Template-directed assembly on an ordered microsphere array

We investigated the capability of an ordered array of microspheres to act as a template for deposition and ordering of a subsequent layer of microspheres. An evaporation-based technique was used to deposit monolayers of large colloidal spheres. A novel technique for selective deposition of polyelectrolyte film was used to stabilize the arrays and optimize the bead-substrate interaction. The template behavior of face-centered cubic and body-centered cubic (bcc) microsphere arrays was studied by optical and scanning electron microscopy, and the packing geometry was found to have a dramatic effect on the arrangement of the subsequent layer. A geometrical interpretation of the experimental data explains why a bcc bead array is well suited to act as a template for an additional layer of microspheres.     

A system for low field imaging of laser-polarized noble gas

We describe a device for performing MRI with laser-polarized noble gas at low magnetic fields (<50 G). The system is robust, portable, inexpensive, and provides gas-phase imaging resolution comparable to that of high field clinical instruments. At 20.6 G, we have imaged laser-polarized (3)He (Larmor frequency of 67 kHz) in both sealed glass cells and excised rat lungs, using approximately 0.1 G/cm gradients to achieve approximately 1 mm(2) resolution. In addition, we measured (3)He T(2)(*) times greater than 100 ms in excised rat lungs, which is roughly 20 times longer than typical values observed at high ( approximately 2 T) fields. We include a discussion of the practical considerations for working at low magnetic fields and conclude with evidence of radiation damping in this system.     

Direct-detected rapid-scan EPR at 250 MHz

EPR spectra at 250 MHz for a single crystal of lithium phthalocyanine (LiPc) in the absence of oxygen and for a deoxygenated aqueous solution of a Nycomed triarylmethyl (trityl-CD3) radical were obtained at scan rates between 1.3 x 10(3) and 3.4 x 10(5)G/s. These scan rates are rapid relative to the reciprocals of the electron spin relaxation times (LiPc: T1 = 3.5 micros and T2 = 2.5 micros; trityl: T1 = 12 micros and T2 = 11.5 micros) and cause characteristic oscillations in the direct-detected absorption spectra. For a given scan rate, shorter values of T2 and increased inhomogeneous broadening cause less deep oscillations that damp out more quickly than for longer T2. There is excellent agreement between experimental and calculated lineshapes and signal amplitudes as a function of radiofrequency magnetic field (B1) and scan rate. When B1 is adjusted for maximum signal amplitude as a function of scan rate, signal intensity for constant number of scans is enhanced by up to a factor of three relative to slow scans. The number of scans that can be averaged in a defined period of time is proportional to the scan rate, which further enhances signal amplitude per unit time. Longer relaxation times cause the maximum signal intensity to occur at slower scan rates. These experiments provide the first systematic characterization of direct-detected rapid-scan EPR signals.     

The synthesis of ketolide antibiotic ABT-773 (cethromycin)

A practical and efficient synthesis of ketolide antibiotic cethromycin (ABT-773) (1) is described. An effective protection strategy allows high yielding, regioselective C6-O-alkylation and subsequent stereoselective modification of the erythromycin nucleus. ABT-773 was prepared in 10 steps from commercially available erythromycin A oxime.     

An unusual intramolecular hetero-Diels-Alder cycloaddition

A new reaction of erythronolides, an intramolecular hetero-Diels-Alder, has been discovered. Heated aqueous alcoholic solutions of ABT-773 (1) and its cis isomer (3) convert slowly to cycloadducts 2 and 4, respectively. Optimal reaction conditions, mechanistic studies supported by molecular modeling, and biological activity data are reported. Single-crystal X-ray structures for both adducts 2 and 4 have been obtained.     

A planar electromagnet microwiggler for free electron lasers

The authors have designed and tested an electromagnet planar microwiggler for use in free electron lasers (FELs), constructed of current conductors wound on ferromagnetic cores. A prototype with a period of 1 cm and a gap of 0.5 cm produced a peak field on axis in excess of 4.6 kG, with a linear B/H characteristic to about 3.2 kG. The field of each half-period of the wiggler is independently tunable by adjusting the current delivered to each, thus allowing for precision tuning and/or wiggler tapering. General scaling laws are employed to predict the performance of a geometrically similar design with a period of 5 mm     

Enthalpies of transfer of amino acids and sodium chloride from water to aqueous fructose at 25°C

Enthalpies of transfer H ₂ of three amino acids glycine, L-alanine, and L-phenylalanine, and of NaCl, from water to aqueous fructose at low fructose mole fraction were determined at 25°C. Comparison with H ₂ for glycine in the presence and absence of KCl shows that the effect of additional ion-dipole interactions is apparent only in more concentrated fructose solutions. Limiting slopes indicate that glycine resembles NaCl more than it does the other two amino acids. The effect on H ₂ of the polarity of the zwitterion is offset and dominated by the growing non-polar side chain. Values of H ₂ for glycine, L-phenylalanine, and NaCl from water to aqueous urea are compared with the results of the present study. A rough colligative effect similiar to that observed for NaCl in a variety of polar mixed aqueous solvents may also be possible for glycine, since the change of the enthalpies of transfer with mole fraction for glycine are the same for transfer to both aqueous urea and fructose.     

Molecular mechanics study of myelin basic protein

Myelin basic protein (MBP) is the major extrinsic protein of the myelin sheath in the central nervous system. We have examined the predicted structure of segments of MBP using the molecular mechanics program ECEPP83 developed by Scheraga and coworkers as modified by Chuman, Momany, and Schafer. We have focused upon segments containing the Pro-Pro-Pro sequence (residues 100-102), which have been predicted from standard algorithms to exist in a hairpin loop connecting anti-parallel beta-strands. Both the shorter (98-105, 99-105, and 100-105) and longer segments (87-109, 87-118, and 87-120) have been examined. These results indicate potential for a chain reversal in this region. The shorter segments have been studied by others using NMR techniques and the results are compared.     

Bound on Lorentz and CPT violating boost effects for the neutron

A search for an annual variation of a daily sidereal modulation of the frequency difference between colocated 129Xe and 3He Zeeman masers sets a stringent limit on boost-dependent Lorentz and CPT violation involving the neutron, consistent with no effect at the level of 150 nHz. In the framework of the general standard-model extension, the present result provides the first clean test for the fermion sector of the symmetry of spacetime under boost transformations at a level of 10(-27) GeV.