2,012 research outputs found
Recoiling DNA Molecule: Simulation & Experiment
Single molecule DNA experiments often generate data from force versus
extension measurements involving the tethering of a microsphere to one end of a
single DNA molecule while the other is attached to a substrate. We show that
the persistence length of single DNA molecules can also be measured based on
the recoil dynamics of these DNA-microsphere complexes if appropriate
corrections are made to the friction coefficient of the microsphere in the
vicinity of the substrate. Comparison between computer simulated recoil curves,
generated from the corresponding Langevin equation, and experimental recoils is
used to assure the validity of data analysis.Comment: 14 pages (single column preprint), 7 figures. Major changes: data
analysis method improved; dna-ethidium bromide results removed (dna-ethidium
bromide protocol affected microspheres and coverglass behavior
Transition on the entropic elasticity of DNA induced by intercalating molecules
We use optical tweezers to perform stretching experiments on DNA molecules
when interacting with the drugs daunomycin and ethidium bromide, which
intercalate the DNA molecule. These experiments are performed in the low-force
regime from zero up to 2 pN. Our results show that the persistence length of
the DNA-drug complexes increases strongly as the drug concentration increases
up to some critical value. Above this critical value, the persistence length
decreases abruptly and remains practically constant for larger drug
concentrations. The contour length of the molecules increases monotonically and
saturates as drugs concentration increases. Measured in- tercalants critical
concentrations for the persistence length transition coincide with reported
values for the helix-coil transition of DNA-drug complexes, obtained from
sedimentation experiments.Comment: This experimental article shows and discuss a transition observed in
the persistence length of DNA molecules when studied as a function of some
intercalating drug concentrations, like daunomycin and ethidium bromide. It
has 15 pages and 4 figures. The article presented here is in preprint forma
DNA-psoralen: single-molecule experiments and first principles calculations
The authors measure the persistence and contour lengths of DNA-psoralen
complexes, as a function of psoralen concentration, for intercalated and
crosslinked complexes. In both cases, the persistence length monotonically
increases until a certain critical concentration is reached, above which it
abruptly decreases and remains approximately constant. The contour length of
the complexes exhibits no such discontinuous behavior. By fitting the relative
increase of the contour length to the neighbor exclusion model, we obtain the
exclusion number and the intrinsic intercalating constant of the psoralen-DNA
interaction. Ab initio calculations are employed in order to provide an
atomistic picture of these experimental findings.Comment: 9 pages, 4 figures in re-print format 3 pages, 4 figures in the
published versio
In-line phosphate pre-concentration in a flow injection method platform for monitoring fertilizers excess in soil leachates
info:eu-repo/semantics/publishedVersio
Improvement of a sequential injection method for the determination of chromium (VI) in waters using a GlobalFIA platform.
info:eu-repo/semantics/publishedVersio
Towards absolute calibration of optical tweezers
Aiming at absolute force calibration of optical tweezers, following a
critical review of proposed theoretical models, we present and test the results
of MDSA (Mie-Debye-Spherical Aberration) theory, an extension of a previous
(MD) model, taking account of spherical aberration at the glass/water
interface. This first-principles theory is formulated entirely in terms of
experimentally accessible parameters (none adjustable). Careful experimental
tests of the MDSA theory, undertaken at two laboratories, with very different
setups, are described. A detailed description is given of the procedures
employed to measure laser beam waist, local beam power at the transparent
microspheres trapped by the tweezers, microsphere radius and the trap
transverse stiffness, as a function of radius and height in the (inverted
microscope) sample chamber. We find generally very good agreement with MDSA
theory predictions, for a wide size range, from the Rayleigh domain to large
radii, including the values most often employed in practice, and at different
chamber heights, both with objective overfilling and underfilling. The results
asymptotically approach geometrical optics in the mean over size intervals, as
they should, and this already happens for size parameters not much larger than
unity. MDSA predictions for the trapping threshold, position of stiffness peak,
stiffness variation with height, multiple equilibrium points and `hopping'
effects among them are verified. Remaining discrepancies are ascribed to focus
degradation, possibly arising from objective aberrations in the infrared, not
yet included in MDSA theory.Comment: 15 pages, 20 figure
Gas diffusion sequential injection system for the spectrophotometric determination of free chlorine with o-dianisidine
A gas diffusion sequential injection system for spectrophotometric determination of free chlorine is described. The detection is based in the colorimetric reaction between free chlorine and a low toxicity reagent o-dianisidine. A gas diffusion unit is used to isolate free chlorine from the sample in order to avoid possible interferences. This feature results from the conversion of free chlorine to molecular chlorine (gaseous) with sample acidification. With minor changes in the operating conditions, two different dynamic ranges were obtained enhancing the application both to water samples and bleaches. The results obtained with the developed system were compared to the reference method, iodometric titration and proved not to be statistically different. A detection limit of 0.6 mg ClO−/L was achieved. Repeatability was evaluated from 10 consecutive determinations being the results better than 2%. The two dynamic ranges presented different determination rates: 15 h−1 for 0.6–4.8 mg ClO−/L (water samples) and 30 h−1 for 0.047–0.188 g ClO−/L (bleaches).info:eu-repo/semantics/acceptedVersio
Microfluidic paper-based devices as disposable, easy-to-use, real time quantification methods
info:eu-repo/semantics/publishedVersio
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