Carriers versus Adapters in Stochastic Sensing

Abstract

Sensors based on analyte-dependent changes in the conduction of transmembrane protein pores are an interesting prospect. Stochastic sensing, that is, the detection of individual analyte molecules by single pores, has several potential advantages and a variety of sensor elements based on engineered forms of staphylococcal a-hemolysin (aHL) has been tested. [1] In one manifestation of this approach, organic molecules have been detected by using molecular adapters, such as cyclodextrins, which continue to engage in dynamic host-guest interactions after they have become lodged within the lumen of the aHL pore.[2] Here, we show that small organic molecules can also be detected by a scheme in which a host, cucurbit[6]uril (CB6), [3] acts as a carrier rather than an adapter. In this case, the rate of exchange of the guest between the host·guest complex and bulk solvent is slower [4] than the rate of exchange of the host between the solvent and its binding site within the aHL pore. Besides providing a useful means for sensing organic molecules, the carrier system allows the determination of dissociation constants and, independently, association and dissociation rate constants for CB6·guest pairs.CB6 ( Figure 1) contains a hydrophobic cavity, % 5.5 in diameter, which can host small neutral organic molecules.[3] Six carbonyl groups are located at each of the two 4--diameter entrances to the cavity. CB6 is sparingly soluble in water, but its solubility increases in the presence of alkali-metal salts because the carbonyl groups coordinate to the metal cations to form positively charged complexes. [5,6] An X-ray structure of CB6, which was crystallized from an aqueous solution containing Na + ions, shows two sodium ions and their coordinated water molecules at each entrance. [5,6] By contrast, when CB6 is crystallized in the presence of Cs + , only one cesium ion is associated with each entrance.[6] The dimensions of CB6 (maximum external diameter % 14.4 ) suggested that it might become lodged in the lumen of the aHL pore (Figure 1), similar to b-cyclodextrin (diameter % 15.5 ), and mediate the detection of analytes through host-guest interactions.The interaction of CB6 with the aHL pore was first examined in the absence of analyte by monitoring the current through a single pore in a planar bilayer apparatus, as described previously. [7] At + 100 mV in 1.0 m CsCl/10 mm K phosphate buffer (pH 7.4), the current measured through a fully open aHL pore (level L1) was 93.3 AE 1.2 pA (n = 7), and the current with CB6 bound from the trans side (level L2, aHL·CB6) was 41.7 AE 1.5 pA (n = 7) (Figure 2). Binding events were not detected when CB6 was applied to the cis side of the bilayer. The mean interevent intervals (t on ) and the mean durations of the binding events (t off ) were determined from dwell-time histograms.[8]The value of 1/t on increased linearly with [CB6], where [CB6] is the concentration of the cucurbituril, which suggests a simple bimolecular interaction (Scheme 1).[8] The rate constants for association and dissociat...

Nanopore Detection of Single‐Molecule Binding within a Metallosupramolecular Cage

“…The ability of this method to resolve subtle differences between cage enantiomers and cage–guest complexes is striking, particularly given that they are too large to enter the pore . Thus, this approach extends the molecular adapter principle beyond established nanopore‐detection strategies in which smaller adapters reside deep within the pore . In common with other nanopore‐based detection methods, the non‐equilibrium (voltage‐driven) nature of the approach facilitates the detection of small molecules at sub‐micromolar concentrations.…”

“…[10] Thus, this approach extends the molecular adapter principle beyond established nanopore-detections trategies in which smallera dapters reside deep within the pore. [13][14][15] In common with other nanopore-based detectionm ethods, the non-equilibrium (voltagedriven)n ature of the approachf acilitates the detection of small molecules at sub-micromolar concentrations. Similarly, direct observation of guest binding within an individual cage grants access not only to thermodynamic data, but also hardto-obtain single-molecule distributions of kinetic data.…”

Cucurbiturilas

“…CB [6] e CB [5] foram utilizados também como ionóforos em membranas de vesículas pelo mesmo grupo e apresentaram permeabilidade seletiva para H + e K + , formando assim estruturas automontadas biomiméticas [116][117][118] . O grupo de Kim desenvolveu uma metodologia de modificação de superfícies de eletrodos utilizando pseudo-rotaxanos de CB [6] com diamino butano funcionalizado com grupos 1,2-ditiolanos terminais em uma de suas extremidades 119,120 .…”

A Universal Strategy for Aptamer‐Based Nanopore Sensing through Host–Guest Interactions inside α‐Hemolysin

“…[10] Thep rinciple that underlies the strategy is shown in Figure 1. [10] Thep rinciple that underlies the strategy is shown in Figure 1.…”