The equilibrium acidity scale (pK_a scale) in acetonitrile has been supplemented by numerous new compounds and new ΔpK_a measurements. It now contains altogether 231 acids – over twice more than published previously – linked by 569 ΔpK_a measurements and spans between the pK_a values of hydrogen iodide (2.8) and indole (32.57), covering close to 30 orders of magnitude. Measurement results acquired over the last 15 years were added to the scale and new least‐squares treatment was carried out. The treatment yielded revised pK_a values for the compounds published previously, with the root mean square difference between revised and previous values 0.04, demonstrating very good stability of the scale. Correlation equations were developed for estimating pK_a values for the studied types of compounds in water, DMSO, DMF, and 1,2‐dichloroethane on the basis of pK_a values in acetonitrile. These equations enable predicting pK_a values with an average error around or less than 1 pK_a unit, which is a sufficient accuracy for many applications. The scale is expected to be a useful tool for the widest possible research areas in organic chemistry, electrochemical power sources, catalysis, etc.

Biomolecular systems are able to respond to their chemical environment through reversible, selective, noncovalent intermolecular interactions. Typically, these interactions induce conformational changes that initiate a signaling cascade, allowing the regulation of biochemical pathways. In this work, we describe an artificial molecular system that mimics this ability to translate selective noncovalent interactions into reversible conformational changes. An achiral but helical foldamer carrying a basic binding site interacts selectively with the most acidic member of a suite of chiral ligands. As a consequence of this noncovalent interaction, a global absolute screw sense preference, detectable by ¹³C NMR, is induced in the foldamer. Addition of base, or acid, to the mixture of ligands competitively modulates their interaction with the binding site, and reversibly switches the foldamer chain between its left and right-handed conformations. As a result, the foldamer–ligand mixture behaves as a biomimetic chemical system with emergent properties, functioning as a “proton-counting” molecular device capable of providing a tunable, pH-dependent conformational response to its environment.