Detailed active site configuration of a new crystal form of methanol dehydrogenase from Methylophilus W3A1 at 1.9 A resolution.

The three-dimensional structure of a new crystal form of methanol dehydrogenase from Methylophilus W3A1 has been obtained in the presence of substrate using data recorded at a synchrotron. The structure of this approximately 140 kDa heterotetramer, refined at 1. 9 A resolution, reveals the detailed configuration of its redox cofactor, pyrroloquinoline quinone (PQQ). C4, one of the oxygen-bearing atoms of this orthoquinone is in a planar configuration while C5, which bears the other quinone oxygen, is tetrahedral, suggesting that the PQQ is in the semiquinone redox state. The substrate binding site has been identified close to PQQ and to the side chain of Asp297, the putative active site base. The proximity of the hydroxyl of methanol to C5 of PQQ compared to the greater separation of the substrate methyl group from C5 supports the addition-elimination reaction mechanism involving a hemiketal intermediate.

Determination of the gene sequence and the three-dimensional structure at 2.4 angstroms resolution of methanol dehydrogenase from Methylophilus W3A1.

The DNA sequences for the genes encoding the heavy and light subunits of methanol dehydrogenase from Methylophilus methylotrophus W3A1 have been determined. The deduced amino acid sequence has enabled the structure of the enzyme to be refined at 2.4 angstrom resolution against X-ray data collected on a Hamlin area detector. The structure was refined using the programs PROFFT and X-PLOR with several model building step interspersed. The final model contains two heavy chains (571 amino acids), two light chains (69 amino acids), two molecules of pyrroloquinoline quinone, two Ca2+ and 521 solvent molecules. Each half molecule contains four disulfide linkages and four cis peptides. One of the disulfides is formed from two adjacent cysteine residues linked by a trans peptide which creates a novel eight-membered ring. The heavy subunit is an 8-fold beta-propeller, each "blade" of which is a four-stranded antiparallel twisted beta-sheet. The light chain is an elongated subunit stretching across the surface of the heavy subunit, with residues 1 to 32 containing four beta-turns and residues 33 to 62 forming a helix; however, it neither interacts with the active site, nor the other HL dimer and its functional role is obscure. Around the 8-fold beta-propeller there is a repeating pattern of tryptophan residues located in the outer strand of seven of the eight beta-leaflets, each packed between adjacent leaflets. Each of these tryptophan residues is centered in the beta-strand and participates in the main chain hydrogen bonding of the sheet. Five of the seven tryptophan residues have closely similar interactions with the adjacent beta-leaflet including stacking of the tryptophan indole rings against a peptide plane and formation of a hydrogen bond from NE1 of the indole ring to a main-chain carbonyl. This repeating pattern is conserved over a number of MEDH sequences. The PQQ is located on the pseudo 8-fold rotation axis of the heavy subunit, in a funnel-shaped internal cavity, sandwiched between the indole ring of Trp237 and the two sulfur atoms of the Cys103-Cys104 vicinal disulfide. A hexacoordinate Ca2+ is bound in the active site by one nitrogen and five oxygen ligands, three from the PQQ and the others from two protein side-chains. In the active site an isolated solvent molecule is bound to the O5 of PQQ and to a nearby aspartate side-chain; its position may be the binding site for methanol. The aspartate might than serve as a general base for proton abstraction from the substrate hydroxyl. The C5 atom of PQQ could be activated by electrophilic catalysis by a nearby argenine side-chain or by the calcium ion bound to PQQ.

Wavelength optimization of quantum-well modulators in smart pixels.

We extend our recent general discussion of electroabsorption and refraction in multiple-quantum-well modulators to determine the optimum modulator design for smart-pixel applications. In addition to the optimum operating wavelength shift, from that of the zero-voltage exciton, we determine the optimum number of quantum wells, and we calculate the reflectivity change and the contrast ratio obtainable. This analysis is undertaken for both simple detectors and modulators, meaning that they are antireflection coated, as well as for devices that include Fabry-Perot resonators. The optimization is performed on a figure of merit that is inversely proportional to the incident optical read energy required on a device to switch another, downstream device. We maximize the figure of merit to minimize the optical read energy. An interesting result is that there should be no significant improvement in our smart-pixel circuit figure of merit with the use of Fabry-Perot resonant modulators and detectors. Our results are, of course, material-system specific, but for the 850-nm AlGalAs/GaAs quantum-well system the optimum wavelength shift from the exciton location is approximately 6 nm. The general trends and approach are applicable to other material systems.

Picosecond switching of GaAs/AlGaAs smart pixels.

We demonstrate that GaAs/AlGaAs smart pixels can be operated with picosecond and subpicosecond laser pulses. Switching times as short as 200 ps are measured. Our results confirm the theoretical prediction that input mode-locked pulses are more advantageous than the more typical square-wave pulses. They also suggest that use of picosecond mode-locked pulses for optical output could produce operating speeds in the gigahertz range.

Self-linearized analog differential self-electro-optic-effect device.

We describe, demonstrate, and characterize an analog self-electro-optic-effect device that gives a difference between two optical output powers that is linearly proportional to an electrical or an optical drive. Such a device should permit bipolar (positive and negative) processing in novel image processing arrays. The device is able to operate over a range of more than 4 orders of magnitude of optical power from 50 nW to 2.5 mW, corresponding to uniform incident intensities as low as 3.3 mW/cm(2). The frequency response (3-dB limit) varies linearly from 7 kHz at 1-µW absorber power to 3.5 MHz at 1 mW of absorbed powers.

Analog differential self-linearized quantum-well self-electro-optic-effect modulator.

We describe an analog self-electro-optic-effect device that gives a difference between two optical output powers that is linearly proportional to electrical or optical drive, permitting bipolar processing in novel image-processing arrays. The device is able to operate over a range of more than four orders of magnitude optical power, from 50 nW to 2.5 mW, corresponding to uniform incident intensities as low as 3.3 mW/cm(2).

Suspected buttercup (Ranunculus bulbosus) toxicosis with secondary photosensitization in a Charolais heifer.

A presumptive diagnosis of buttercup toxicosis with photosensitization secondary to hepatotoxicity was made in an 18-mo-old Charolais heifer. The differential diagnosis included salmonellosis, aflatoxicosis, bovine virus diarrhea, internal parasite infestation, and plant toxicosis with either primary or secondary photosensitization. All these possibilities were excluded except buttercup toxicosis with photosensitization secondary to hepatotoxicity. While this diagnosis was not absolutely confirmed, it was the most likely cause of the disease and raised the intriguing possibility that protoanemonin, buttercup's toxic principle, is hepatotoxic.

Solid-state low-loss intracavity saturable absorber for Nd:YLF lasers: an antiresonant semiconductor Fabry-Perot saturable absorber.

We introduce a new low-loss fast intracavity semiconductor Fabry-Perot saturable absorber operated at anti-resonance both to start and sustain stable mode locking of a cw-pumped Nd:YLF laser. We achieved a 3.3-ps pulse duration at a 220-MHz repetition rate. The average output power was 700 mW with 2 W of cw pump power from a Ti:sapphire laser. At pump powers of less than 1.6 W the laser self-Q switches and produces 4-ps pulses within a 1.4-micros Q-switched pulse at an approximately 150-kHz repetition rate determined by the relaxation oscillation of the Nd:YLF laser. Both modes of operation are stable. In terms of coupled-cavity mode locking, the intra-cavity antiresonant Fabry-Perot saturable absorber corresponds to monolithic resonant passive mode locking.

Optically excited synapse for neural networks.

What can optics with its promise of parallelism do for neural networks which require matrix multipliers? An all optical approach requires optical logic devices which are still in their infancy. An alternative is to retain electronic logic while optically addressing the synapse matrix. This paper considers several versions of an optically addressed neural network compatible with VLSI that could be fabricated with the synapse connection unspecified. This optical matrix multiplier circuit is compared to an all electronic matrix multiplier. For the optical version a synapse consisting of back-to-back photodiodes is found to have a suitable i-v characteristic for optical matrix multiplication (a linear region) plus a clipping or nonlinear region as required for neural networks. Four photodiodes per synapse are required. The strength of the synapse connection is controlled by the optical power and is thus an adjustable parameter. The synapse network can be programmed in various ways such as a shadow mask of metal, imaged mask (static), or light valve or an acoustooptic scanned laser beam or array of beams (dynamic). A milliwatt from LEDs or lasers is adequate power. The neuron has a linear transfer function and is either a summing amplifier, in which case the synapse signal is current, or an integrator, in which case the synapse signal is charge, the choice of which depends on the programming mode. Optical addressing and settling times of microseconds are anticipated. Electronic neural networks using single-value resistor synapses or single-bit programmable synapses have been demonstrated in the high-gain region of discrete single-value feedback. As an alternative to these networks and the above proposed optical synapses, an electronic analog-voltage vector matrix multiplier is considered using MOSFETS as the variable conductance in CMOS VLSI. It is concluded that a shadow mask addressed (static) optical neural network is promising.

Large Cryptococcus neoformans isolated from brain abscess.

Cryptococcus neoformans yeast cells 40 to 60 micron in diameter were seen in an India ink preparation made from a human brain abscess specimen. In culture at 25 degrees C, uniform 5-micron-diameter yeast cells were produced. Inoculation into mice produced yeast cells up to 40 micron in diameter, and brain heart infusion broth culture at 35 degrees C produced yeast cells about 25 micron in diameter. A relationship of yeast cell diameter to incubation temperature is suggested.

Theory Z: a review of management styles and a proposal for change.

Japanese management techniques have been the subject of much interest over the past few years. However, American culture does not lend itself to strict adherence to Japanese practices. Theory Z is a management philosophy that blends Japanese and American techniques and is currently being practiced in many areas of the business world. With some work, Theory Z can be incorporated into health care organizations in general,and radiology departments in particular.