Biomembranes assume nonlamellar structures in many cellular events, with the tendency of forming a nonlamellar structure quantified by the monolayer spontaneous curvature, C₀, and with many of these events involving the acts of Ca²⁺. Despite this biologically important intimacy, how C₀ is affected by [Ca²⁺] is unknown. In this study, we use the X-ray diffraction technique and the reconstruction of electron density profiles to measure the C₀s of a zwitterionic phospholipid, DOPE, and two anionic phospholipids, DOPA and 18 : 1 (9Z) cardiolipin, at temperatures from 20 °C to 40 °C and [Ca²⁺]s from 0 mM to 100 mM; these phospholipids are chosen to examine the contributions of the electric charge density per molecule. While showing a strong dependence on temperature, C_0,DOPE is nearly independent of [Ca²⁺]. In contrast, C_0,DOPA and C_{0,cardiolipin} are almost unresponsive to the temperature change but affected by the [Ca²⁺] variation; and C_0,DOPA varies with [Ca²⁺] ∼1.5 times more strongly than C_{0,cardiolipin}, with the phase preferences of DOPA and cardiolipin shifting to the H_II phase and remaining on the L_α phase, respectively, at [Ca²⁺] = 100 mM. From these observations, we reveal the effects of modulating the strength of the inter-headgroup repulsion and discuss the mechanisms underlying the phase behaviour and cellular functions of the investigated phospholipids. Most importantly, this study recognizes that the headgroup charge density is dominant in dictating the phase behaviour of the anionic phospholipids, and that the unique molecular characteristics of cardiolipin are critically needed both for maintaining the structural integrity of cardiolipin-rich biomembranes and for fulfilling the biological roles of the phospholipid.

Membrane fusion is a vital process in key cellular events. The fusion capability of a membrane depends on its elastic properties and varies with its lipid composition. It is believed that as the composition varies, the consequent change in C₀ (monolayer spontaneous curvature) is the major factor dictating fusion, owing to the associated variation in G_Es (elastic energies) of the fusion intermediates (e.g. stalk). By exploring the correlations among fusion, C₀ and K_cp (monolayer bending modulus), we revisit this long-held belief and re-examine the fusogenic contributions of some relevant factors. We observe that not only C₀ but also K_cp variations affect fusion, with depression in K_cp leading to suppression in fusion. Variations in G_Es and inter-membrane interactions cannot account for the K_cp-fusion correlation; fusion is suppressed even as the G_Es decrease with K_cp, indicating the presence of factor(s) with fusogenic importance overtaking that of G_E. Furthermore, analyses find that the C₀ influence on fusion is effected via modulating G_E of the pre-fusion planar membrane, rather than stalk. The results support a recent proposition calling for a paradigm shift from the conventional view of fusion and may reshape our understanding to the roles of fusogenic proteins in regulating cellular fusion machineries.

In this study, a novel organic/inorganic hybrid material containing cadmium sulfide (CdS) nanoparticles was synthesized in situ by using an all π-conjugated block copolymer as both a synergistic long-range ordered structure-directing template and an efficient exciton quencher for the nanohybrid. The conducting–conducting block copolymer of poly(2,5-dihexyloxy-para-phenylene)-b-poly(3-hexyl thiophene) (PPP-P3HT) was used as a linear nanoreactor for the quantum dot synthesis to yield nanohybrids with highly ordered donor/acceptor (D/A) nanowire morphology that exhibit enhanced optoelectronic properties. The synthesized PPP-P3HT/CdS-OH nanohybrid possessed a one-dimensional D/A coaxial nanowire structure of uniform width (∼10 nm), wherein the synthesized CdS quantum dots were embedded, resulting in a display of superior long-term stability in a solution medium. By a simple drop casting process, the hybrid materials with highly ordered organic/inorganic nanochannel arrays could also be preserved in a thin film state, as evidenced by TEM and X-ray measurements, for device applications. The PPP-P3HT block copolymer exhibited significantly better hole mobility than its P3HT homopolymer counterpart. In addition, the resulting PPP-P3HT/CdS-OH hybrid systems displayed a considerably enhanced PL quench effect and shorter exciton lifetime due to efficient charge transfer between the P3HT block and the confining CdS nanoparticles in the P3HT/CdS coaxial domain of the hybrid nanowire structure. Therefore, the PPP-P3HT/CdS-OH nanohybrids can be used as bicontinuous nanochannels for efficient charge separation and charge transport, which show potential for use in future optoelectronic applications.

Human amniotic fluid-derived stem cells (hAFCs) are pluripotent fetal cells capable of differentiating into multiple lineages, including cell types of each of the three embryonic germ layers. Proper differentiation and maintenance of pluripotency, the defining characteristics of stem cells, are regulated not only by the cells themselves but also by their microenvironment. Furthermore, the physical characteristics of the cell culture materials, such as material elasticity, influence the results of stem cell differentiation. We investigated the osteogenic differentiation efficiency of hAFCs cultured on cell culture materials with different elasticities that were grafted with extracellular matrix-derived oligopeptides. Polyvinyl alcohol-co-itaconic acid (PV) hydrogels with different elasticities were prepared by controlling the crosslinking intensity, and the resulting PV hydrogels were grafted with and without extracellular matrix (ECM)-derived oligopeptides. Specific ECM-derived oligopeptides were used to maintain the pluripotency of AFCs and were determined by evaluation of pluripotent gene expression (Sox2 and Oct4). The osteogenic differentiation efficiency of the hAFCs, cultured on PV hydrogels grafted with and without ECM-derived oligopeptides, was analyzed by alkaline phosphatase activity, Alizarin Red S staining, and von Kossa staining. Unmodified PV hydrogels induced osteoblast differentiation of hAFCs with high efficiency. We conclude that the hAFCs interacting with ECM-derived oligopeptides tended to maintain an undifferentiated state.

There is considerable evidence for the essential role of surface water in protein function and structure. However, it is unclear to what extent the hydration water and protein are coupled and interact with each other. Here, we show by ESR experiments (cw, DEER, ESEEM, and ESE techniques) with spin-labeling and nanoconfinement techniques that the vitrified hydration layers can be evidently recognized in the ESR spectra, providing nanoscale understanding for the biological interfacial water. Two peptides of different secondary structures and lengths are studied in vitrified bulk solvents and in water-filled nanochannels of different pore diameter (6.1∼7.6 nm). The existence of surface hydration and bulk shells are demonstrated. Water in the immediate vicinity of the nitroxide label (within the van der Waals contacts, ∼0.35 nm) at the water-peptide interface is verified to be non-crystalline at 50 K, and the water accessibility changes little with the nanochannel dimension. Nevertheless, this water accessibility for the nanochannel cases is only half the value for the bulk solvent, even though the peptide structures remain largely the same as those immersed in the bulk solvents. On the other hand, the hydration density in the range of ∼2 nm from the nitroxide spin increases substantially with decreasing pore size, as the density for the largest pore size (7.6 nm) is comparable to that for the bulk solvent. The results demonstrate that while the peptides are confined but structurally unaltered in the nanochannels, their surrounding water exhibits density heterogeneity along the peptide surface normal. The causes and implications, especially those involving the interactions between the first hydration water and peptides, of these observations are discussed. Spin-label ESR techniques are proven useful for studying the structure and influences of interfacial hydration.