional in adverse conditions and is able to recover quickly after a failure. It is demonstrated that for addressing the above task it is critical to use instruments that nurture interdisciplinary competences in individual professionals and in teams that manage the development of complex systems and implement major innovation projects. As part of the study, the authors also look at the possibility of using the principles of resilient systems design and the fundamental principles for agile systems engineering when managing critical infrastructure facilities. © 2018 WIT Press.ACKNOWLEDGEMENTS The work was supported by Act 211 of the Government of the Russian Federation, contract No. 02.A03.21.0006
Effects of molecular motion on charge transfer/transport through DNA duplexes with and without base pair mismatch
Multiple-charge transfer and trapping in DNA dimers
We investigate the charge transfer characteristics of one and two excess
charges in a DNA base-pair dimer using a model Hamiltonian approach. The
electron part comprises diagonal and off-diagonal Coulomb matrix elements such
a correlated hopping and the bond-bond interaction, which were recently
calculated by Starikov [E. B. Starikov, Phil. Mag. Lett. {\bf 83}, 699 (2003)]
for different DNA dimers. The electronic degrees of freedom are coupled to an
ohmic or a super-ohmic bath serving as dissipative environment. We employ the
numerical renormalization group method in the nuclear tunneling regime and
compare the results to Marcus theory for the thermal activation regime. For
realistic parameters, the rate that at least one charge is transferred from the
donor to the acceptor in the subspace of two excess electrons significantly
exceeds the rate in the single charge sector. Moreover, the dynamics is
strongly influenced by the Coulomb matrix elements. We find sequential and pair
transfer as well as a regime where both charges remain self-trapped. The
transfer rate reaches its maximum when the difference of the on-site and
inter-site Coulomb matrix element is equal to the reorganization energy which
is the case in a GC-GC dimer. Charge transfer is completely suppressed for two
excess electrons in AT-AT in an ohmic bath and replaced by damped coherent
electron-pair oscillations in a super-ohmic bath. A finite bond-bond
interaction alters the transfer rate: it increases as function of when
the effective Coulomb repulsion exceeds the reorganization energy (inverted
regime) and decreases for smaller Coulomb repulsion
Investigation of a Kubo-formula-based approach to estimate DNA conductance in an atomistic model
Voltage controlled terahertz transmission through GaN quantum wells
We report measurements of radiation transmission in the 0.220--0.325 THz
frequency domain through GaN quantum wells grown on sapphire substrates at room
and low temperatures. A significant enhancement of the transmitted beam
intensity with the applied voltage on the devices under test is found. For a
deeper understanding of the physical phenomena involved, these results are
compared with a phenomenological theory of light transmission under electric
bias relating the transmission enhancement to changes in the differential
mobility of the two-dimensional electron gas
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