Producing gallium arsenide crystals in space

The production of high quality crystals in space is a promising near-term application of microgravity processing. Gallium arsenide is the selected material for initial commercial production because of its inherent superior electronic properties, wide range of market applications, and broad base of on-going device development effort. Plausible product prices can absorb the high cost of space transportation for the initial flights provided by the Space Transportation System. The next step for bulk crystal growth, beyond the STS, is planned to come later with the use of free flyers or a space station, where real benefits are foreseen. The use of these vehicles, together with refinement and increasing automation of space-based crystal growth factories, will bring down costs and will support growing demands for high quality GaAs and other specialty electronic and electro-optical crystals grown in space

NASA's high efficiency and radiation damage solar cell program

The conversion efficiency and the life expectancy of solar cells and arrays were evaluated for space applications. Efforts were made to improve the understanding of the conversion of electromagnetic radiation to useful forms of energy. A broad range of advanced concepts were evaluated

An analytical basis for assaying buried biological contamination Interim report

Assay techniques for determining biological contamination of spacecraft material

Voltage Sensing Mechanism in the Voltage-gated and Proton (H+)-selective Ion Channel Hv1

Activation of the intrinsic aqueous water-wire proton conductance (GAQ) in Hv1 channels is controlled by changes in membrane potential and the transmembrane pH gradient (ΔpH). The mechanism by which changes in ΔpH affect the apparent voltage dependence of GAQ activation is not understood. In order to measure voltage sensor (VS) activation in Hv1, we mutated a conserved Arg residue in the fourth helical segment (S4) to His and measured H+ currents under whole-cell voltage clamp in transfected HEK-293 cells. Consistent with previous studies in VS domain containing proteins, we find that Hv1 R205H mediates a robust resting-state H+ ‘shuttle’ conductance (GSH) at negative membrane potentials. Voltage-dependent GSH gating is measured at more negative voltages than the activation GAQ, indicating that VS activation is thermodynamically distinct from opening of the intrinsic H+ permeation pathway. A hallmark biophysical feature of Hv1 channels is a ~-40 mV/pH unit shift in the apparent voltage dependence of GAQ gating. We show here that changes pHO are sufficient to cause similar shifts in GSH gating, indicating that GAQ inherits its pH dependence from an early step in the Hv1 activation pathway. Furthermore, we show for the first time that Hv1 channels manifest a form of electromechanical coupling VS activation and GAQ pore opening. Second-site mutations of D185 markedly alter GAQ gating without affecting GSH gating, indicating that D185 is required for a late step in the activation pathway that controls opening of the aqueous H+ permeation pathway. In summary, this work demonstrates that the Hv1 activation pathway contains multiple transitions with distinct voltage and pH dependencies that have not been previously identified. The results reported here novel insight into the mechanism of VS activation in Hv1 and raise fundamental questions about the nature of pH-dependent gating and electromechanical coupling in related VS domain-containing ion channels and phosphatases