Simultaneous measurement and quantitation of 4-hydroxyphenylacetic acid and dopamine with fast-scan cyclic voltammetry

In Situ Electrochemical Monitoring of Caged Compound Photochemistry: An Internal Actinometer for Substrate Release

Caged compounds are molecules that release a protective substrate to free a biologically active substrate upon treatment with light of sufficient energy and duration. A notable limitation of this approach is difficulty in determining the degree of photoactivation in tissues or opaque solutions because light reaching the desired location is obstructed. Here, we have addressed this issue by developing an in situ electrochemical method in which the amount of caged molecule photorelease is determined by fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes. Using p-hydroxyphenyl glutamate (pHP-Glu) as our model system, we generated a linear calibration curve for oxidation of 4-hydroxyphenylacetic acid (4HPAA), the group from which the glutamate molecule leaves, up to a concentration of 1000 μM. Moreover, we are able to correct for the presence of residual pHP-Glu in solution as well as the light artifact that is produced. A corrected calibration curve was constructed by photoactivation of pHP-Glu in a 3 μL photoreaction vessel and subsequent analysis by high-performance liquid chromatography. This approach has yielded a linear relationship between 4HPAA concentration and oxidation current, allowing the determination of released glutamate independent of the amount of light reaching the chromophore. Moreover, we have successfully validated the newly developed method by in situ measurement in a whole, intact zebrafish brain. This work demonstrates for the first time the in situ electrochemical monitoring of caged compound photochemistry in brain tissue with FSCV, thus facilitating analyses of neuronal function.

Amperometric Detection of the Urinary Disease Biomarker p-HPA by Allosteric Modulation of a Redox Polymer-Embedded Bacterial Reductase

We report an amperometric biosensor for the urinary disease biomarker para-hydroxyphenylacetate ( p-HPA) in which the allosteric reductase component of a bacterial hydroxylase, C1-hpah, is electrically wired to glassy carbon electrodes through incorporation into a low-potential Os-complex modified redox polymer. The proposed biosensing strategy depends on allosteric modulation of C1-hpah by the binding of the enzyme activator and analyte p-HPA, stimulating oxidation of the cofactor NADH. The pronounced concentration-dependence of allosteric C1-hpah modulation in the presence of a constant concentration of NADH allowed sensitive quantification of the target, p-HPA. The specific design of the immobilizing redox polymer with suitably low working potential allowed biosensor operation without the risk of co-oxidation of potentially interfering substances, such as uric acid or ascorbic acid. Optimized sensors were successfully applied for p-HPA determination in artificial urine, with good recovery rates and reproducibility and sub-micromolar detection limits. The proposed application of the allosteric enzyme C1-hpah for p-HPA trace electroanalysis is the first successful example of simple amperometric redox enzyme/redox polymer biosensing in which the analyte acts as an effector, modulating the activity of an immobilized biocatalyst. A general advantage of the concept of allosterically modulated biosensing is its ability to broaden the range of approachable analytes, through the move from substrate to effector detection.