Switching mechanisms in flexible solution-processed TiO2 memristors.

Memristors are emerging as unique electrical devices with potential applications in memory, reconfigurable logic and biologically inspired computing. Due to the novelty of these devices, the complete details of their switching mechanism is not yet well established. In this work, the switching mechanism of our solution-processed titanium dioxide-based memristor is investigated by studying how variations in the device area and film thickness affect electrical behavior and correlating these behavioral changes to proposed switching mechanisms. The conduction path of the switching is also investigated through electrical characterization of devices both before and after physically cutting the devices in half, as well as through infrared imaging of the devices during operation. The results suggest that the electrical behavior of these devices is dominated by a localized, charge-based phenomenon that exhibits a dependence on device area.

The photocatalytic production of organic-free water for molecular biological and pharmaceutical applications.

The inability of conventional water-purification systems to meet the ultra-high purity needs of molecular biology and biopharmaceuticals reliably was attributed to their almost exclusive utilization of phase-transfer technologies. Water quality may unpredictably degrade when confronted by microorganism blooms or altered feed water characteristics. Photocatalytic point-of-use water-purification systems fed by deionized water were demonstrated to meet the most stringent water-purity needs of the molecular biologist. The reliability of the photocatalytic water-purification technology was attributed to its ability to destroy organic contaminants rather than just effect their phase transfer. Photocatalytically produced water was shown to be free of detectable microorganisms, DNA, endotoxins and RNAses. It is suitable for immunological studies involving tissue and other cell cultures because of its lack of detectable endotoxins. Because DNA was also undetectable, it is suitable for DNA and endotoxin zero-standards as well as pharmaceutical formulation. The photocatalytic water is a reliable substitute for diethyl pyrocarbonate-treated water used in RNA work, compatible with PCR and sufficiently free from other contaminants to be useful for most biochemical and enzymatic assays.

Expression of fully functional tetrameric human hemoglobin in Escherichia coli.

Synthetic genes encoding the human alpha- and beta-globin polypeptides have been expressed from a single operon in Escherichia coli. The alpha- and beta-globin polypeptides associate into soluble tetramers, incorporate heme, and accumulate to greater than 5% of the total cellular protein. Purified recombinant hemoglobin has the correct stoichiometry of alpha- and beta-globin chains and contains a full complement of heme. Each globin chain also contains an additional methionine as an extension to the amino terminus. The recombinant hemoglobin has a C4 reversed-phase HPLC profile essentially identical to that of human hemoglobin A0 and comigrates with hemoglobin A0 on SDS/PAGE. The visible spectrum and oxygen affinity are similar to that of native human hemoglobin A0. The recombinant protein shows a reduction in Bohr and phosphate effects, which may be attributed to the presence of methionine at the amino termini of the alpha and beta chains. We have also expressed the alpha- and beta-globin genes separately and found that the expression of the alpha-globin gene alone results in a marked decrease in the accumulation of alpha-globin in the cell. Separate expression of the beta-globin gene results in high levels of insoluble beta-globin. These observations suggest that the presence of alpha- and beta-globin in the same cell stabilizes alpha-globin and aids the correct folding of beta-globin. This system provides a simple method for expressing large quantities of recombinant hemoglobin and allows facile manipulation of the genes encoding hemoglobin to produce functionally altered forms of this protein.

Molecular receptors and their potential for artificial transduction.

The thesis is presented that molecular receptor physical chemistry offers an interesting model for the design of biosensors with respect to chemical recognition and transduction. In order to appreciate the desirable features of this system and the inherent difficulties, the structures and binding state energetics of molecular receptors are considered via a direct comparison with enzyme chemistry. Detailed analyses of the candidate species nicotinic and beta-adrenergic receptors are provided to illustrate the complexity of molecular receptor binding properties. A revised ternary-complex model, which combines enzymatic and receptor energetics, is proposed to explain the free-energy changes which drive ligand-binding reactions of coupled systems. Throughout this discussion the relevance of the various arguments to applications in biosensor development is highlighted. Finally, a brief appraisal of attempts to produce devices ready for marriage to molecular receptor material is presented.

Affinity purification of the mammalian beta-2-adrenergic receptor.

The efficiency of covalently linking alprenolol to Sepharose via a 14-atom spacer prepared from 1,4-butanediol diglycidyl ether has been increased. This in turn has aided in increasing the specific yield of beta-2-adrenergic receptor by a factor of 3 over earlier results. Further development of extraction and solubilization protocols are also described. The adsorption of the affinity-purified receptor to stainless steel was measured, and is cited as a potential problem in further purification by high-pressure liquid chromatography.

Dopamine D2 receptor binding sites for agonists. A tetrahedral model.

In order to develop a model for the putative binding sites between the D2 dopamine receptor and many of its agonists, we obtained the dissociation constants of many dopaminergic agonists at the high affinity state, D2high, as well as at the low affinity state, D2low, of the receptor. [3H]Spiperone was used to label the D2 dopamine receptors in porcine anterior pituitary tissue. Agonists without any hydroxyl groups, such as 2-aminotetralin, effectively inhibited the binding of [3H]spiperone; the addition of a hydroxyl group corresponding to the "meta" position in dopamine, however, enhanced the potency (in four series of agonists) by an order of magnitude. The R-(-)-enantiomers of the aporphines and 5,6,-dihydroxy-2-dipropylaminotetralin were more potent than the S-(+)-enantiomers. Although the 4-methoxy-2-dipropylaminoindans were potent, the R-(-)-11-methoxyaporphines were not. A tetrahedral model is proposed; this has two sites for agonist attachment, the extremities of the sites being separated by 8 A, and their functional groups directed between 15 degrees and 30 degrees off the orthogonal from the receptor surface. Several steric obstacles are required to account for the inactivity of several congeners.

Tissue-specific and substrate-specific detection of aldehyde and pyridoxal oxidase in larval and imaginal tissues of Drosophila melanogaster.

The substrate specificities of aldehyde and pyridoxal oxidases in Drosophila melanogaster have been determined with a variety of aliphatic and aromatic aldehydes. This analysis has led to the discovery that 2,4,5-trimethoxybenzaldehyde is a specific substrate for pyridoxal oxidase, as based on the histochemical distribution of oxidase activity, the absence of enzymatic activity in the lpo strains, and the dosage dependence on the number of 1po+ genes present. The tissue-specific localization of aldehyde oxidase (AO) and pyridoxal oxidase (PO) in the larval and adult structures showed that AO was present in all the major internal organs of the larvae and adults, including brain, imaginal discs, Malpighian tubules, digestive system, and reproductive structures. Pyridoxal oxidase is present in many of the same structures which possess AO, but is missing from the cardia, crop, imaginal discs, ovarian follicle cells, paragonia, pericardial cells, and wreath cells. The only structure which possesses PO but lacks AO is the larval salivary gland. These histochemical differences in AO and PO distribution were also confirmed by enzymatic analysis of the activities present in homogenates of ovaries, paragonia, and salivary glands. The general pattern of enzyme expression appears to be established during embryogenesis and maintained throughout the life of the individual.

Dopamine receptors in the central nervous system.

Dopamine receptors in the central nervous system can be studied by measuring the specific binding of [3H]dopamine, [3H]haloperidol, d-[3H]LSD, [3H]dihydroergocryptine or [3H]apomorphine. The receptors are stereoselectively blocked by +)-butaclamol, a neuroleptic. All neuroleptics inhibit the specific binding of [3H]haloperidol in relation to their clinical potencies. The radioligand that desorbs most slowly from the receptor is [3H]apomorphine, thus making it a reliable ligand for dopamine receptors. Dopamine agonists that compete for [3H]apomorphine binding do so at concentrations that correlate with their potency in stimulating striatal adenylate cyclase. Structure-activity analysis, using [3H]apomorphine, confirms that the active dopamine-mimetic conformation is the beta rotamer of dopamine. Prolonged exposure in vitro of caudate homogenate to high concentrations of dopamine leads to increased binding of [3H]apomorphine or [3H]haloperidol, suggesting receptor "sensitization." Chronic haloperidol treatment of rats leads to an increased number of dopamine/neuroleptic receptors in the striatum, but a decrease in the pituitary.