We investigate isothermal diffusion and growth of micron-scale liquid domains within membranes of free-floating giant unilamellar vesicles with diameters between 80 and 250 μm. Domains appear after a rapid temperature quench, when the membrane is cooled through a miscibility phase transition such that coexisting liquid phases form. In membranes quenched far from a miscibility critical point, circular domains nucleate and then progress within seconds to late stage coarsening in which domains grow via two mechanisms 1), collision and coalescence of liquid domains, and 2), Ostwald ripening. Both mechanisms are expected to yield the same growth exponent, α = 1/3, where domain radius grows as time(α). We measure α = 0.28 ± 0.05, in excellent agreement. In membranes close to a miscibility critical point, the two liquid phases in the membrane are bicontinuous. A quench near the critical composition results in rapid changes in morphology of elongated domains. In this case, we measure α = 0.50 ± 0.16, consistent with theory and simulation.

the phase diagram itself. Brewster angle microscopy allows one to image the domains in a Langmuir monolayer with and without the probe molecules to directly test their effect. The combined Brewster angle (BAM)/ fluorescence microscope allows us to image simultaneously with the two techniques exactly the same domains in the Langmuir film. In general, the images taken by the two microscopes compare well. Comparison of the techniques can then make it easier to correlate the different domain properties leading to contrast in the two techniques. Some types of domains may however be much more evident with BAM than with FM. When placed in water, lipid molecules form multilamellar vesicles (MLVs) in which lipid layers are separated by water regions. These interlamellar water regions have thicknesses on the order of 1 to 10 nanometers depending on lipid type. What happens if water contains buffer molecules? Will buffer molecules be taken inside the MLVs? If yes, then to what extent? These questions arise because the physical properties of water regions next to lipid membranes have been shown to differ from bulk water [1]. We would like to know how buffer molecules and other solutes partition between MLVs and the outer solution. To determine this partitioning, we use lipid membranes that sink in pure water but float in buffer solutions of certain concentrations. We then find the exact concentration for which the mass density of the solution matches that of the MLVs. This density matching allows us to calculate the ratio between water and buffer molecules present inside MLVs by using data from smallangle x-ray scattering. For KCl solutions, as well as for solutions of common buffers, we find that solutes are excluded from the interlamellar water regions creating a solute deficit inside the MLVs. [1] Petrache et al., Biophys.

To help students who traditionally underperform in general chemistry, we created a supplementary instruction (SI) course and called it the STEM-Dawgs Workshops. These workshops are an extension of the Peer-led Team Learning (PLTL) SI. In addition to peer-facilitated problem-solving, we incorporated two components inspired by learning sciences: (1) training in research-based study skills, and (2) evidence-based interventions targeting psychological and emotional support. Here we use an explanatory mixed methods approach to measure the impact of the STEM-Dawgs Workshops, with a focus on four sub-populations that are historically underrepresented in Chemistry: underrepresented minorities, females, low-income students, and first-generation students. Specifically, we compared three groups of students in the same General Chemistry course: students in general chemistry and not the workshops (“Gen Chem students”), students in the workshops (“STEM-Dawgs”), and students who volunteered for the workshops but did not get in (“Volunteers”). We tested hypotheses with regression models and conducted a series of focus group interviews with STEM-Dawgs. Compared to the Gen Chem population, the STEM-Dawg and Volunteer populations were enriched with students in all four under-represented sub-populations. Compared to Volunteers, STEM-Dawgs had increased exam scores, sense of belonging, perception of relevance, self-efficacy, and emotional satisfaction about chemistry. URM STEM-Dawgs had lower failure rates, and exam score achievement gaps that impacted first-generation and female Gen Chem students were eliminated in the STEM-Dawg population. Finally, female STEM-Dawgs had an increased sense of belonging and higher emotional satisfaction about chemistry than women Volunteers. Focus groups suggested that successes came in part from the supportive peer-learning environment and the relationships with peer facilitators. Together, our results indicate that this supplementary instruction model can raise achievement and improve affect for students who are underrepresented in chemistry.

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morphologies and the transitions between them are important to many cellular processes. Supported lipid bilayer (SLB) provides a model system in which to quantitatively investigate transitions from planar to tubular and tubular to spherical morphologies. Following a small increase in temperature (~5-10 C) flexible filaments extrude from a fluid SLB. Individual filaments can reach hundreds of microns in length before spontaneously collapsing into discs. We demonstrate that the filaments are tubular by decreasing the external buffer concentration, which causes them to swell, first into resolvable tubules with capped ends and then into giant vesicles. At high ionic strength, the sub-resolution tubules are adsorbed to the SLB, enabling the measurement of their radius to within 55 nm using conventional fluorescence microscopy. The radius depends on the lipid tail composition and varies <10% along the tubule length. Under tension, tubules are even more uniform, having no measurable variation in radius.