Janet Layne Marshall

Partition of sodium dodecyl sulfate into stratum corneum lipid liposomes.

Synthetic detergents produce deleterious effects on human skin as the result of being taken up by the stratum corneum (SC). The present study aimed to determine to what extent a typical detergent enters the SC lipid lamellae, and what effect this might have on the physical properties of the lipids. These effects were studied in large unilamellar liposomes prepared from SC lipids (50% by weight of epidermal ceramides, 28% cholesterol, 17% free fatty acids, and 5% cholesteryl sulfate) by extrusion through successive polycarbonate filters of decreasing pore size, finally 400 nm. Freeze-fracture electron microscopy and light-scattering particle size analysis indicated a uniform liposome diameter averaging 230 nm. Partitioning of sodium dodecyl sulfate (SDS) into the lipid phase from aqueous buffer solutions was measured using the SC lipid liposomes and [U-14C]SDS. The partition coefficient was 416, 450, and 588 at pH 8.5 and 524, 507, and 807 at pH 7 for three different concentrations (0.1%, 0.02%, and 0.004%) of SDS. This high degree of partitioning into the liposomes is consistent with the high level of SDS partitioning seen in full SC. At the maximum, the SDS represented 18% of the liposomal lipids. Preparation of stable liposomes from SC lipids to which 10% or 20% of SDS had been added confirmed the ability of the liposomes to survive these high concentrations of surfactant. The permeability of the liposomes was enhanced as a result of SDS partitioning into the bilayers, as measured by the increased release of trapped [U-14C]glucose from these vesicles, and by their increased permeability to water in osmotic shock experiments. These studies demonstrate that SDS in dilute solution can partition into lipid bilayers at high concentration so as to affect the properties of the lipid lamellae that constitute the epidermal permeability barrier.

Picosecond spectroscopic studies of (d8-d8) binuclear rhodium and iridium complexes: a comparison of 1B2 and 3B2 reactivity in bis(1,5-cyclooctadiene)bis(.mu.-pyrazolyl)diiridium(I)

Picosecond transient kinetics and difference spectra have been recorded for the singlet and triplet (dsigma*psigma) excited states in the d/sup 8/-d/sup 8/ dimers Rh/sub 2/(TMB)/sub 4//sup 2 +/ (TMB = 2,5-dimethyl-2,5-diisocyanohexane) and (Ir(..mu..-pz)(COD))/sub 2/ (pz = pyrazolyl, COD = 1,5-cyclooctadiene). The singlet excited state in the rhodium dimer (tau=- 820 ps) displays a strong transient absorption feature maximizing at 440 nm that is not present in the spectrum of the triplet excited state. This intense absorption feature, characteristic of a /sup 1/(dsigma-psigma) electronic configuration, is assigned to a /sup 1/(dsigma*psigma) ..-->.. /sup 1/(psigma/sup 2/) excitation. This singlet excited state lifetime of the iridium dimer in cyclohexane is less than 20 ps. Though the solvent 1,2-dichloroethane (DCE) quenches luminescence from both singlet and triplet excited states in (Ir(..mu..-pz)(COD))/sub 2/ and oxidatively adds to the dimer upon steady-state illumination, picosecond spectroscopy finds no evidence for any chemical reactivity of the very short-lived singlet excited state. The quenching of (Ir(..mu..-pz)(COD))/sub 2/ singlet luminescence in DCE appears to result from enhanced singlet ..-->.. triplet intersystem crossing in DCE relative to that in cyclohexane. Also, the invariance of triplet yield in these two solvents indicates that the efficiency of intersystem crossing is near unity.