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Harris, Andrew

Publications and source records attributed to Harris, Andrew.

5 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

The crystal structures of a chloride-pumping microbial rhodopsin and its proton-pumping mutant illuminate proton transfer determinants

Microbial rhodopsins are versatile and ubiquitous retinal-binding proteins that function as light-driven ion pumps, light-gated ion channels, and photosensors, with potential utility as optogenetic tools for altering membrane potential in target cells. Insights from crystal structures have been central for understanding proton, sodium, and chloride transport mechanisms of microbial rhodopsins. Two of three known groups of anion pumps, the archaeal halorhodopsins (HRs) and bacterial chloride-pumping rhodopsins, have been structurally characterized. Here we report the structure of a representative of a recently discovered third group consisting of cyanobacterial chloride and sulfate ion-pumping rhodopsins, the Mastigocladopsis repens rhodopsin (MastR). Chloride-pumping MastR contains in its ion transport pathway a unique Thr-Ser-Asp (TSD) motif, which is involved in the binding of a chloride ion. The structure reveals that the chloride-binding mode is more similar to HRs than chloride-pumping rhodopsins, but the overall structure most closely resembles bacteriorhodopsin (BR), an archaeal proton pump. The MastR structure shows a trimer arrangement reminiscent of BR-like proton pumps and shows features at the extracellular side more similar to BR than the other chloride pumps. We further solved the structure of the MastR-T74D mutant, which contains a single amino acid replacement in the TSD motif. We provide insights into why this point mutation can convert the MastR chloride pump into a proton pump but cannot in HRs. Our study points at the importance of precise coordination and exact location of the water molecule in the active center of proton pumps, which serves as a bridge for the key proton transfer.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2018

Incremental States for Precise On-Orbit Relative Knowledge in Formation Flight

High precision and close proximity formation flight is an enabling technology for future space missions and requires an on-board relative navigation capability that is accurate to the mm-level and robust to formation parameters. Common estimation techniques linearize the entire formation about one spacecraft’s position, resulting in degraded accuracy due to linearization errors when separation distances become large. Additionally, methods which decouple absolute and relative state estimates usually require ad-hoc methods to incorporate the estimates together. This work discusses an alternative ”incremental” formulation of the relative navigation problem which is in variant to formation size, robust to coupling between absolute and relative dynamics, and can undergo similarity transformations to smoothly incorporate either absolute or relative information without numerical issues. A specific example of this architecture is presented in the context of formation navigation using Carrier-Differential Global Positioning System measurements, and is compared to a traditional leader-linearized filter. In the presence of accurate measurements, the incremental architecture is shown to reduce linearization errors and mean estimate errors by up to three orders of magnitude without the incorporation of new sensor information.

Seubert, Carl↗
NASA NTRS2004

A Wide-Bandwidth, Low-Noise SIS Receiver Design for Millimeter and Submillimeter Wavelengths

In principle, millimeter and submillimeter heterodyne receivers using state-of-the-art SIS detectors are capable of extremely large instantaneous bandwidths with noise temperatures within a few Kelvin of the quantum limit. We are applying modem design tools, such as 3D electromagnetic simulators and Caltech's SuperMix SIS analysis package, to develop a new generation of waveguide SIS mixers with very broad RF and IF bandwidths. Our initial design consists of a double-sideband mixer targeted for the 180- 300 GHz band that uses a single SIS junction excited by a full bandwidth, fixed-tuned waveguide probe on a silicon substrate. The IF output band, limited by the MMIC low-noise IF preamplifier, is 6-18 GHz, providing an instantaneous RF bandwidth of 24 GHz (double-sideband). The SIS mixer conversion loss is predicted to be no more than 1-2 dB (single-sideband) with mixer noise temperatures across the band within 10 Kelvin of the quantum limit. The single-sideband receiver noise temperature goal is 70 Kelvin. The wide instantaneous bandwidth and low noise will result in an instrument capable of a variety of important astrophysical observations beyond the capabilities of current instruments. Lab testing of the receiver will begin in the summer of 2002, and a demonstration on the CSO should occur in the spring of 2003.

Sumner, Matthew↗
NASA NTRS2002

An Extremely Wide Bandwidth, Low-Noise SIS Heterodyne Receiver Design for Millimeter and Submillimeter Observations

Millimeter and submillimeter heterodyne receivers using state-of-the-art SIS detectors are capable of extremely large instantaneous bandwidths with noise temperatures within a few Kelvin of the quantum limit. We present the design for a broadband, sensitive, heterodyne spectrometer under development for the Caltech Submillimeter Observatory (CSO). The 180-300 GHz double-sideband design uses a single SIS device excited by a full bandwidth, fixed-tuned waveguide probe on a silicon substrate. The IF output frequency (limited by the MMIC low noise IF preamplifier) is 6-18 GHz, providing an instantaneous RF bandwidth of 24 GHz (double-sideband). The SIS mixer conversion loss should be no more than 1-2 dB with mixer noise temperatures across the band within 10 K of the quantum limit. The single-sideband receiver noise temperature goal is 70 K. The wide instantaneous bandwidth and low noise will result in an instrument capable of a variety of important astrophysical observations beyond the capabilities of current instruments. Lab testing of the receiver will begin in the summer of 2002, and the first use on the CSO should occur in the spring of 2003.

Sumner, Matthew↗