Switching the spin cycloid in BiFeO3 with an electric field

Bismuth ferrite (BiFeO3) is a multiferroic material that exhibits both ferroelectricity and canted antiferromagnetism at room temperature, making it a unique candidate in the development of electric-field controllable magnetic devices. The magnetic moments in BiFeO3 are arranged into a spin cycloid, resulting in unique magnetic properties which are tied to the ferroelectric order. Previous understanding of this coupling has relied on average, mesoscale measurements. Using nitrogen vacancy-based diamond magnetometry, we observe the magnetic spin cycloid structure of BiFeO3 in real space. This structure is magnetoelectrically coupled through symmetry to the ferroelectric polarization and this relationship is maintained through electric field switching. Through a combination of in-plane and out-of-plane electrical switching, coupled with ab initio studies, we have discovered that the epitaxy from the substrate imposes a magnetoelastic anisotropy on the spin cycloid, which establishes preferred cycloid propagation directions. The energy landscape of the cycloid is shaped by both the ferroelectric degree of freedom and strain-induced anisotropy, restricting the spin spiral propagation vector to changes to specific switching events.

Experimentally Probing the Effect of Confinement Geometry on Lipid Diffusion

The lateral mobility of molecules within the cell membrane is ultimately governed by the local environment of the membrane. Confined regions induced by membrane structures, such as protein aggregates or the actin meshwork, occur over a wide range of length scales and can impede or steer the diffusion of membrane components. However, a detailed picture of the origins and nature of these confinement effects remains elusive. Here, we prepare model lipid systems on substrates patterned with confined domains of varying geometries constructed with different materials to explore the influences of physical boundary conditions and specific molecular interactions on diffusion. We demonstrate a platform that is capable of significantly altering and steering the long-range diffusion of lipids by using simple oxide deposition approaches, enabling us to systematically explore how confinement size and shape impact diffusion over multiple length scales. While we find that a "boundary condition" description of the system captures underlying trends in some cases, we are also able to directly compare our systems to analytical models, revealing the unexpected breakdown of several approximate solutions. Our results highlight the importance of considering the length scale dependence when discussing properties such as diffusion.

Indistinguishable telecom band photons from a single Er ion in the solid state

Atomic defects in the solid state are a key component of quantum repeater networks for long-distance quantum communication1. Recently, there has been significant interest in rare earth ions2-4, in particular Er3+ for its telecom band optical transition5-7 that allows long-distance transmission in optical fibres. However, the development of repeater nodes based on rare earth ions has been hampered by optical spectral diffusion, precluding indistinguishable single-photon generation. Here, we implant Er3+ into CaWO4, a material that combines a non-polar site symmetry, low decoherence from nuclear spins8 and is free of background rare earth ions, to realize significantly reduced optical spectral diffusion. For shallow implanted ions coupled to nanophotonic cavities with large Purcell factor, we observe single-scan optical linewidths of 150 kHz and long-term spectral diffusion of 63 kHz, both close to the Purcell-enhanced radiative linewidth of 21 kHz. This enables the observation of Hong-Ou-Mandel interference9 between successively emitted photons with a visibility of V = 80(4)%, measured after a 36 km delay line. We also observe spin relaxation times T1,s = 3.7 s and T2,s > 200 μs, with the latter limited by paramagnetic impurities in the crystal instead of nuclear spins. This represents a notable step towards the construction of telecom band quantum repeater networks with single Er3+ ions.

Neutral Silicon Vacancy Centers in Undoped Diamond via Surface Control

Neutral silicon vacancy centers (SiV^{0}) in diamond are promising candidates for quantum applications; however, stabilizing SiV^{0} requires high-purity, boron-doped diamond, which is not a readily available material. Here, we demonstrate an alternative approach via chemical control of the diamond surface. We use low-damage chemical processing and annealing in a hydrogen environment to realize reversible and highly stable charge state tuning in undoped diamond. The resulting SiV^{0} centers display optically detected magnetic resonance and bulklike optical properties. Controlling the charge state tuning via surface termination offers a route for scalable technologies based on SiV^{0} centers, as well as charge state engineering of other defects.

Tautomerisation Mechanisms in the Adenine-Thymine Nucleobase Pair during DNA Strand Separation

The adenine-thymine tautomer (A*-T*) has previously been discounted as a spontaneous mutagenesis mechanism due to the energetic instability of the tautomeric configuration. We study the stability of A*-T* while the nucleobases undergo DNA strand separation. Our calculations indicate an increase in the stability of A*-T* as the DNA strands unzip and the hydrogen bonds between the bases stretch. Molecular Dynamics simulations reveal the time scales and dynamics of DNA strand separation and the statistical ensemble of opening angles present in a biological environment. Our results demonstrate that the unwinding of DNA, an inherently out-of-equilibrium process facilitated by helicase, will change the energy landscape of the adenine-thymine tautomerization reaction. We propose that DNA strand separation allows the stable tautomerization of adenine-thymine, providing a feasible pathway for genetic point mutations via proton transfer between the A-T bases.

The effect of iron deficiency on quality of life outcomes after surgery for obstructive sleep apnoea

AIM

This study aimed to assess the effect of iron deficiency on parent-reported changes in quality of life (QOL) among children receiving an adenotonsillectomy for paediatric obstructive sleep apnoea (OSA).

METHODS

This was a retrospective review study that recruited children under 16 years of age undergoing an adenotonsillectomy, adenoidectomy, or tonsillectomy for clinically diagnosed paediatric OSA between June 2020 and January 2021 inclusive, in Western Australia. The main outcome measures for this study were changes in QOL by age group and iron status, defined by an absolute change of more than 3 points on OSA-18 survey domains.

RESULTS

About 249 participants had iron studies performed on perioperative blood samples drawn at operation and completed both pre-operative and post-operative OSA-18 QOL questionnaires at initial consultation and 8-12 weeks post-surgery, respectively. 41.8% were iron deficient, 53.8% were borderline iron deficient and 4.4% had normal iron levels. Following surgery, a decrease was observed for all OSA-18 score domains in post-operation scores compared to pre-operation scores in both iron-deficient and borderline iron-deficient cohorts. 'Daytime Problems' in the <2 years group, within the iron-deficient cohort, was the only domain that found to be non-superior (i.e. 'not better') following surgery.

CONCLUSIONS

Following adenotonsillectomy, patients with paediatric OSA reported significant improvements in QOL regardless of their iron status. Those undergoing an adenotonsillectomy for paediatric OSA had a high prevalence of iron deficiency at operation, especially those under 6 years of age.

DETECT Schools Study Protocol: A Prospective Observational Cohort Surveillance Study Investigating the Impact of COVID-19 in Western Australian Schools

Introduction: Amidst the evolving COVID-19 pandemic, understanding the transmission dynamics of the SARS-CoV-2 virus is key to providing peace of mind for the community and informing policy-making decisions. While available data suggest that school-aged children are not significant spreaders of SARS-CoV-2, the possibility of transmission in schools remains an ongoing concern, especially among an aging teaching workforce. Even in low-prevalence settings, communities must balance the potential risk of transmission with the need for students' ongoing education. Through the roll out of high-throughput school-based SARS-CoV-2 testing, enhanced follow-up for individuals exposed to COVID-19 and wellbeing surveys, this study investigates the dynamics of SARS-CoV-2 transmission and the current psychosocial wellbeing impacts of the pandemic in school communities. Methods: The DETECT Schools Study is a prospective observational cohort surveillance study in 79 schools across Western Australia (WA), Australia. To investigate the incidence, transmission and impact of SARS-CoV-2 in schools, the study comprises three "modules": Module 1) Spot-testing in schools to screen for asymptomatic SARS-CoV-2; Module 2) Enhanced surveillance of close contacts following the identification of any COVID-19 case to determine the secondary attack rate of SARS-CoV-2 in a school setting; and Module 3) Survey monitoring of school staff, students and their parents to assess psycho-social wellbeing following the first wave of the COVID-19 pandemic in WA. Clinical Trial Registration: Trial registration number: ACTRN12620000922976.

Optically Detected Magnetic Resonance in Neutral Silicon Vacancy Centers in Diamond via Bound Exciton States

Neutral silicon vacancy (SiV^{0}) centers in diamond are promising candidates for quantum networks because of their excellent optical properties and long spin coherence times. However, spin-dependent fluorescence in such defects has been elusive due to poor understanding of the excited state fine structure and limited off-resonant spin polarization. Here we report the realization of optically detected magnetic resonance and coherent control of SiV^{0} centers at cryogenic temperatures, enabled by efficient optical spin polarization via previously unreported higher-lying excited states. We assign these states as bound exciton states using group theory and density functional theory. These bound exciton states enable new control schemes for SiV^{0} as well as other emerging defect systems.

Narrow Optical Line Widths in Erbium Implanted in TiO2

Atomic and atomlike defects in the solid state are widely explored for quantum computers, networks, and sensors. Rare earth ions are an attractive class of atomic defects that feature narrow spin and optical transitions that are isolated from the host crystal, allowing incorporation into a wide range of materials. However, the realization of long electronic spin coherence times is hampered by magnetic noise from abundant nuclear spins in the most widely studied host crystals. Here, we demonstrate that Er³⁺ ions can be introduced via ion implantation into TiO₂, a host crystal that has not been studied extensively for rare earth ions and has a low natural abundance of nuclear spins. We observe efficient incorporation of the implanted Er³⁺ into the Ti⁴⁺ site (>50% yield) and measure narrow inhomogeneous spin and optical line widths (20 and 460 MHz, respectively) that are comparable to bulk-doped crystalline hosts for Er³⁺. This work demonstrates that ion implantation is a viable path to studying rare earth ions in new hosts and is a significant step toward realizing individually addressed rare earth ions with long spin coherence times for quantum technologies.

Improving diagnostic accuracy of skin biopsies

BACKGROUND

The skin biopsy is a simple but essential clinical skill of the general practitioner. Performed properly, it can be quick and comfortable for the patient, and yield a very high level of diagnostic information. Performed incorrectly, it can lead to delays in diagnosis and treatment for the patient.

OBJECTIVE

This article reviews some simple but effective steps the clinician can take to ensure proper technique and maximise the diagnostic accuracy of their skin biopsies.

DISCUSSION

Diagnostic accuracy can be improved through optimal selection of biopsy site, correct biopsy technique, requesting ancillary tests where appropriate, proper handling of specimens, and providing detailed clinical information for the dermatopathologist.

Sepsis Mortality in the U.S. Correctional System: An Underappreciated Disparity

The effect of incarceration on sepsis outcomes in the United States is infrequently described in the literature. This study sought to investigate whether being incarcerated affected mortality rates in sepsis. The retrospective study used data from October 1, 2013, to November 30, 2016, of patients admitted with a diagnosis of sepsis at a tertiary care center with a primary outcome of in-hospital mortality. The study cohort included 8,568 cases of sepsis, of which 8,448 were noninmates and 120 were inmates. Overall mortality was 15.7%; for noninmates, the rate was 15.3%, and for inmates, 42.5%. The risk of death among inmates was 2.8 times that of noninmates. Neither age, sex, nor race were significant confounders. Findings suggest a direct association between incarceration and sepsis mortality. Larger regional or nationwide case-control studies should be conducted to confirm these findings.

Observation of an environmentally insensitive solid-state spin defect in diamond

Engineering coherent systems is a central goal of quantum science. Color centers in diamond are a promising approach, with the potential to combine the coherence of atoms with the scalability of a solid-state platform. We report a color center that shows insensitivity to environmental decoherence caused by phonons and electric field noise: the neutral charge state of silicon vacancy (SiV⁰). Through careful materials engineering, we achieved >80% conversion of implanted silicon to SiV⁰ SiV⁰ exhibits spin-lattice relaxation times approaching 1 minute and coherence times approaching 1 second. Its optical properties are very favorable, with ~90% of its emission into the zero-phonon line and near-transform-limited optical linewidths. These combined properties make SiV⁰ a promising defect for quantum network applications.

An Analysis of Medico-legal Claims against Dermatologists in Australia from a Single Medical Indemnity Insurer

Risk mitigation practices are essential to protecting patients from harm and reducing medical practitioner exposure to unnecessary reputational damage and economic loss. Despite traditionally being perceived as a low-risk specialty, published data on medico-legal claims against dermatologists in Australia are currently lacking. This article reviews the sources of medico-legal claims against dermatologists in Australia from a single medical indemnity insurer over the most recent three years. The failure to meet patient expectations was the largest source of claims against dermatologists, followed by adverse outcomes. Improved communication from practitioner to patient remains the most effective step to preventing medico-legal claims. Medico-legal claims, when they occur, are more successfully defended when thorough documentation processes are in place.

An experimental study of OH(A2Σ+) + H2: Electronic quenching, rotational energy transfer, and collisional depolarization

Zeeman quantum beat spectroscopy has been used to determine the thermal (300 K) rate constants for electronic quenching, rotational energy transfer, and collisional depolarization of OH(A²Σ⁺) by H₂. Cross sections for both the collisional disorientation and collisional disalignment of the angular momentum in the OH(A²Σ⁺) radical are reported. The experimental results for OH(A²Σ⁺) + H₂ are compared to previous work on the OH(A²Σ⁺) + He and Ar systems. Further comparisons are also made to the OH(A²Σ⁺) + Kr system, which has been shown to display significant non-adiabatic dynamics. The OH(A²Σ⁺) + H₂ experimental data reveal that collisions that survive the electronic quenching process are highly depolarizing, reflecting the deep potential energy wells that exist on the excited electronic state surface.

Impact of Electrolyte on Adsorption at the Air-Water Interface for Ternary Surfactant Mixtures above the Critical Micelle Concentration

The composition of the air-water adsorbed layer of the ternary surfactant mixture, octaethylene monododecyl ether, C₁₂E₈, sodium dodecyl 6-benzenesulfonate, LAS, and sodium dioxyethylene glycol monododecyl sulfate, SLES, and of each of the binary mixtures, with varying amounts of electrolyte, has been studied by neutron reflectivity. The measurements were made above the mixed critical micelle concentration. In the absence of electrolyte adsorption is dominated by the nonionic component C₁₂E₈ but addition of electrolyte gradually changes this so that SLES and LAS dominate at higher electrolyte concentrations. The composition of the adsorbed layer in both binary and ternary mixtures can be quantitatively described using the pseudo-phase approximation with quadratic and cubic interactions in the excess free energy of mixing (G_E) at both the surface and in the micelles. A single set of parameters fits all the experimental data. A similar analysis is effective for a mixture in which SDS replaces SLES. Addition of electrolyte weakens the synergistic SLES-C₁₂E₈ and LAS-C₁₂E₈ interactions, consistent with them being dominated by electrostatic interactions. The SLES-LAS (and SDS-LAS) interaction is moderately strong at the surface and is little affected by addition of electrolyte, suggesting that it is controlled by structural or packing factors. Most of the significant interactions in the mixtures are unsymmetrical with respect to composition, with the minimum in G_E at the 1:2 or 2:1 composition. There is a small structural contribution to the LAS-C₁₂E₈ interaction that leads to a minimum intermediate in composition between 1:2 and 1:1 (LAS:C₁₂E₈) and to a significant residual G_E in strong electrolyte.

Application of a monoclonal antibody-based antigen detection enzyme-linked immunosorbent assay (antigen ELISA) for field diagnosis of bovine trypanosomiasis at Nguruman, Kenya

A monoclonal antibody-based, enzyme immunoassay (antigen ELISA) for the detection of species-specific invariant antigens of Trypanosoma congolense, T. vivax or T. brucei in the serum of infected animals was evaluated as a means of diagnosis using bovine field sera from a trypanosomiasis endemic area, Nguruman, Kenya. Circulating trypanosome antigens were detected in 126 (96.2%) of 131 serum samples from animals with parasitologically confirmed diagnosis: 74.8% were positive for antigens of two or three trypanosome species, while 21.4% tested positive for one trypanosome species. When 70 sera from animals (at Nguruman), which had tested negative for trypanosomes by the buffy coat technique, were tested, 35 (50.0%) of them were shown to be antigen-ELISA positive: 24 (34.3%) showing infection with a single species and 11 (15.7%) with mixed infections. The predominant trypanosome species diagnosed in the two herds by antigen ELISA was T. vivax, which was detected in 133 (82.6%) of the 161 sera that tested positive for antigens, followed by T. congolense in 122 (75.8%) sera, with 109 (67.7%) showing evidence of mixed infections with two or three trypanosome species. In single infections, T. vivax exceeded T. congolense by a ratio of 2:1, with T. brucei accounting for less than 1.0%. Evidence for the specificity of the test was provided by analysis of field sera from 100 cattle, from a trypanosomiasis-free area, infected with other haemoparasites (anaplasmosis, babesiosis and theileriosis), which all tested negative in the assay.

Sequence-Dependent Mechanism of DNA Oligonucleotide Dehybridization Resolved through Infrared Spectroscopy

Despite its important role in biology and nanotechnology, many questions remain regarding the molecular mechanism and dynamics by which oligonucleotides recognize and hybridize to their complementary sequence. The thermodynamics and kinetics of DNA oligonucleotide hybridization and dehybridization are often assumed to involve an all-or-nothing two-state dissociation pathway, but deviations from this behavior can be considerable even for short sequences. We introduce a new strategy to characterize the base-pair-specific thermal dissociation mechanism of DNA oligonucleotides through steady-state and time-resolved infrared spectroscopy. Experiments are interpreted with a lattice model to provide a structure-specific interpretation. This method is applied to a model set of self-complementary 10-base-pair sequences in which the placement of GC base pairs is varied in an otherwise AT strand. Through a combination of Fourier transform infrared and two-dimensional infrared spectroscopy, experiments reveal varying degrees of deviation from simple two-state behavior. As the temperature is increased, duplexes dissociate through a path in which the terminal bases fray, without any significant contribution from loop configurations. Transient temperature jump experiments reveal time scales of 70-100 ns for fraying and 10-30 μs for complete dissociation near the melting temperature. Whether or not frayed states are metastable intermediates or short-lived configurations during the full dissociation of the duplex is dictated by the nucleobase sequence.

Teledermatology and clinical photography: safeguarding patient privacy and mitigating medico-legal risk

Capturing clinical images is becoming more prevalent in everyday clinical practice, and dermatology lends itself to the use of clinical photographs and teledermatology. "Store-and-forward", whereby clinical images are forwarded to a specialist who later responds with an opinion on diagnosis and management is a popular form of teledermatology. Store-and-forward teledermatology has proven accurate and reliable, accelerating the process of diagnosis and treatment and improving patient outcomes. Practitioners' personal smartphones and other devices are often used to capture and communicate clinical images. Patient privacy can be placed at risk with the use of this technology. Practitioners should obtain consent for taking images, explain how they will be used, apply appropriate security in their digital communications, and delete images and other data on patients from personal devices after saving these to patient health records. Failing to use appropriate security precautions poses an emerging medico-legal risk for practitioners.

Antibacterial properties of biosurfactants against selected Gram-positive and -negative bacteria

The antibacterial properties and ability to disrupt biofilms of biosurfactants (rhamnolipids, sophorolipids) and sodium dodecyl sulphate (SDS) in the presence and absence of selected organic acids were investigated. Pseudomonas aeruginosa PAO1 was inhibited by sophorolipids and SDS at concentrations >5% v/v, and the growth of Escherichia coli NCTC 10418 was also inhibited by sophorolipids and SDS at concentrations >5% and 0.1% v/v, respectively. Bacillus subtilis NCTC 10400 was inhibited by rhamnolipids, sophorolipids and SDS at concentrations >0.5% v/v of all three; the same effect was observed with Staphylococcus aureus ATCC 9144. The ability to attach to surfaces and biofilm formation of P. aeruginosa PAO1, E. coli NCTC 10418 and B. subtilis NCTC 10400 was inhibited by sophorolipids (1% v/v) in the presence of caprylic acid (0.8% v/v). In the case of S. aureus ATCC 9144, the best results were obtained using caprylic acid on its own. It was concluded that sophorolipids are promising compounds for the inhibition/disruption of biofilms formed by Gram-positive and Gram-negative microorganisms and this activity can be enhanced by the presence of booster compounds such as caprylic acid.

Distinguishing gramicidin D conformers through two-dimensional infrared spectroscopy of vibrational excitons

Gramicidin D is a short peptide which dimerizes to form helical pores, adopting one of two conformations in the process. These conformations differ primarily in number of residues per turn and the hydrogen-bond registry between rungs of the helix. Using amide I 2D infrared (IR) and FTIR, we have demonstrated that it is possible to distinguish between the different conformers of gramicidin D in solution. We show that the spectra observed for this helical peptide bear no resemblance to the spectra of α- or 310-helices and that while the FTIR spectra appear similar to spectra of β-sheets, 2D IR reveals that the observed resonances arise from vibrational modes unlike those observed in β-sheets. We also present an idealized model which reproduces the experimental data with high fidelity. This model is able to explain the polarization-dependence of the experimental 2D IR data. Using this model, we show the coupling between the rungs of the helix dominates the spectra, and as a consequence of this, the number of residues per turn can greatly influence the amide I spectra of gramicidin D.

Visualizing KcsA conformational changes upon ion binding by infrared spectroscopy and atomistic modeling

The effect of ion binding in the selectivity filter of the potassium channel KcsA is investigated by combining amide I Fourier-transform infrared spectroscopy with structure-based spectral modeling. Experimental difference IR spectra between K(+)-bound KcsA and Na(+)-bound KcsA are in good qualitative agreement with spectra modeled from structural ensembles generated from molecular dynamics simulations. The molecular origins of the vibrational modes contributing to differences in these spectra are determined not only from structural differences in the selectivity filter but also from the pore helices surrounding this region. Furthermore, the coordination of K(+) or Na(+) to carbonyls in the selectivity filter effectively decouples the vibrations of those carbonyls from the rest of the protein, creating local probes of the electrostatic environment. The results suggest that it is necessary to include the influence of the surrounding helices in discussing selectivity and transport in KcsA and, on a more general level, that IR spectroscopy offers a nonperturbative route to studying the structure and dynamics of ion channels.

Pharmacokinetics of micafungin in subjects with severe hepatic dysfunction

Micafungin is an echinocandin with potent activity against a broad range of fungal species, including Candida species. The pharmacokinetic and safety profiles of micafungin have been evaluated in individuals with mild-to-moderate hepatic dysfunction, but not in individuals with severe hepatic dysfunction. Therefore, the present study assessed the pharmacokinetics and safety of a single 100 mg dose of micafungin in healthy subjects (n = 8) and subjects with severe hepatic dysfunction (n = 8). Mean maximum plasma concentration of micafungin and mean area under the plasma micafungin concentration-time curve extrapolated to infinity were lower in subjects with severe hepatic dysfunction (7.3 ± 2.4 µg/mL and 100.1 ± 34.5 h·μg/mL, respectively) than in subjects with normal hepatic function (10.3 ± 2.5 µg/mL and 142.4 ± 28.9 h·μg/mL, respectively). Mean clearance was higher in subjects with severe hepatic dysfunction (1,098 ± 347 mL/h) than in subjects with normal hepatic function (728 ± 149 mL/h). Concentrations of albumin in subjects with severe hepatic dysfunction were lower. Assessments of micafungin plasma protein binding suggested that the higher clearance in subjects with severe hepatic dysfunction may be due to higher unbound concentrations. However, the magnitude of the differences was not considered clinically meaningful and is comparable with exposures reported elsewhere for a 100-mg dose in patients treated for invasive candidiasis. Thus, dose adjustment in subjects with severe hepatic dysfunction is not warranted. Micafungin was well tolerated in all subjects throughout the study.

In vitro bactericidal activity of diterpenoids isolated from Aframomum melegueta K.Schum against strains of Escherichia coli, Listeria monocytogenes and Staphylococcus aureus

ETHNOPHARMACOLOGICAL RELEVANCE

The ethnobotanical use of Aframomum melegueta in the treatment of urinary tract and soft tissue infection suggested that the plant has antimicrobial activity.

MATERIALS AND METHODS

To substantiate the folkloric claims, an acetone, 50:50 acetone:methanol and 2:1 chloroform:methanol extracts were tested against Escherichia coli K12; acetone extract and the fractions of acetone extracts were tested against Listeria monocytogenes. Bioassay-guided fractionation was performed on the extract using L. monocytogenes as the test organism to isolate the bioactive compounds which were then tested against all the other organisms.

RESULTS

Four known labdane diterpenes (G3 and G5) were isolated for the first time from the rhizomes of A. melegueta and purified. These were tested against E. coli, L. monocytogenes, methicillin resistant Staphylococus aureus (MRSA) and S. aureus to determine antibacterial activity. The result showed that two compounds G3 and G5 exhibited more potent antibacterial activity compared to the current clinically used antibiotics ampicillin, gentamicin and vancomycin and can be potential antibacterial lead compounds. The structure of the labdane diterpenes were elucidated using nuclear magnetic resonance (NMR) spectroscopy and Mass spectrometry. A possible mode of action of the isolated compound G3 and its potential cytotoxicity towards mammalian cells were also discussed.

CONCLUSION

The results confirmed the presence of antibacterial compounds in the rhizomes of A. melegueta with a favourable toxicity profile which could be further optimized as antibacterial lead compounds.

Influence of calcium ions on rhamnolipid and rhamnolipid/anionic surfactant adsorption and self-assembly

The impact of Ca(2+) counterions on the adsorption at the air-water interface and self-assembly in aqueous solution of the rhamnolipid biosurfactant and its mixture with the anionic surfactant sodium dodecylbenzenesulfonate, LAS, has been studied using neutron reflectometry and small-angle neutron scattering. The results illustrate how rhamnolipids are calcium tolerant and how their blending with conventional anionic surfactants improves the calcium tolerance of the anionic surfactant. Ca(2+) has relatively little effect upon the adsorption and self-assembly of the monorhamnose, R1, and dirhamnose, R2, rhamnolipids, even at high pH, due to their predominantly nonionic nature. For R1/R2 mixtures the addition of Ca(2+) has little impact upon the adsorbed amount or the surface composition. For R2/LAS mixtures the addition of Ca(2+) results in an increased adsorption and a surface slightly richer in R2. The weak binding of Ca(2+) to R1 and R2 does result in a change to the degree of ionization of the micelles and especially for mixed R1/R2 micelles at R1-rich solution compositions. The stronger binding of Ca(2+) to LAS results in the addition of Ca(2+) having a much greater impact on the self-assembly of R1/LAS and R2/LAS mixtures. For R1/LAS mixtures the addition of Ca(2+) promotes the formation of more planar structures, even at low surfactant concentrations where in the absence of Ca(2+) mixed globular micelle formation dominates. In R2/LAS mixtures, where there is a greater contrast between the high and low preferred curvatures associated with R2 and LAS, the addition of Ca(2+) results in a more complex evolution in micellar aggregation and the degree of ionization of the micelles. This results in variations in Ca(2+) binding that promotes micellar structures in which a spatial segregation of the two surfactant components within the micelle occurs.

Effect of environmental humidity on static foam stability

The quality of foaming products (such as beer and shampoo) and the performance of industrial processes that harness foam (such as the froth flotation of minerals or the foam fractionation of proteins) depend upon foam stability. In this study, experiments are performed to study the effect of environmental humidity on the collapse of static foams. The dependency of the rate at which a foam collapses upon humidity is demonstrated, and we propose a hypothesis for bubble bursting due to Marangoni instability induced by nonuniform evaporation to help explain the dependency. This hypothesis is supported by direct experimental observations of the bursting process of isolated bubbles by high speed video recording and the thinning of isolated foam films under different values of humidity and temperature by microinterferometric methods.

Adsorption of sophorolipid biosurfactants on their own and mixed with sodium dodecyl benzene sulfonate, at the air/water interface

The adsorption of the lactonic (LS) and acidic (AS) forms of sophorolipid and their mixtures with the anionic surfactant sodium dodecyl benzene sulfonate (LAS) has been measured at the air/water interface by neutron reflectivity, NR. The AS and LS sophorolipids adsorb with Langmuir-like adsorption isotherms. The more hydrophobic LS is more surface active than the AS, with a lower critical micellar concentration, CMC, and stronger surface adsorption, with an area/molecule ∼70 Å(2) compared with 85 Å(2) for the AS. The acidic sophorolipid shows a maximum in its adsorption at the CMC which appears to be associated with a mixture of different isomeric forms. The binary LS/AS and LS/LAS mixtures show a strong surface partitioning in favor of the more surface active and hydrophobic LS component but are nevertheless consistent with ideal mixing at the interface. In contrast, the surface composition of the AS/LAS mixture is much closer to the solution composition, but the surface mixing is nonideal and can be accounted for by regular solution theory, RST. In the AS/LS/LAS ternary mixtures, the surface adsorption is dominated by the sophorolipid, and especially the LS component, in a way that is not consistent with the observations for the binary mixtures. The extreme partitioning in favor of the sophorolipid for the LAS/LS/AS (1:2) mixtures is attributed to a reduction in the packing constraints at the surface due to the AS component. Measurements of the surface structure reveal a compact monolayer for LS and a narrow solvent region for LS, LS/AS, and LS/LAS mixtures, consistent with the more hydrophobic nature of the LS component. The results highlight the importance of the relative packing constraints on the adsorption of multicomponent mixtures, and the impact of the lactonic form of the sophorolipid on the adsorption of the sophorolipid/LAS mixtures.