9 research outputs found
Spectroscopic studies of a phosphoinositide-binding peptide from gelsolin: behavior in solutions of mixed solvent and anionic micelles
The peptide G(150–169) corresponds to a phosphatidylinositol 4,5-bisphosphate (PIP2) and filamentous actin (F-actin) binding site on gelsolin (residues 150–169, with the sequence KHVVPNEVVVQRLFQVKGRR). The conformation of this peptide in trifluoroethanol (TFE) aqueous solution was determined by 1H nuclear magnetic resonance as the first step toward understanding the structural aspects of the interaction of G(150–169) and PIP2. The circular dichroism experiments show that G(150–169) adopts a predominantly alpha-helical form in both 50% TFE aqueous solution and in the presence of PIP2 micelles, therefore establishing a connection between the two conformations. 1H nuclear magnetic resonance experiments of G(150–169) in TFE co-solvent show that the helical region extends from Pro-154 to Lys-166. The amphiphilic nature of this helical structure may be the key to understanding the binding of the peptide to lipids. Sodium dodecyl sulfate micelle solution is used as a model for anionic lipid environments. Preliminary studies of the conformation of G(150–169) in sodium dodecyl sulfate micelle solution show that the peptide forms an alpha-helix similar to but with some structural differences from that in TFE co-solvent. Fluorescence experiments provide evidence of peptide clustering over a narrow range of peptide/PIP2 ratios, which is potentially relevant to the biological function of PIP2
The Persistence Length of a Strongly Charged, Rod-like, Polyelectrolyte in the Presence of Salt
The persistence length of a single, intrinsically rigid polyelectrolyte
chain, above the Manning condensation threshold is investigated theoretically
in presence of added salt. Using a loop expansion method, the partition
function is consistently calculated, taking into account corrections to
mean-field theory. Within a mean-field approximation, the well-known results of
Odijk, Skolnick and Fixman are reproduced. Beyond mean-field, it is found that
density correlations between counterions and thermal fluctuations reduce the
stiffness of the chain, indicating an effective attraction between monomers for
highly charged chains and multivalent counterions. This attraction results in a
possible mechanical instability (collapse), alluding to the phenomenon of DNA
condensation. In addition, we find that more counterions condense on slightly
bent conformations of the chain than predicted by the Manning model for the
case of an infinite cylinder. Finally, our results are compared with previous
models and experiments.Comment: 13 pages, 2 ps figure
Adsorption of mono- and multivalent cat- and anions on DNA molecules
Adsorption of monovalent and multivalent cat- and anions on a deoxyribose
nucleic acid (DNA) molecule from a salt solution is investigated by computer
simulation. The ions are modelled as charged hard spheres, the DNA molecule as
a point charge pattern following the double-helical phosphate strands. The
geometrical shape of the DNA molecules is modelled on different levels ranging
from a simple cylindrical shape to structured models which include the major
and minor grooves between the phosphate strands. The densities of the ions
adsorbed on the phosphate strands, in the major and in the minor grooves are
calculated. First, we find that the adsorption pattern on the DNA surface
depends strongly on its geometrical shape: counterions adsorb preferentially
along the phosphate strands for a cylindrical model shape, but in the minor
groove for a geometrically structured model. Second, we find that an addition
of monovalent salt ions results in an increase of the charge density in the
minor groove while the total charge density of ions adsorbed in the major
groove stays unchanged. The adsorbed ion densities are highly structured along
the minor groove while they are almost smeared along the major groove.
Furthermore, for a fixed amount of added salt, the major groove cationic charge
is independent on the counterion valency. For increasing salt concentration the
major groove is neutralized while the total charge adsorbed in the minor groove
is constant. DNA overcharging is detected for multivalent salt. Simulations for
a larger ion radii, which mimic the effect of the ion hydration, indicate an
increased adsorbtion of cations in the major groove.Comment: 34 pages with 14 figure