Picosecond pulses from a cryogenically cooled, composite amplifier using Yb:YAG and Yb:GSAG

A cryogenic Yb amplifier using two laser materials, Gd3Sc2Al3O12 and Y3Al5O12 (YAG), has been used to obtain 70 W average power at 5 kHz pulse repetition frequency; the output was compressed to 1.6 ps, compared with an input compressible to 1.4 ps. The gain broadening obtained by combining two media enables shorter pulses than using Yb:YAG alone but retains the power-scaling advantages of cryogenic Yb:YAG.     

A metacognitive-based instruction for Primary Four students to approach non-routine mathematical word problems

Many Primary Four students (fourth graders) in Singapore have difficulties initiating or persevering in the problem-solving process even though the curriculum has focused on problem solving since 1992. This study served to examine the role of metacognition in self-regulated problem solving. The study, a quasi-experimental pretest–posttest design involving a convenience sample of 63 students from two intact mixed-ability Primary Four classes, examined the impact of using a metacognitive scheme that focuses on the understanding and planning stages of Pòlya’s four-stage approach on students’ mathematical problem-solving behavior, performance and attitudes. The findings revealed that the metacognitive-based scheme had a positive impact on students’ understanding of the problem posed, solution planning, confidence in and personal control of problem-solving behavior and emotions. It had also helped them to initiate and persevere in the problem-solving process to achieve a higher level of problem-solving success. Limitations and instructional implications are discussed.     

Limit on monopole flux in the Mont Blanc NUSEX experiment

Summary A trigger dedicated to the search of superheavy monopoles operates since about two years in the NUSEX apparatus, installed in the Mont Blanc tunnel. Monopoles are identified by time of flight and visualization of the pulses involved in a trigger. The present upper limit on monopole flux is 2.3·10⁻¹³ cm⁻² s⁻¹ sr⁻¹ (90% c.l.).

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Riassunto Da circa due anni è operativo sull'apparato NUSEX, installato nel tunnel del Monte Bianco, un trigger dedicato alla ricerca dei monopoli previsti dalle teorie di grande unificazione. L'identificazione dei monopoli è basata sul tempo di volo e sulla localizzazione nello spazio degli impulsi coinvolti nel trigger. Il limite superiore fino ad oggi raggiunto per il flusso di monopoli è 2.3·10⁻¹³ cm⁻²s⁻¹ sr⁻¹ (90% c.l.).

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Резюме Триггерная схема, предназначенная для поиска сверхтяжелых монополей, действует в течение почти двух лет в установке NUSEX, расположенной в туннеле под Мон Бланом. Идентификация монополей основана на времени пролета, а локализация в пространстве осуществляется с помощью импульсов в триггерной схеме. Настоящий верхний предел на поток монополей составляет 2.3· ·10⁻¹³ см⁻² с⁻¹ ср⁻¹ (к 90% вероятностью).

     

Wide dynamic range detection of bidirectional flow in Doppler optical coherence tomography using a two-dimensional Kasai estimator

We demonstrate extended axial flow velocity detection range in a time-domain Doppler optical coherence tomography (DOCT) system using a modified Kasai velocity estimator with computations in both the axial and transverse directions. For a DOCT system with an 8 kHz rapid-scanning optical delay line, bidirectional flow experiments showed a maximum detectable speed of >56 cm/s using the axial Kasai estimator without the occurrence of aliasing, while the transverse Kasai estimator preserved the approximately 7 microm/s minimum detectable velocity to slow flow. By using a combination of transverse Kasai and axial Kasai estimators, the velocity detection dynamic range was over 100 dB. Through a fiber-optic endoscopic catheter, in vivoM-mode transesophageal imaging of the pulsatile blood flow in rat aorta was demonstrated, for what is for the first time to our knowledge, with measured peak systolic blood flow velocity of >1 m/s, while maintaining good sensitivity to detect aortic wall motion at <2 mm/s, using this 2D Kasai technique.     

Facile Synthesis and Properties of 2‐λ5‐Phosphaquinolines and 2‐λ5‐Phosphaquinolin‐2‐ones

Treatment of 2‐ethynylanilines with P(OPh)₃ gives either 2,2‐diphenoxy‐2‐λ⁵‐phosphaquinolines or 2‐phenoxy‐2‐λ⁵‐phosphaquinolin‐2‐ones under transition‐metal‐free conditions. This reaction offers access to an underexplored heterocycle, which opens up the study of the fundamental nature of the N?P^V double bond and its potential for delocalization within a cyclic π‐electron system. This heterocycle can serve as a carbostyril mimic, with application as a bioisostere for pharmaceuticals based on the 2‐quinolinone scaffold. It also holds promise as a new fluorophore, since initial screening reveals quantum yields upwards of 40 %, Stokes shifts of 50–150 nm, and emission wavelengths of 380–540 nm. The phosphaquinolin‐2‐ones possess one of the strongest solution‐state dimerization constants for a D–A system (130 M ^?1) owing to the close proximity of a strong acceptor (P?O) and a strong donor (phosphonamidate N? H), which suggests that they might hold promise as new hydrogen‐bonding hosts for optoelectronic sensing.     

Direct Catalytic Enantio‐ and Diastereoselective Ketone Aldol Reactions of Isocyanoacetates

A catalytic asymmetric aldol addition/cyclization reaction of unactivated ketones with isocyanoacetate pronucleophiles has been developed. A quinine‐derived aminophosphine precatalyst and silver oxide were found to be an effective binary catalyst system and promoted the reaction to afford chiral oxazolines possessing a fully substituted stereocenter with good diastereoselectivities and excellent enantioselectivities.     

Synthesis of C-9 oxidised N-acetylneuraminic acid derivatives as biological probes

Sialic acids are 9-carbon acidic sugars involved in a number of important biological processes and human diseases. As part of our ongoing interest in the development of novel sialic acids as biological probes, we have developed an efficient and simple synthesis of C-9 oxidised sialic acid derivatives. The key oxidative step involves the use of TEMPO under carefully controlled aqueous pH conditions.     

Using molecular dynamics and quantum mechanics calculations to model fluorescence observables

We provide a critical examination of two different methods for generating a donor-acceptor electronic coupling trajectory from a molecular dynamics (MD) trajectory and three methods for sampling that coupling trajectory, allowing the modeling of experimental observables directly from the MD simulation. In the first coupling method we perform a single quantum-mechanical (QM) calculation to characterize the excited state behavior, specifically the transition dipole moment, of the fluorescent probe, which is then mapped onto the configuration space sampled by MD. We then utilize these transition dipoles within the ideal dipole approximation (IDA) to determine the electronic coupling between the probes that mediates the transfer of energy. In the second method we perform a QM calculation on each snapshot and use the complete transition densities to calculate the electronic coupling without need for the IDA. The resulting coupling trajectories are then sampled using three methods ranging from an independent sampling of each trajectory point (the independent snapshot method) to a Markov chain treatment that accounts for the dynamics of the coupling in determining effective rates. The results show that the IDA significantly overestimates the energy transfer rate (by a factor of 2.6) during the portions of the trajectory in which the probes are close to each other. Comparison of the sampling methods shows that the Markov chain approach yields more realistic observables at both high and low FRET efficiencies. Differences between the three sampling methods are discussed in terms of the different mechanisms for averaging over structural dynamics in the system. Convergence of the Markov chain method is carefully examined. Together, the methods for estimating coupling and for sampling the coupling provide a mechanism for directly connecting the structural dynamics modeled by MD with fluorescence observables determined through FRET experiments.     

Structure-activity relationships for organometallic osmium arene phenylazopyridine complexes with potent anticancer activity

We report the synthesis and characterisation of 32 half sandwich phenylazopyridine Os(II) arene complexes [Os(η(6)-arene)(phenylazopyridine)X](+) in which X is chloride or iodide, the arene is p-cymene or biphenyl and the pyridine and phenyl rings contain a variety of substituents (F, Cl, Br, I, CF(3), OH or NO(2)). Ten X-ray crystal structures have been determined. Cytotoxicity towards A2780 human ovarian cancer cells ranges from high potency at nanomolar concentrations to inactivity. In general the introduction of an electron-withdrawing group (e.g. F, Cl, Br or I) at specific positions on the pyridine ring significantly increases cytotoxic activity and aqueous solubility. Changing the arene from p-cymene to biphenyl and the monodentate ligand X from chloride to iodide also increases the activity significantly. Activation by hydrolysis and DNA binding appears not to be the major mechanism of action since both the highly active complex [Os(η(6)-bip)(2-F-azpy)I]PF(6) (9) and the moderately active complex [Os(η(6)-bip)(3-Cl-azpy)I]PF(6) (23) are very stable and inert towards aquation. Studies of octanol-water partition coefficients (log P) and subcellular distributions of osmium in A2780 human ovarian cancer cells suggested that cell uptake and targeting to cellular organelles play important roles in determining activity. Although complex 9 induced the production of reactive oxygen species (ROS) in A2780 cells, the ROS level did not appear to play a role in the mechanism of anticancer activity. This class of organometallic osmium complexes has new and unusual features worthy of further exploration for the design of novel anticancer drugs.