Volumes of activation for dissociation of pentacyanoferrates(II) through pressure and salt effects on reactivity

Summary Dependences of rate constants on pressure (up to 1 kbar) and on added salt concentration (up to 6.0 mol dm^–3 LiNO₃, NaNO₃, NaCl, Na₂SO₄ or KNO₃) have been established for dissociative substitution of pentacyanoferrates(II), [Fe(CN)₅L]^3– with L = 4-cyanopyridine, 4,4-bipyridyl, 4-phenylpyridine and 4-t-butylpyridine. Activation volumes derived directly from pressure effects, and indirectly from salt effects via surface tension dependences and derived surfaces of activation, are reported, compared and discussed.     

Giant vesicles formed by gentle hydration and electroformation: a comparison by fluorescence microscopy

Giant unilamellar vesicles (diameter of a few tens of micrometers) are commonly produced by hydration of a dried lipidic film. After addition of the aqueous solution, two major protocols are used: (i) the gentle hydration method where the vesicles spontaneously form and (ii) the electroformation method where an ac electric field is applied. Electroformation is known to improve the rate of unilamellarity of the vesicles though it imposes more restricting conditions for the lipidic composition of the vesicles. Here we further characterize these methods by using fluorescence microscopy. It enables not only a sensitive detection of the defects but also an evaluation of the quantity of lipids in these defects. A classification of the defects is proposed and statistics of their relative importance in regard to both methods and lipid composition are presented: it shows for example that 80% of the vesicles obtained by electroformation from 98% 1,2-dioleoyl-sn-glycero-3-phosphocholine are devoid of significant defects against only 40% of the vesicles with the gentle hydration method. It is also shown that the presence of too many negatively charged lipids does not favor the formation of unilamellar vesicles with both methods. For the gentle hydration, we checked if the negatively charged lipids were inserted in the vesicles membrane in the same proportion as that of the lipid mixture from which they are formed. The constant incorporation of a negatively charged labeled lipid despite an increasing presence of negatively charged 1,2-dioleoyl-sn-glycero-3-[phospho-l-serine] tends to confirm that the composition of vesicles is indeed close to that of the initial mixture.