44,704 research outputs found
DNA: From rigid base-pairs to semiflexible polymers
The sequence-dependent elasticity of double-helical DNA on a nm length scale
can be captured by the rigid base-pair model, whose strains are the relative
position and orientation of adjacent base-pairs. Corresponding elastic
potentials have been obtained from all-atom MD simulation and from
high-resolution structural data. On the scale of a hundred nm, DNA is
successfully described by a continuous worm-like chain model with homogeneous
elastic properties characterized by a set of four elastic constants, which have
been directly measured in single-molecule experiments. We present here a theory
that links these experiments on different scales, by systematically
coarse-graining the rigid base-pair model for random sequence DNA to an
effective worm-like chain description. The average helical geometry of the
molecule is exactly taken into account in our approach. We find that the
available microscopic parameters sets predict qualitatively similar mesoscopic
parameters. The thermal bending and twisting persistence lengths computed from
MD data are 42 and 48 nm, respectively. The static persistence lengths are
generally much higher, in agreement with cyclization experiments. All
microscopic parameter sets predict negative twist-stretch coupling. The
variability and anisotropy of bending stiffness in short random chains lead to
non-Gaussian bend angle distributions, but become unimportant after two helical
turns.Comment: 13 pages, 6 figures, 6 table
Strategies for synthesis of yardsticks and abaci for nanometre distance measurements by pulsed EPR
Silvia Valera is grateful for support by EPSRC and Bela E. Bode acknowledges support by EastCHEM.Pulsed electron paramagnetic resonance (EPR) techniques have been found to be an efficient tool for elucidation of structure in complex biological systems as they give access to distances in the nanometre range. These measurements can provide additional structural information such as relative orientations, structural flexibility or aggregation states. A wide variety of model systems for calibration and optimisation of pulsed experiments has been synthesised. Their design is based on mimicking biological systems or materials in specific properties such as the distances themselves and the distance distributions. Here, we review selected approaches to the synthesis of chemical systems bearing two or more spin centres, such as nitroxide or trityl radicals, metal ions or combinations thereof and sketch their application in pulsed EPR distance measurements.Publisher PDFPeer reviewe
Role of Internal Motions and Molecular Geometry on the NMR Relaxation of Hydrocarbons
The role of internal motions and molecular geometry on H NMR relaxation
times in hydrocarbons is investigated using MD (molecular dynamics)
simulations of the autocorrelation functions for in{\it tra}molecular
and in{\it ter}molecular H-H dipole-dipole interactions
arising from rotational () and translational () diffusion, respectively.
We show that molecules with increased molecular symmetry such as neopentane,
benzene, and isooctane show better agreement with traditional hard-sphere
models than their corresponding straight-chain -alkane, and furthermore that
spherically-symmetric neopentane agrees well with the Stokes-Einstein theory.
The influence of internal motions on the dynamics and relaxation of
-alkanes are investigated by simulating rigid -alkanes and comparing with
flexible (i.e. non-rigid) -alkanes. Internal motions cause the rotational
and translational correlation-times to get significantly shorter
and the relaxation times to get significantly longer, especially for
longer-chain -alkanes. Site-by-site simulations of H's along the chains
indicate significant variations in and across the chain,
especially for longer-chain -alkanes. The extent of the stretched (i.e.
multi-exponential) decay in the autocorrelation functions are
quantified using inverse Laplace transforms, for both rigid and flexible
molecules, and on a site-by-site bases. Comparison of measurements
with the site-by-site simulations indicate that cross-relaxation (partially)
averages-out the variations in and across the chain of
long-chain -alkanes. This work also has implications on the role of
nano-pore confinement on the NMR relaxation of fluids in the organic-matter
pores of kerogen and bitumen
Exploring the flexibility of MIL-47(V)-type materials using force field molecular dynamics simulations
The flexibility of three MIL-47(V)-type materials (MIL-47, COMOC-2, and COMOC-3) has been explored by constructing the pressure versus volume and free energy versus volume profiles at various temperatures ranging from 100 to 400 K This is done with first-principles-based force fields using the recently proposed QuickFF parametrization protocol. Specific terms were added for the materials at hand to describe the asymmetry of the one-dimensional vanadium oxide chain and to account for the flexibility of the organic linkers. The force fields are used in a series of molecular dynamics simulations at fixed volumes but varying unit cell shapes. The three materials show a distinct pressure-volume behavior, which underlines the ability to tune the mechanical properties by varying the linkers toward different applications such as nanosprings, dampers, and shock absorbers
Anharmonic Torsional Stiffness of DNA Revealed under Small External Torques
DNA supercoiling plays an important role in a variety of cellular processes.
The torsional stress related with supercoiling may be also involved in gene
regulation through the local structure and dynamics of the double helix. To
check this possibility steady torsional stress was applied to DNA in the course
of all-atom molecular dynamics simulations. It is found that small static
untwisting significantly reduces the torsional persistence length () of
GC-alternating DNA. For the AT-alternating sequence a smaller effect of the
opposite sign is observed. As a result, the measured values are similar
under zero stress, but diverge with untwisting. The effect is traced to
sequence-specific asymmetry of local torsional fluctuations, and it should be
small in long random DNA due to compensation. In contrast, the stiffness of
special short sequences can vary significantly, which gives a simple
possibility of gene regulation via probabilities of strong fluctuations. These
results have important implications for the role of local DNA twisting in
complexes with transcription factors.Comment: 8 pages, 5 figures, to appear in Phys. Rev. Let
Rational design and dynamics of self-propelled colloidal bead chains: from rotators to flagella
The quest for designing new self-propelled colloids is fuelled by the demand
for simple experimental models to study the collective behaviour of their more
complex natural counterparts. Most synthetic self-propelled particles move by
converting the input energy into translational motion. In this work we address
the question if simple self-propelled spheres can assemble into more complex
structures that exhibit rotational motion, possibly coupled with translational
motion as in flagella. We exploit a combination of induced dipolar interactions
and a bonding step to create permanent linear bead chains, composed of
self-propelled Janus spheres, with a well-controlled internal structure. Next,
we study how flexibility between individual swimmers in a chain can affect its
swimming behaviour. Permanent rigid chains showed only active rotational or
spinning motion, whereas longer semi-flexible chains showed both translational
and rotational motion resembling flagella like-motion, in the presence of the
fuel. Moreover, we are able to reproduce our experimental results using
numerical calculations with a minimal model, which includes full hydrodynamic
interactions with the fluid. Our method is general and opens a new way to
design novel self-propelled colloids with complex swimming behaviours, using
different complex starting building blocks in combination with the flexibility
between them.Comment: 27 pages, 10 figure
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