The Potential of Selenium-Based Therapies for Ocular Oxidative Stress.

Oxidative stress plays a critical role in the development of chronic ocular conditions including cataracts, age-related macular degeneration, and diabetic retinopathy. There is a need to explore the potential of topical antioxidants to slow the progression of those conditions by mediating oxidative stress and maintaining ocular health. Selenium has attracted considerable attention because it is a component of selenoproteins and antioxidant enzymes. The application of selenium to a patient can increase selenoprotein expression, counteracting the effect of reactive oxygen species by increasing the presence of antioxidant enzymes, and thus slowing the progression of chronic ocular disorders. Oxidative stress effects at the biomolecular level for prevalent ocular conditions are described in this review along with some of the known defensive mechanisms, with a focus on selenoproteins. The importance of selenium in the eye is described, along with a discussion of selenium studies and uses. Selenium's antioxidant and anti-inflammatory qualities may prevent or delay eye diseases. Recent breakthroughs in drug delivery methods and nanotechnology for selenium-based ocular medication delivery are enumerated. Different types of selenium may be employed in formulations aimed at managing ocular oxidative stress conditions.

Sterol composition in plants is specific to pollen, leaf, pollination and pollinator.

Sterols have several roles in planta, including as membrane components. Sterols are also essential nutrients for insects. Based on this, and the different functions of leaves and pollen, we tested the hypotheses that (a) the sterolome is different in leaves and pollen from the same plant, (b) pollens from wind- and insect pollinated plants comprise different sterols, and (c) sterol provision in pollen-rewarding angiosperms differs from nectar-rewarding species. A novel approach to sterolomics was developed, using LCMS to determine the sterol profile of leaf and pollen from a taxonomically diverse range of 36 plant species. Twenty-one sterols were identified unambiguously, with several more identified in trace amounts. C29 sterols dominated the sterolome in most plants. The sterol composition was significantly different in leaf and pollen and their main sterols evolved in different ways. The sterolome of pollen from animal- and wind-pollinated was also significantly different, but not between nectar- and pollen-rewarding species. Our results suggest that the sterol composition in different plant tissues is linked to their biological functions. Sterol composition in pollen might be driven by physical role rather than the nutrient needs of pollinating insects.

Do lipids shape the eukaryotic cell cycle?

Successful passage through the cell cycle presents a number of structural challenges to the cell. Inceptive studies carried out in the last five years have produced clear evidence of modulations in the lipid profile (sometimes referred to as the lipidome) of eukaryotes as a function of the cell cycle. This mounting body of evidence indicates that lipids play key roles in the structural transformations seen across the cycle. The accumulation of this evidence coincides with a revolution in our understanding of how lipid composition regulates a plethora of biological processes ranging from protein activity through to cellular signalling and membrane compartmentalisation. In this review, we discuss evidence from biological, chemical and physical studies of the lipid fraction across the cell cycle that demonstrate that lipids are well-developed cellular components at the heart of the biological machinery responsible for managing progress through the cell cycle. Furthermore, we discuss the mechanisms by which this careful control is exercised.

Fast dissolving paracetamol/caffeine nanofibers prepared by electrospinning.

A series of polyvinylpyrrolidone fibers loaded with paracetamol (PCM) and caffeine (CAF) was fabricated by electrospinning and explored as potential oral fast-dissolving films. The fibers take the form of uniform cylinders with smooth surfaces, and contain the drugs in the amorphous form. Drug-polymer intermolecular interactions were evidenced by infrared spectroscopy and molecular modeling. The properties of the fiber mats were found to be highly appropriate for the preparation of oral fast dissolving films: their thickness is around 120-130 µm, and the pH after dissolution in deionized water lies in the range of 6.7-7.2. Except at the highest drug loading, the folding endurance of the fibers was found to be >20 times. A flavoring agent can easily be incorporated into the formulation. The fiber mats are all seen to disintegrate completely within 0.5s when added to simulated saliva solution. They release their drug cargo within around 150s in a dissolution test, and to undergo much more rapid dissolution than is seen for the pure drugs. The data reported herein clearly demonstrate that electrospun PCM/CAF fibers comprise excellent candidates for oral fast-dissolving films, which could be particularly useful for children and patients with swallowing difficulties.

Amorphous formulations of indomethacin and griseofulvin prepared by electrospinning.

Following an array of optimization experiments, two series of electrospun polyvinylpyrrolidone (PVP) fibers were prepared. One set of fibers contained various loadings of indomethacin, known to form stable glasses, and the other griseofulvin (a poor glass former). Drug loadings of up to 33% w/w were achieved. Electron microscopy data showed the fibers largely to comprise smooth and uniform cylinders, with evidence for solvent droplets in some samples. In all cases, the drug was found to exist in the amorphous physical state in the fibers on the basis of X-ray diffraction and differential scanning calorimetry (DSC) measurements. Modulated temperature DSC showed that the relationship between a formulation's glass transition temperature (Tg) and the drug loading follows the Gordon-Taylor equation, but not the Fox equation. The results of Gordon-Taylor analysis indicated that the drug/polymer interactions were stronger with indomethacin. The interactions between drug and polymer were explored in more detail using molecular modeling simulations and again found to be stronger with indomethacin; the presence of significant intermolecular forces was further confirmed using IR spectroscopy. The amorphous form of both drugs was found to be stable after storage of the fibers for 8 months in a desiccator (relative humidity <25%). Finally, the functional performance of the fibers was studied; in all cases, the drug-loaded fibers released their drug cargo very rapidly, offering accelerated dissolution over the pure drug.

A 3-D hexagonal inverse micellar lyotropic phase.

Lipids that are found in cell membranes form a variety of self-assembled phases in the presence of water. Many of these structures are liquid-crystalline with structural motifs mirrored in cells and organelles and can be exploited in the delivery of drugs and genes. We report the discovery of a lyotropic liquid crystalline phase based on a 3-D hexagonal close-packed arrangement of inverse micelles, of space group P6(3)/mmc. This is the first new inverse lyotropic liquid-crystalline phase to be reported for two decades and is the only known lyotropic phase whose structure consists of a close packing of identical inverse micelles.

Factors controlling the stability of a kinetically hindered lamellar-lamellar transition.

We show that we can manipulate the stability of a metastable gel phase, either to enhance its transitory nature or to "lock" it in. Using simple additives such as salt and fatty alcohol we were able to examine both the long-range effect, acting between charged bilayers, and short-range effects on the metastability. We found that the addition of salt to the cationic surfactant diethanolamine ester dimethyl ammonium chloride destabilized the gel phase, and at high concentrations it was able to decrease the length of time taken for the gel phase to revert to a hydrated solid "coagel" phase by an order of magnitude. The growth of the coagel phase was also found to be affected by increasing salt concentration, changing from needle-like (1D) to spherical growth. In contrast to the marked destabilization of the gel phase by salt, the addition of 1-octadecanol was found to prolong the lifetime of the gel phase almost indefinitely by disrupting the short-range packing between the surfactant molecules. This suggests that counterion binding plays a major role in the stability of metastable lamellar gel phases.

The lyotropic phase behaviour of ester quaternary surfactants.

The lyotropic phase behaviour of two analogues of dioctadecyl dimethylammonium chloride was investigated. Both the inclusion of ester groups and subsequent minor structural rearrangement of the interfacial region of the surfactant were found to increase the chain melting temperature, although the overall phase behaviour remained similar for both compounds. Both of the two analogues were found to underswell, due to the formation of multi-lamellar vesicles. We also found that the inclusion of these ester linkages substantially reduced the metastability of the 'gel phase' in which the surfactants usually reside, accelerating the rate of collapse to a coagel state. This occurred via a nucleation-growth mechanism, where the growth was found to be one-dimensional, i.e. needle-like.

Calculations of and evidence for chain packing stress in inverse lyotropic bicontinuous cubic phases.

Inverse bicontinuous cubic lyotropic phases are a complex solution to the dilemma faced by all self-assembled water-amphiphile systems: how to satisfy the incompatible requirements for uniform interfacial curvature and uniform molecular packing. The solution reached in this case is for the water-amphiphile interfaces to deform hyperbolically onto triply periodic minimal surfaces. We have previously suggested that although the molecular packing in these structures is rather uniform the relative phase behavior of the gyroid, double diamond, and primitive inverse bicontinuous cubic phases can be understood in terms of subtle differences in packing frustration. In this work, we have calculated the packing frustration for these cubics under the constraint that their interfaces have constant mean curvature. We find that the relative packing stress does indeed differ between phases. The gyroid cubic has the least packing stress, and at low water volume fraction, the primitive cubic has the greatest packing stress. However, at very high water volume fraction, the double diamond cubic becomes the structure with the greatest packing stress. We have tested the model in two ways. For a system with a double diamond cubic phase in excess water, the addition of a hydrophobe may release packing frustration and preferentially stabilize the primitive cubic, since this has previously been shown to have lower curvature elastic energy. We have confirmed this prediction by adding the long chain alkane tricosane to 1-monoolein in excess water. The model also predicts that if one were able to hydrate the double diamond cubic to high water volume fractions, one should destabilize the phase with respect to the primitive cubic. We have found that such highly swollen metastable bicontinuous cubic phases can be formed within onion vesicles. Data from monoelaidin in excess water display a well-defined transition, with the primitive cubic appearing above a water volume fraction of 0.75. Both of these results lend support to the proposition that differences in the packing frustration between inverse bicontinuous cubic phases play a pivotal role in their relative phase stability.

Degradative transport of cationic amphiphilic drugs across phospholipid bilayers.

Drug molecules must cross multiple cell membrane barriers to reach their site of action. We present evidence that one of the largest classes of pharmaceutical drug molecules, the cationic amphiphilic drugs (CADs), does so via a catalytic reaction that degrades the phospholipid fabric of the membrane. We find that CADs partition rapidly to the polar-apolar region of the membrane. At physiological pH, the protonated groups on the CAD catalyse the acid hydrolysis of the ester linkage present in the phospholipid chains, producing a fatty acid and a single-chain lipid. The single-chain lipids rapidly destabilize the membrane, causing membranous fragments to separate and diffuse away from the host. These membrane fragments carry the drug molecules with them. The entire process, from drug adsorption to drug release within micelles, occurs on a time-scale of seconds, compatible with in vivo drug diffusion rates. Given the rate at which the reaction occurs, it is probable that this process is a significant mechanism for drug transport.

Pressure-jump X-ray studies of liquid crystal transitions in lipids.

In this paper, we give an overview of our studies by static and time-resolved X-ray diffraction of inverse cubic phases and phase transitions in lipids. In [section sign] 1, we briefly discuss the lyotropic phase behaviour of lipids, focusing attention on non-lamellar structures, and their geometric/topological relationship to fusion processes in lipid membranes. Possible pathways for transitions between different cubic phases are also outlined. In [section sign] 2, we discuss the effects of hydrostatic pressure on lipid membranes and lipid phase transitions, and describe how the parameters required to predict the pressure dependence of lipid phase transition temperatures can be conveniently measured. We review some earlier results of inverse bicontinuous cubic phases from our laboratory, showing effects such as pressure-induced formation and swelling. In [section sign] 3, we describe the technique of pressure-jump synchrotron X-ray diffraction. We present results that have been obtained from the lipid system 1:2 dilauroylphosphatidylcholine/lauric acid for cubic-inverse hexagonal, cubic-cubic and lamellar-cubic transitions. The rate of transition was found to increase with the amplitude of the pressure-jump and with increasing temperature. Evidence for intermediate structures occurring transiently during the transitions was also obtained. In [section sign] 4, we describe an IDL-based 'AXcess' software package being developed in our laboratory to permit batch processing and analysis of the large X-ray datasets produced by pressure-jump synchrotron experiments. In [section sign] 5, we present some recent results on the fluid lamellar-Pn3m cubic phase transition of the single-chain lipid 1-monoelaidin, which we have studied both by pressure-jump and temperature-jump X-ray diffraction. Finally, in [section sign] 6, we give a few indicators of future directions of this research. We anticipate that the most useful technical advance will be the development of pressure-jump apparatus on the microsecond time-scale, which will involve the use of a stack of piezoelectric pressure actuators. The pressure-jump technique is not restricted to lipid phase transitions, but can be used to study a wide range of soft matter transitions, ranging from protein unfolding and DNA unwinding and transitions, to phase transitions in thermotropic liquid crystals, surfactants and block copolymers.

Adsorption of ionic surfactants at an expanding air-water interface.

A quantitative model for the kinetics of adsorption of ionic surfactants to an expanding liquid surface is presented for surfactant concentrations below and above the critical micelle concentration (cmc). For surfactant concentrations below the cmc, the electrostatic double layer is accounted for explicitly in the adsorption isotherm. An overflowing cylinder (OFC) was used to create nonequilibrium liquid surfaces under steady-state conditions. Experimental measurements of the surface excess for solutions of cationic surfactants CH3(CH2)n-1N+(CH3)3 Br- (CnTAB, n = 12, 14, 16) and the anionic fluorocarbon surfactant sodium bis(1H,1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) in the OFC are in excellent agreement with the theoretical predictions for diffusion-controlled adsorption for all concentrations studied below the cmc. For surfactant concentrations above cmc, the diffusion ofmicelles and monomers are handled separately under the assumption of fast micellar breakdown. This simplified model gives excellent agreement for the system C14TAB + 0.1 M NaBr above the cmc. Agreement between theory and experiment for C16TAB + 0.1 M NaBr is less good. A plausible explanation for the discrepancy is that micellar breakdown is no longer fast on the time scale of the OFC (ca. 0.1 s).