Friday, May 17, 2013

A Promising Drug Controlled-Release System Based on Diacetylene/ Phospholipid Polymerized Vesicles

Caixin Guo, Shaoqin Liu, Chang Jiang, Wenyuan Li, and Zhifei Dai
Langmuir 2009, 25(22), 13114–13119


A novel polymerized vesicular carrier loaded with paclitaxel was developed by introducing the ultraviolet (UV) crosslinkable 10,12-pentacosadiynoic acid (PCDA) into bilayered phospholipid vesicles with the purpose of improving the physicochemical stability as well as the controlled-release property of liposomes. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) results revealed the enhanced stability of PCDA-polymerized vesicles against Triton X-100. In particular, alteration in PCDA/phospholipids ratios and UV-irradiation time can modulate the cumulative paclitaxel released. For instance, vesicles composed of phospholipids only released 98.0(2.1% of paclitaxel within 24 h. Over the same time period, 72.0 ( 5.8%, 43.9 ( 6.5%, and 20.1 ( 5.4% of paclitaxel was released from polymerized PCDA/phospholipid vesicles at molar ratios of 1:3, 1:1, and 3:1, respectively. Likewise, by increasing the UV-irradiation time from 20 to 40 min, the cumulative release of paclitaxel from polymerized  CDA/phospholipid vesicles at molar ratio of 1:1 decreased from 90.5 ( 3.7% to 37.6 ( 2.3% over a time period of experimental observation of 24 h. The influences of vesicle composition (i.e., PCDA/phospholipids ratio) and UV-irradiation time on the release rates of paclitaxel were further examined by finite element (FE) analyzed using Abaqus. Our results demonstrate that novel polymerized vesicles capable of regulating the release of anticancer drugs such as paclitaxel have been developed.

Liposome−Quantum Dot Complexes Enable Multiplexed Detection of Attomolar DNAs without Target Amplification

Juan Zhou, Qiang-xin Wang, and Chun-yang Zhang
J. Am. Chem. Soc. 2013, 135, 2056−2059


Sensitive detection of DNA usually relies on target amplification approaches such as polymerase chain reaction and rolling circle amplification. Here we describe a new approach for sensitive detection of lowabundance DNA using liposome−quantum dot (QD) complexes and single-particle detection techniques. This assay allows for detection of single-stranded DNA at attomolar concentrations without the involvement of target amplification. Importantly, this strategy can be employed for simultaneous detection of multiple DNA targets.

Saturday, May 11, 2013

Interaction of Ionic Surfactants with Cornea-Mimicking Anionic Liposomes

Chhavi Gupta, Andrew K. Daechsel, and Anuj Chauhan
Langmuir 2011, 27, 10840–10846


The interaction of surface-active molecules with lipid bilayers is ubiquitous both in biological systems and also in several technological applications.Here we explore the interaction of ionic surfactants with liposomes whose composition mimics the ocular epithelia. In this study, liposomes with a composition mimicking ocular epithelia are loaded with calcein dye above the self-quenching concentration. The liposomes are then exposed to surfactants, and the rate of dye leaked from the liposomes due to the interaction of surfactants is measured. Both cationic and anionic surfactants at various concentrations and ionic strengths are explored. Results show that the liposome bilayer permeability to the dye increases on exposure to the surfactants, leading to the release of the dye trapped in the core. However, the dye release stops after a finite time, suggesting a transient increase in permeability followed by healing. The leakage profiles exhibit two different timescales for the cationic surfactant but only one timescale for the anionic surfactant. The total dye leakage increases with surfactant concentration, and at a given concentration, the dye leakage is significantly higher for the cationic surfactants. The timescale for the healing decreases with increasing surfactant concentration, and increasing ionic strength increases the dye leakage for the anionic surfactant. These results show that the surfactant binding to the lipid bilayer increases the permeability while the bilayers heal likely because of the surfactant jump from the outer to the inner leaflet and/or rearrangement into tighter aggregates.

Shrink-Wrap Vesicles

Shelly M. Fujikawa, Irene A. Chen, and Jack W. Szostak
Langmuir 2005, 21, 12124-12129


We describe a simple approach to the controlled removal of molecules from the membrane of large unilamellar vesicles made of fatty acids. Such vesicles shrink dramatically upon mixing with micelles composed of a mixture of fatty acid and a phospholipid (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)), as fatty acid molecules leave the vesicle membrane and accumulate within the mixed micelles. Vesicle shrinkage was confirmed by dynamic light scattering, fluorescence recovery after photobleaching of labeled vesicles, and fluorescence resonance energy transfer between lipid dyes incorporated into the vesicle membrane. Most of the encapsulated impermeable solute is retained during shrinkage, becoming concentrated by a factor of at least 50-fold in the final small vesicles. This unprecedented combination of vesicle shrinkage with retention of contents allows for the preparation of small vesicles containing high solute concentrations, and may find applications in liposomal drug delivery.

On Effective Leadership


MANY HAVE PONDERED WHY SOME NATIONS ARE MUCH MORE SUCCESSFUL THAN OTHERS. IS THE central factor an abundance of natural resources, wise traditional systems of governance, or the absence of serious confl icts? Or does a nation thrive merely as the result of a series of historical accidents? I believe that a primary determinant of success, often neglected, is how the leaders of its major institutions,* governmental and nongovernmental, are selected; how they in turn choose their deputies; and under what incentives they must operate. The incentives part is perhaps the easiest to describe. To achieve anything of great importance takes time, but in an increasingly rapid-paced world of quarterly reports and sound bites, leaders often become hamstrung by short-term goals. And in large public organizations such as government agencies, leaders need to resist the inevitable pressures from employees to expand the responsibilities and budget of their particular division, even when other parts of the government (or society) are more qualifi ed to meet a goal. A core assumption here is that the leaders are qualifi ed to lead. In a merit-based society, the selection of the most qualifi ed person for each position of responsibility, independent of personal connections, background, sex, or age, has the obvious advantage of placing critical decision-making into capable hands. Much less obvious is the confounding fact that oustanding “A” individuals commonly have the selfconfidence and discrimination needed to hire A, and whenever possible, A-plus, deputies below them. Conversely, when a poorly qualified person is selected as a leader, the oppposite occurs. Feeling insecure, such B individuals will hire only B-minus or C deputies, giving rise to a propagating chain of mediocrity that degrades the entire institution. When serving as president of the U.S. National Academy of Sciences (NAS), I befriended a very wise NAS Foreign Associate from Nigeria, Dr. Akin Mabogunje, who wrote an autobiography on the occasion of his 80th birthday.† In the final chapter, he describes the deplorable situation in his country created by “ascriptive rights”: rights that “just depend on being able to assert that one belongs to a group or part of the country to be able to lay claim to positions of authority or power.” As he starkly states: “The first consequence of putting unqualified individuals in important positions is their failure to grasp the nature of the opportunity being given to them to excel. Lacking the requisite mental capacity to cope with the challenges of the leadership position, they invariably turn their attention inward to weed out those whose presence reminds them of their own inadequacies. The mission of the institutions is thus lost in the pervasive fear of victimization for diligence. Any attempt at pursuing excellence at work is seen as a way of showing off the weakness of management and is punished rather than rewarded. In no time, the institution is manned by individuals who are willing to kowtow to the whims and caprices of the so-called managers or heads.” I have seen the tragic consequences of the situation Mabogunje describes repeated over and over in nations around the world, from important U.S. institutions such as large school districts to international organizations associated with the United Nations. But I have also witnessed the opposite: the flourishing of institutions in which the deputies outshine the boss. This is why my advice to managers is to aim at hiring people who, at least in some respects, appear to be more talented than you are. This requires honest humility. But only in this way can a leader hope to achieve his or her goals.
It is only through a meritocracy in which leaders encourage creativity from outstanding subordinates and are primarily rewarded for long-term, rather than short-term achievements that a nation—and the world—can expect to meet the many challenges that lie before us.

– Bruce Alberts

A study on the interactions of surfactants with phospholipid/ polydiacetylene vesicles in aqueous solutions

Colloids and Surfaces A: Physicochem. Eng. Aspects 257–258 (2005) 25–30
Yan-Lei Su, Jin-Ru Li, Long Jiang


Abstract: A colorimetric method for studying the interactions of surfactants with lipid vesicles, formed from dimyristoylphosphatidycholine (DMPC) and polymerized 10,12-pentacosadiynoic acid (PCDA), in aqueous solutions was reported. The electrostatic interaction and hydrophobic interaction play a key role in the interaction of the mixed lipid vesicles with surfactant. The hydrophobic interaction results in the insertion of the alkyl chain of surfactants into the hydrophobic domain of the vesicles. The insertion perturbs the conformation of the polymerized PCDA backbone in the mixed vesicles, which induces the chromatic response. The value of chromatic response induced by the addition of surfactants was found to correlate with the DMPC content in the vesicles; an increase of DMPC content would enhance the interaction of surfactants with the mixed vesicles. 

Sunday, April 21, 2013

How Do Amphiphiles Form Ion-Conducting Channels in Membranes? Lessons from Linear Oligoesters


THOMAS M. FYLES

ACCOUNTS OF CHEMICAL RESEARCH

The X-ray crystal structures of biological ion channels are exquisitely complex, but not all natural products capable of forming ion-conducting channels are equally elaborate. Examples such as the peptides gramicidin or alamethicin or the polyene antibiotics amphotericin and nystatin clearly form well-defined channels without requiring a massive protein superstructure. These molecules form the starting point for a supramolecular chemistry challenge: how to create synthetic compounds and systems that catalyze the translocation of ionic species across bilayer membranes mimicking naturally occurring channels. Over the past three decades, supramolecular chemists have developed numerous examples of systems with transport rates and efficiencies that rival natural channels. As the field developed, researchers discovered many compounds that are functional for ion transport but bear very little resemblance to any imagined architectures of ion channels. We and others have followed these lead compounds extensively in a quest to focus on the mechanisms such simple compounds use to achieve their function. These compounds show all the hallmarks of ion channels including high activity, ion specificity, regular time-dependent conductance changes, and in some cases higher-order phenomena such as voltage-dependent activity.