Surface studies of quinhydrone pH sensors

A simplified methodology: pH sensing using an in situ fabricated Ir electrode under neutral conditions

Herein, a simplified fabrication method for the producing of a pH-sensitive iridium electrode is developed. The in situ electrochemical fabrication of an iridium oxide film is optimized and shown to be achievable under neutral conditions rather than the acidic conditions hitherto employed. The formation of a pH sensitive Ir(III/IV) hydrous film is confirmed via XPS. The amperometric pH-sensing properties of this electrochemically generated material were investigated using square wave voltammetry. In the pH range 2–13, the iridium oxide redox signal has a pH dependency of 86.1 ± 1.1 mV per pH unit for midpoint potentials with uncertainties being ± 0.01–0.05 pH. Finally, the newly developed pH sensor was used to measure the pH of a natural water sample with excellent results as compared to a conventional glass pH probe.

Positional adaptability in the design of mutation-resistant nonnucleoside HIV-1 reverse transcriptase inhibitors: a supramolecular perspective

Drug resistance is a key cause of failed treatment of HIV infection. The efficacy of nonnucleoside reverse transcriptase-inhibiting (NNRTI) drugs is impaired by the rapid emergence of drug-resistant mutations. The literature supports the idea that purposefully designed flexible NNRTIs at an active site may help overcome drug resistance. It is proposed here that the usual "lock and key" model, with respect to NNRTI drug design, be expanded to consider creating "master keys" that would automatically adjust conformations to fit all of the "locks" mutations may make. The present work introduces the novel perspective of designing and creating supramolecular assemblies as potential NNRTIs (instead of the relatively more rigid single-molecule inhibitors). Specifically, flexible self-assembling quinhydrone supramolecular dimers formed from quinonoid monomers (designed to be highly flexible NNRTIs themselves) will be offered as a working example of this new perspective in NNRTI drug design. Quinonoid compounds have demonstrated binding interactions at various sites of the HIV-1 RT enzyme, including the elusive ribonuclease H area. Quinhydrone self-organized dimers have at some point in their molecular architecture a noncovalently interacting donor-acceptor ring pair complex. This complex is at the heart of the increased torsional, rotational, and translational motion this species will experience at a particular active site. Flexible supramolecular assemblies, together with their flexible monomer components, may offer a critical advantage in retaining potency against a wide range of drug-resistant HIV-1 RTs. This new supramolecular perspective may also have broader implications in the general field of antimicrobial drug design.

Disulfide bond formation involves a quinhydrone-type charge-transfer complex

The chemistry of disulfide exchange in biological systems is well studied. However, the detailed mechanism of how oxidizing equivalents are derived to form disulfide bonds in proteins is not clear. In prokaryotic organisms, it is known that DsbB delivers oxidizing equivalents through DsbA to secreted proteins. DsbB becomes reoxidized by reducing quinones that are part of the membrane-bound electron-transfer chains. It is this quinone reductase activity that links disulfide bond formation to the electron transport system. We show here that purified DsbB contains the spectral signal of a quinhydrone, a charge-transfer complex consisting of a hydroquinone and a quinone in a stacked configuration. We conclude that disulfide bond formation involves a stacked hydroquinone-benzoquinone pair that can be trapped on DsbB as a quinhydrone charge-transfer complex. Quinhydrones are known to be redox-active and are commonly used as redox standards, but, to our knowledge, have never before been directly observed in biological systems. We also show kinetically that this quinhydrone-type charge-transfer complex undergoes redox reactions consistent with its being an intermediate in the reaction mechanism of DsbB. We propose a simple model for the action of DsbB where a quinhydrone-like complex plays a crucial role as a reaction intermediate.