A fiber-optic pH-imaging sensor array capable of both visualizing remote corrosion sites and measuring local chemical concentrations at these sites was applied to realtime corrosion monitoring. The imaging fiber's distal face, containing an immobilized pH-sensitive fluorescent dye, was brought into contact with metal surfaces submerged in aqueous buffers and fluorescence images were acquired as a function of time. Heterogeneous fluorescence signals were observed due to both pH increases at cathodic surface sites and pH decreases at anodic surface sites. These fluorescence signals showed both localization and rates of corrosion activity. Three corrosion processes were investigated, galvanic corrosion at a copper/aluminum interface and crevice corrosion and pitting at a stainless steel surface. The spatial resolution of the technique was limited by proton/hydroxide diffusion and the diameter of the individually clad optical fibers comprising the imaging bundle.
A chemoenzymatic process for the production of high-purity glycolic acid has been demonstrated, starting with the reaction of formaldehyde and hydrogen cyanide to produce glycolonitrile in > 99 % yield and purity. The resulting aqueous glycolonitrile was used without further purification in a subsequent biocatalytic conversion of glycolonitrile to ammonium glycolate. A high-activity biocatalyst based on an Acidovorax facilis 72W nitrilase was developed, where protein engineering and optimized protein expression in an E. coli transformant host were used to improve microbial nitrilase specific activity by 33-fold compared to the wild-type strain. A biocatalyst productivity of > 1000 g glycolic acid/g dry cell weight was achieved using a glutaraldehyde/ polyethylenimine cross-linked carrageenan-immobilized E. coli MG1655 transformant expressing the A. facilis 72W nitrilase mutant, where 3.2 M ammonium glycolate was produced in consecutive batch reactions with biocatalyst recycle, or in a continuous stirred-tank reactor. Direct conversion of the unpurified ammonium glycolate product solution to highpurity aqueous glycolic acid was accomplished by fixed-bed ion exchange over a strong acid cation resin.
Lipase-catalyzed polymerization of caprolactone (CL) in toluene with methoxy-poly(ethylene glycol) (MPEG) and water as initiators was characterized in detail for mechanistic insight. (1)H NMR analysis of polycaprolactone chains (PCL), dicaprolactone, degree of esterification of MPEG, and fractions of PCL chains initiated by MPEG and water were used to follow the reactions. The data were analyzed with the kinetic scheme involving formation of the acylenzyme and its consequent reaction with MPEG, water, or PCL to yield the MPEG- or water-initiated PCL chains, or increase in PCL length. A limit for MPEG initiator esterification in lipase-catalyzed CL polymerization was observed and was explained by preferential reaction of PCL propagation over MPEG esterification at long reaction times and low MPEG concentrations. Slower monomer conversion in concentrated monomer solutions was explained by decreased partitioning of PCL between the solvent and the enzyme. This effect resulted in inhibition of the lipase by the reaction product, PCL chains, and/or insufficient diffusion of monomer to the enzyme active site. High monomer/initiators ratio in these solutions did not yield longer polymer chains due to decreased monomer conversion and the corresponding decrease in product yields; lower yields were also observed for chain initiation by MPEG and water. A shift in the reaction rate-limiting step from formation of acylenzyme in dilute CL solutions to its deacylation in concentrated CL solutions yielded higher PCL polydispersity due to increased initiation by water. Enhanced intramolecular cyclization was also observed. Endgroup composition of PCL chains was influenced by the concentration of monomer, ratio of initiators (MPEG and water), and reaction time, yielding PCL chains initiated exclusively by MPEG at "infinite reaction times."
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