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DNA as a polyelectrolyte

par douarche - publié le , mis à jour le

 

 

DNA AS A POLYELECTROLYTE


 

If DNA is usually known to be the support of the genetic information. It is also a polymer carrying a series of phosphate groups (red circles on the picture). In contact with water molecules, the phosphate groups usually hydrate and ionise in a such way that the DNA chain may be considered as a long chain of negatively charged monomers (i.e. a polyelectrolyte or a poly-anion).

 

 

Because of electro-neutrality condition, the same number of positive charges has to counterbalance the DNA charges. In vivo the positive charges may be simple ions (potassium, sodium, calcium,…) and/or belong to other cell components (proteins,..).
For many years, the team has been interested in DNA as a polyelectrolyte and has been working on model systems. According to the valency of the cations for example, various phenomena can be observed. In the presence of monovalent ions, the interactions between DNA molecules in solution are repulsive. When the valency of the cations is higher or equal to three, or in presence of basic proteins, an attraction between DNA molecules gives rise to a phase separation i.e. to the formation of a phase very concentrated in DNA coexisting with a dilute DNA solution.

 

  • DNA-polycations interactions

We studied the interaction of DNA with polycations of various valences (monovalent and multivalent). We chose ubiquitous polycations, polyamines (3+ or 4+), present in the cell in millimolar concentrations. These cations are implied in many activities related to cell division and growth. Polyamines can induce the collapse or the aggregation of DNA and the formation of ordered phases, either cholesteric or hexagonal. We worked out the phase diagram, taking account the DNA and ions concentrations and showed the general behaviour of the phenomenon. Using Xray diffraction methods, we followed variations of the distances between helices in these aggregates, at various points of the phase diagram (col. D. Durand). This variation would come from the local concentrations of the different types of ions in the precipitates. In addition, we estimated the energy of cohesion per nucleotide by using the model of Nguyen et al. (2000) based on the ionic correlations. If one expresses cohesive energy according to the distance between helices in the precipitate, all the values are aligned on the same curve, thus hiding the ionic specificities. This graph could be used to estimate the cohesive energy of precipitates induced by ions.
 

  • DNA-protamine interactions

Protamines are oligo peptides with high charge density, responsible for sperm chromatin condensation. Soluble DNA-protamine complexes are formed at low DNA and salt concentration and remain stable for months. We explored the mechanisms involved in their formation and their stability by using a combination of multiple experimental and simulation approaches. We suggest that inhomogeneous mixing conditions are required to form and stabilize charged selfassembled nano-aggregates in large excess of DNA. More generally, these results should help re-interpreting puzzling behaviors reported for a large class of strongly charged polyelectrolyte systems.

 
(A) Simulated bundle structure. Snapshots showing the longitudinal and the lateral fluctuations (second peripheral DNA “layer” colored in green and core in blue) (B) DNA-protamines complexes observed by cryoTEM. Bundles are seen in side view (white arrows), or in top view (insert 1 for example) or in slightly oblique view (insert 2) and coexist with larger aggregates (*).
 

 
REFERENCES


LANSAC Y, DEGROUARD J, RENOUARD M, TOMA AC, LIVOLANT F, RASPAUD E. (2016) A route to self assemble suspended DNA nano-complexes. Sci Rep. 6, 21995.

VULETIC T, BABIC SD, GRGICIN D, AUMILER D, RADLER J, LIVOLANT F, TOMIC S. (2011) Manning free counterion fraction for a rodlike polyion : aqueous solutions of short DNA fragments in presence of very low added salt. Phys Rev E Stat Nonlin Soft Matter Phys. 83, 041803

TOMA AC, de FRUTOS M, LIVOLANT F, RASPAUD E. (2009) DNA condensed by protamine : a "short" or "long" polycation behavior. Biomacromolecules. 10(8), 2129-34.

RASPAUD E., DURAND D., LIVOLANT F. (2005) Interhelical spacing in liquid crystalline spermine and spermidine-DNA precipitates. Biophys. J, 88, 392-403.

RASPAUD E., CHAPERON I., LEFORESTIER A., LIVOLANT F. (1999) General rules and specific effects in spermine induced precipitation of DNA, nucleosome core particles and chromatin. Biophys. J. 77, 1547-1555.

RASPAUD E., OLVERA DE LA CRUZ M., SIKORAV J.-L., LIVOLANT F. (1998) Precipitation of DNA by polyamines : a polyelectrolyte behavior. Biophys. J. 74, 381-393.