Dijanah Cota Machado

Mechanism of KCl Enhancement in Detection of Nonionic Polymers by Nanopore Sensors

The mechanisms of KCl-induced enhancement in identification of individual molecules of poly(ethylene glycol) using solitary alpha-hemolysin nanoscale pores are described. The interaction of single molecules with the nanopore causes changes in the ionic current flowing through the pore. We show that the on-rate constant of the process is several hundred times larger and that the off-rate is several hundred times smaller in 4 M KCl than in 1 M KCl. These shifts dramatically improve detection and make single molecule identification feasible. KCl also changes the solubility of poly(ethylene glycol) by the same order of magnitude as it changes the rate constants. In addition, the polymer-nanopore interaction is determined to be a strong non-monotonic function of voltage, indicating that the flexible, nonionic poly(ethylene glycol) acts as a charged molecule. Therefore, salting-out and Coulombic interactions are responsible for the KCl-induced enhancement. These results will advance the development of devices with sensor elements based on single nanopores.

Hofmeister Effect in Confined Spaces: Halogen Ions and Single Molecule Detection

Despite extensive research in the nanopore-sensing field, there is a paucity of experimental studies that investigate specific ion effects in confined spaces, such as in nanopores. Here, the effect of halogen anions on a simple bimolecular complexation reaction between monodisperse poly(ethylene glycol) (PEG) and α-hemolysin nanoscale pores have been investigated at the single-molecule level. The anions track the Hofmeister ranking according to their influence upon the on-rate constant. An inverse relationship was demonstrated for the off-rate and the solubility of PEG. The difference among anions spans several hundredfold. Halogen anions play a very significant role in the interaction of PEG with nanopores although, unlike K(+), they do not bind to PEG. The specific effect appears dominated by a hydration-dehydration process where ions and PEG compete for water. Our findings provide what we believe to be novel insights into physicochemical mechanisms involved in single-molecule interactions with nanopores and are clearly relevant to more complicated chemical and biological processes involving a transient association of two or more molecules (e.g., reception, signal transduction, enzyme catalysis). It is anticipated that these findings will advance the development of devices with nanopore-based sensors for chemical and biological applications.

Brain acetylcholinesterase of jaguar cichlid (Parachromis managuensis): From physicochemical and kinetic properties to its potential as biomarker of pesticides and metal ions

This contribution aimed to characterize physicochemical and kinetic parameters of the brain cholinesterases (ChEs) from Parachromis managuensis and investigate the in vitro effects of pesticides and metal ions on its activity intending to propose as biomarker. This species is suitable for this investigation because (1) it was recently introduced in Brazil becoming invasive (no restrictions on capture) and (2) occupies the top of the food chain (being subject to bioaccumulation). The enzyme extract was exposed to 10 metal ions (Al(3+), Ba(2+), Cd(2+), Cu(2+), Hg(2+), Mg(2+), Mn(2+), Pb(2+), Fe(2+) and Zn(2+)) and ChEs selective inhibitors (BW284c51, Iso-OMPA, neostigmine and serine). The extract was also incubated with organophosphate (dichlorvos) and carbamate pesticides (carbaryl and carbofuran). Inhibition parameters (IC20, IC50 and ki) were determined. Selective inhibitors and kinetic parameters confirmed acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) -like as responsible for the ChE activities, most AChE. The IC50 values for pesticides were: 1.68μM (dichlorvos); 4.35μM (carbaryl) and 0.28μM (carbofuran). Most of the analyzed ions did not show significant effect at 1mM (p=0.05), whereas the following ions inhibited the enzyme activity in the order: Hg(2+)>Cu(2+)>Cd(2+)>Zn(2+). Mercury ion strongly inhibited the enzyme activity (IC20=0.7μM). The results about allow to conclude that P. managuensis brain AChE is a potential biomarker for heavy metals and pesticides under study, mainly for the carbamate carbofuran once it was capable to detect 6-fold lower levels than the limit concentration internationally recommended.

BIOSSENSORIAMENTO ESTOCÁSTICO VIA NANOPORO PROTEICO INDIVIDUAL NO DESENVOLVIMENTO DE FERRAMENTAS ANALÍTICAS

Studies employing a single protein nanopore as a molecular recognition element in the development of analytical devices - biosensors, spectrometers, DNA sequencing - have increased considerably in the last decade. Several studies show the potential of these bionanostructures for future stochastic biosensing technology. Stochastic biosensing is an approach that relies on the observation of individual binding events between analyte molecules and a single receptor. This approach is inherent to the organisms that use a single protein nanopore as a key element to start, manage and maintain the chemical and biophysical processes of living cells. Here, we discuss alpha-toxin as a bacterial exotoxin and prototype of a protein nanopore in real-time detection and characterization of molecules in aqueous systems.

Effects of alkali and ammonium ions in the detection of poly(ethyleneglycol) by alpha-hemolysin nanopore sensor

Nanopores have emerged as molecule detectors and more studies are necessary to clarify the analyte–nanopore interaction. Here, we report the influence of cations of the Hofmeister series on a simple bimolecular complexation reaction between poly(ethyleneglycol) (PEG) and α-hemolysin (αHL) nanopore at the single-molecule level. The kinetic constants (on-rate and off-rate) of interaction of PEG molecules with αHL nanopore depend strongly on the type of cation. In the case of on-rate constant, the difference reaches several dozen times. As a consequence of this, both transition rate and detection limit of the nanopore-based sensor were changed. An inverse relationship was demonstrated for the off-rate constant and the solubility of PEG. Except lithium, the other alkali metal cations significantly affect the interaction of PEG with the nanopore and, similarly to potassium, they also bind to this polymer. Thus, the PEG/αHL nanopore interaction can be strongly influenced by the type of cation. The cation-induced specific effect on polymer detection with a single αHL nanopore appears to occur by competition of the cation and PEG by water. Finally, our results represent a guide to select the appropriate physicochemical conditions that enhance the analyte–nanopore interaction, thereby improving the sensitivity of nanopore-based sensors.