Dynamic Second Harmonic Imaging of Proton Translocation Through Water Needles in Lipid Membranes.

Proton translocation through lipid membranes is a fundamental process in the field of biology. Several theoretical models have been developed and presented over the years to explain the phenomenon, yet the exact mechanism is still not well understood. Here, we show that proton translocation is directly related to membrane potential fluctuations. Using high-throughput wide-field second harmonic (SH) microscopy, we report apparently universal transmembrane potential fluctuations in lipid membrane systems. Molecular simulations and free energy calculations suggest that H+ permeation proceeds predominantly across a thin, membrane-spanning water needle and that the transient transmembrane potential drives H+ ions across the water needle. This mechanism differs from the transport of other cations that require completely open pores for transport and follows naturally from the well-known Grotthuss mechanism for proton transport in bulk water. Furthermore, SH imaging and conductivity measurements reveal that the rate of proton transport depends on the structure of the hydrophobic core of bilayer membranes.

Passive transport of Ca2+ ions through lipid bilayers imaged by widefield second harmonic microscopy.

In biology, release of Ca2+ ions in the cytosol is essential to trigger or control many cell functions. Calcium signaling acutely depends on lipid membrane permeability to Ca2+. For proper understanding of membrane permeability to Ca2+, both membrane hydration and the structure of the hydrophobic core must be taken into account. Here, we vary the hydrophobic core of bilayer membranes and observe different types of behavior in high-throughput wide-field second harmonic imaging. Ca2+ translocation is observed through mono-unsaturated (DOPC:DOPA) membranes, reduced upon the addition of cholesterol, and completely inhibited for branched (DPhPC:DPhPA) and poly-unsaturated (SLPC:SLPA) lipid membranes. We propose, using molecular dynamics simulations, that ion transport occurs through ion-induced transient pores, which requires nonequilibrium membrane restructuring. This results in different rates at different locations and suggests that the hydrophobic structure of lipids plays a much more sophisticated regulating role than previously thought.

Ion-Induced Transient Potential Fluctuations Facilitate Pore Formation and Cation Transport through Lipid Membranes.

Unassisted ion transport through lipid membranes plays a crucial role in many cell functions without which life would not be possible, yet the precise mechanism behind the process remains unknown due to its molecular complexity. Here, we demonstrate a direct link between membrane potential fluctuations and divalent ion transport. High-throughput wide-field non-resonant second harmonic (SH) microscopy of membrane water shows that membrane potential fluctuations are universally found in lipid bilayer systems. Molecular dynamics simulations reveal that such variations in membrane potential reduce the free energy cost of transient pore formation and increase the ion flux across an open pore. These transient pores can act as conduits for ion transport, which we SH image for a series of divalent cations (Cu2+, Ca2+, Ba2+, Mg2+) passing through giant unilamellar vesicle (GUV) membranes. Combining the experimental and computational results, we show that permeation through pores formed via an ion-induced electrostatic field is a viable mechanism for unassisted ion transport.

Imaging Cu2+ binding to charged phospholipid membranes by high-throughput second harmonic wide-field microscopy.

The interaction of divalent copper ions (Cu2+) with cell membranes is crucial for a variety of physiological processes of cells, such as hormone synthesis and cellular energy production. These interactions would not be possible without membrane hydration. However, the role of water has not received a lot of attention in membrane studies. Here, we use high-throughput wide-field second harmonic (SH) microscopy to study the interaction between Cu2+ and hydrated freestanding Montal-Müller lipid membranes. The symmetric lipid membranes are composed of 1,2-diphytanoyl-sn-glycero-3-phosphocholine and either 1,2-diphytanoyl-sn-glycero-3-phosphate or 1,2-diphytanoyl-sn-glycero-3-phospho L-serine and are brought into contact with divalent Cu2+, which are added to one leaflet while maintaining the ionic strength balance. We observe transient domains of high SH intensity. In these areas, Cu2+ ions bind to the charged head groups, leading to charge neutralization on one side of the membrane. This exposes the ordered water at the non-interacting side of the membrane interface, which can be used to compute the interfacial membrane potential difference. We find that the domains of lipids with phosphatidic acid head groups display a higher interfacial membrane potential than those with phosphatidylserine head groups, which converts into higher dynamic electrostatic free energies and binding constants.

Highly Rectifying Heterojunctions Formed by Annealed ZnO Nanorods on GaN Substrates.

We study the effect of thermal annealing on the electrical properties of the nanoscale p-n heterojunctions based on single n-type ZnO nanorods on p-type GaN substrates. The ZnO nanorods are prepared by chemical bath deposition on both plain GaN substrates and on the substrates locally patterned by focused ion beam lithography. Electrical properties of single nanorod heterojunctions are measured with a nanoprobe in the vacuum chamber of a scanning electron microscope. The focused ion beam lithography provides a uniform nucleation of ZnO, which results in a uniform growth of ZnO nanorods. The specific configuration of the interface between the ZnO nanorods and GaN substrate created by the focused ion beam suppresses the surface leakage current and improves the current-voltage characteristics. Further improvement of the electrical characteristics is achieved by annealing of the structures in nitrogen, which limits the defect-mediated leakage current and increases the carrier injection efficiency.

Impact of traveling to visit friends and relatives on chronic disease management.

BACKGROUND: Travelers visiting friends and relatives (VFR) are known to be at high risk of acquiring infectious diseases during travel. However, little is known about the impact of VFR travel on chronic diseases. METHODS: This was a nonrandomized, retrospective observational study. Patients were adult VFR travelers who received care from an internal medical clinic serving immigrants and refugees. The primary objective was to determine the impact of VFR travel on markers of chronic disease management including: blood pressure, glycosylated hemoglobin, body mass index, serum creatinine, and anticoagulation. RESULTS: Of the 110 VFR travelers in our study, N = 48 traveled to Africa and N = 62 traveled to Asia for a mean duration of 59 (range 21-303) days. Of the 433 counseling points discussed at pre-travel visits, 71% were infectious disease prevention, 16% chronic disease related, and 13% travel safety. A total of 63 patients (57%) reported one or more health problems while traveling. Of these, 35 patients (32%) experienced a problem related to a chronic condition. In comparison, 24 (22%) patients experienced an acute infection. Sixty percent of patients were nonadherent to medications during travel. An average increase in diastolic blood pressure of 3.6 mmHg among patients with hypertension was the only statistically significant change in a chronic disease marker when values before and after travel were compared. Subgroup analysis revealed that travel to Africa and nonadherence to medications were also associated with worsening blood pressure control, and patients traveling to Africa experienced a decrease in body mass index. CONCLUSIONS: This study identified a high proportion of problems related to chronic conditions experienced during VFR travel, while pre-travel appointments tended to focus on infectious disease prevention. A greater emphasis on medication adherence and chronic disease management during VFR travel is also needed during pre-travel preparations.