Pore Model in the Melting Regime of a Lyotropic Biomembrane with an Anionic Phospholipid.

Aqueous dispersions of the anionic phospholipid dimyristoyl phosphatidyl glycerol (DMPG) exhibit an unusual "melting regime", at the phase transition between the ordered (gel) and the disordered (fluid liquid crystal) state of hydrocarbon chains, depending on the ionic strength and DMPG concentration, previously attributed to the pore formation. Dispersions with 150 mM DMPG present a lamellar phase above 23 °C, within the melting regime. In this study, we present a detailed pore model for the analysis of small-angle X-ray scattering (SAXS) results and their variation with temperature, focused on the surface fractions of pores in the bilayers. Large and small toroidal pores are necessary to explain the SAXS results. Pores have DMPG in the fluid conformation, whereas the flat region of the bilayer has DMPG molecules in fluid and in gel conformations. A particular strategy was developed to estimate the charges due to the localization of mobile ions in the system, which is based on the calculation of electron densities by duly considering all molecular and ionic species that characterize the system, and the temperature dependency of their volumes. The best fit to the model of SAXS curves defines that the gel phase transforms initially, at 19.4 °C, in uncoupled bilayers with large pores (radius 93.2 ± 0.5 Å, with water channel diameter 137 ± 1 Å), which transform into small pores along the lamellar phase. The minimum intensity of the SAXS bilayer peak at 30 °C corresponds to a maximum number of small pores, and above 35 °C, the system enters into the normal lamellar fluid phase, without pores. The charge is estimated and shows that the regions with pores contains less Na+ ions per polar head; hence, when they are forming, there is a release of Na+ ions toward the bulk.

Forearm articular proportions and the antebrachial index in Homo sapiens, Australopithecus afarensis and the great apes.

When hominin bipedality evolved, the forearms were free to adopt nonlocomotor tasks which may have resulted in changes to the articular surfaces of the ulna and the relative lengths of the forearm bones. Similarly, sex differences in forearm proportions may be more likely to emerge in bipeds than in the great apes given the locomotor constraints in Gorilla, Pan and Pongo. To test these assumptions, ulnar articular proportions and the antebrachial index (radius length/ulna length) in Homo sapiens (n=51), Gorilla gorilla (n=88), Pan troglodytes (n=49), Pongo pygmaeus (n=36) and Australopithecus afarensis A.L. 288-1 and A.L. 438-1 are compared. Intercept-adjusted ratios are used to control for size and minimize the effects of allometry. Canonical scores axes show that the proximally broad and elongated trochlear notch with respect to size in H. sapiens and A. afarensis is largely distinct from G. gorilla, P. troglodytes and P. pygmaeus. A cluster analysis of scaled ulnar articular dimensions groups H. sapiens males with A.L. 438-1 ulna length estimates, while one A.L. 288-1 ulna length estimate groups with Pan and another clusters most closely with H. sapiens, G. gorilla and A.L. 438-1. The relatively low antebrachial index characterizing H. sapiens and non-outlier estimates of A.L. 288-1 and A.L. 438-1 differs from those of the great apes. Unique sex differences in H. sapiens suggest a link between bipedality and forearm functional morphology.

Effect of counterions on the shape, hydration, and degree of order at the interface of cationic micelles: the triflate case.

Specific ion effects in surfactant solutions affect the properties of micelles. Dodecyltrimethylammonium chloride (DTAC), bromide (DTAB), and methanesulfonate (DTAMs) micelles are typically spherical, but some organic anions can induce shape or phase transitions in DTA(+) micelles. Above a defined concentration, sodium triflate (NaTf) induces a phase separation in dodecyltrimethylammonium triflate (DTATf) micelles, a phenomenon rarely observed in cationic micelles. This unexpected behavior of the DTATf/NaTf system suggests that DTATf aggregates have unusual properties. The structural properties of DTATf micelles were analyzed by time-resolved fluorescence quenching, small-angle X-ray scattering, nuclear magnetic resonance, and electron paramagnetic resonance and compared with those of DTAC, DTAB, and DTAMs micelles. Compared to the other micelle types, the DTATf micelles had a higher average number of monomers per aggregate, an uncommon disk-like shape, smaller interfacial hydration, and restricted monomer chain mobility. Molecular dynamic simulations supported these observations. Even small water-soluble salts can profoundly affect micellar properties; our data demonstrate that the -CF3 group in Tf(-) was directly responsible for the observed shape changes by decreasing interfacial hydration and increasing the degree of order of the surfactant chains in the DTATf micelles.

Melting regime of the anionic phospholipid DMPG: new lamellar phase and porous bilayer model.

Aqueous dispersions of the anionic phospholipid dimyristoyl phosphatidylglycerol (DMPG) at pH above the apparent pK of DMPG and concentrations in the interval 70-300 mM have been investigated by small (SAXS) and wide-angle X-ray scattering, differential scanning calorimetry, and polarized optical microscopy. The order-disorder transition of the hydrocarbon chains occurs along an interval of about 10 degrees C (between T(m)(on) approximately 20 degrees C and T(m)(off) approximately 30 degrees C). Such melting regime was previously characterized at lower concentrations, up to 70 mM DMPG, when sample transparency was correlated with the presence of pores across the bilayer. At higher concentrations considered here, the melting regime persists but is not transparent. Defined SAXS peaks appear and a new lamellar phase L(p) with pores is proposed to exist above 70 mM DMPG, starting at approximately 23 degrees C (approximately 3 degrees C above T(m)(on)) and losing correlation after T(m)(off). A new model for describing the X-ray scattering of bilayers with pores, presented here, is able to explain the broad band attributed to in-plane correlation between pores. The majority of cell membranes have a net negative charge, and the opening of pores across the membrane tuned by ionic strength, temperature, and lipid composition is likely to have biological relevance.

Extensive bilayer perforation coupled with the phase transition region of an anionic phospholipid.

At low ionic strength dimyristoylphosphatidylglycerol (DMPG) exhibits a broad phase transition region characterized by several superimposed calorimetric peaks. Peculiar properties, such as sample transparency, are observed only in the transition region. In this work we use differential scanning calorimetry (DSC), turbidity, and optical microscopy to study the narrowing of the transition region with the increase of ionic strength (0-500 mM NaCl). Upon addition of salt, the temperature extension of the transition region is reduced, and the number of calorimetric peaks decreases until a single cooperative event at T(m) = 23 degrees C is observed in the presence of 500 mM NaCl. The transition region is always coupled with a decrease in turbidity, but a transparent region is detected within the melting process only in the presence of up to 20 mM NaCl. The vanishing of the transparent region is associated with one of the calorimetric peaks. Optical microscopy of giant vesicles shows that bilayers first rupture when the transition region is reached and subsequently lose optical contrast. Fluorescence microscopy reveals a blurry and undefined image in the transparent region, suggesting a different lipid self-assembly. Overall sample turbidity can be directly related to the bilayer optical contrast. Our observations are discussed in terms of the bilayer being perforated along the transition region. In the narrower temperature interval of the transparent region, dependent on the ionic strength, the perforation is extensive and the bilayer completely loses the optical contrast.

Influence of salt on the structure of DMPG studied by SAXS and optical microscopy.

Aqueous dispersions of 50 mM dimyristoylphosphatidylglycerol (DMPG) in the presence of increasing salt concentrations (2-500 mM NaCl) were studied by small angle X-ray scattering (SAXS) and optical microscopy between 15 and 35 degrees C. SAXS data show the presence of a broad peak around q approximately 0.12 A(-1) at all temperatures and conditions, arising from the electron density contrasts within the bilayer. Up to 100 mM NaCl, this broad peak is the main feature observed in the gel and fluid phases. At higher ionic strength (250-500 mM NaCl), an incipient lamellar repeat distance around d=90-100 A is detected superimposed to the bilayer form factor. The data with high salt were fit and showed that the emergent Bragg peak is due to loose multilamellar structures, with the local order vanishing after approximately 4d. Optical microscopy revealed that up to 20 mM NaCl, DMPG is arranged in submicroscopic vesicles. Giant (loose) multilamellar vesicles (MLVs) start to appear with 50 mM NaCl, although most lipids are arranged in small vesicles. As the ionic strength increases, more and denser MLVs are seen, up to 500 mM NaCl, when MLVs are the prevailing structure. The DLVO theory could account for the experimentally found interbilayer distances.

Mechanical analysis of infant carrying in hominoids.

In all higher nonhuman primates, species survival depends upon safe carrying of infants clinging to body hair of adults. In this work, measurements of mechanical properties of ape hair (gibbon, orangutan, and gorilla) are presented, focusing on constraints for safe infant carrying. Results of hair tensile properties are shown to be species-dependent. Analysis of the mechanics of the mounting position, typical of heavier infant carrying among African apes, shows that both clinging and friction are necessary to carry heavy infants. As a consequence, a required relationship between infant weight, hair-hair friction coefficient, and body angle exists. The hair-hair friction coefficient is measured using natural ape skin samples, and dependence on load and humidity is analyzed. Numerical evaluation of the equilibrium constraint is in agreement with the knuckle-walking quadruped position of African apes. Bipedality is clearly incompatible with the usual clinging and mounting pattern of infant carrying, requiring a revision of models of hominization in relation to the divergence between apes and hominins. These results suggest that safe carrying of heavy infants justify the emergence of biped form of locomotion. Ways to test this possibility are foreseen here.