Charge inversion, condensation and decondensation of DNA and polystyrene sulfonate by polyethylenimine.

We study the formation and structure of stable electrostatic complexes between polyanions (DNA and poly(styrene-sulfonate)) and linear polyethylenimine. The charge ratio x of the mixture is tuned by varying the concentration of the polycation at constant concentration of polyanion. In agreement with recent theories, dynamic light scattering and electrophoretic mobility measurements show two distinct regimes of weak and strong complexation. At low polycation concentration, negatively charged small complexes involving a few polyanion chains are observed first. By further increasing x, these small complexes condense at a precise charge ratio x(c) < 1 to form large anionic aggregates. The inversion of the charge of the condensed complexes coincides with the maximum of complexation and precedes the dissolution of the aggregates which occurs at a well-defined decondensation threshold x(d) > 1. Above x(d), positively charged complexes containing again a few overcharged polyanion chains are observed. The macroscopic phase diagram is qualitatively well corroborated by AFM observation of the complexes. The influence of entropic effects is probed by varying parameters like concentration, polycation molecular weight and ionic strength. Structure of stable negatively charged complexes is investigated at higher concentration using Small Angle Neutron Scattering. In the condensed regime, we observe large soluble bundles with sharp interfaces where the local structure of the polyanions is preserved.

Structure of hydrophobically and hydrogen-bonded complexes between amphiphilic copolymer and polyacid in water.

We observed by SANS and NMR the structure of intermolecular complexes formed through hydrogen bonding and hydrophobic interactions between a polyacid and a neutral copolymer surfactant (PEO-PPO-PEO). The polyacid is perdeuterated and the contrast variation method enables us to measure separately the structure factor of each component in the complex and the cross structure factor as a function of the pH and the temperature. The evolution of NMR spectra and relaxation times with pH and temperature give a complementary insight into the inner structure of the aggregates. The interaction between the aggregates and the aggregation number of polyacid chains are governed by electrostatic effects; they depend on the polyacid charge and are insensitive to temperature. The number of copolymer chains which results from the cooperative action of hydrogen bonding and hydrophobic interactions is sensitive to both pH and temperature. The complexation preserves the micellar core-corona structure of the copolymer and shrinks the polyacid chains which adopt a compact structure. The non-dissociated polyacid sequences are bound to the PPO part of the copolymer forming the hydrophobic core of the complex, whereas the ionized polyacid sequences form with the PEO a stabilizing hydrated corona.

[Corneal lesions caused by caterpillar hairs: four case studies]. / Lésions cornéennes dues aux poils de chenille.

Caterpillar hairs disseminated by the wind can cause serious ocular problems in humans. We present 4 cases of corneal lesions caused by caterpillar hairs. Intense functional signs observed in exposed subjects should bring this diagnosis to mind. The severity of this affection is due to the possibility of intraocular migration, even many years after the first accident. Treatment consists of removing the caterpillar hair associated with an antibiotic and local anti-inflammatory treatment.

Age-dependent axonal loss in nerve biopsy of patients with xeroderma pigmentosum.

We describe the histomorphometric changes in the superficial peroneal nerve biopsy from 13 patients with xeroderma pigmentosum (XP). The mean age at time of biopsy was 15 yr (range 2.5-24 yr). The clinical examination was normal in the two youngest patients and showed absence of the deep tendon reflexes, with choreo-athetosis in ten patients. In addition, in the three oldest patients there were cerebellar signs, ataxia and Babinski signs. The nerve biopsy showed an age-dependent decrease of myelinated fibres, which was mild in the youngest and severe in the oldest patients. This was associated with rare acute axonal degeneration, sparse axonal regeneration, rare axonal atrophy and few onion bulb formations. These findings suggest a neuropathic process. This neuronal degeneration seems to be a progressive and stereotyped phenomenon in XP.