An RNA aptamer possessing a novel monovalent cation-mediated fold inhibits lysozyme catalysis by inhibiting the binding of long natural substrates

Ion uptake and displacement in the aptamer–lysozyme complex. In the dual log plot of K_A (affinity constant) vs. Na⁺ concentration a positive slope of 0.28 is measured at concentrations of Na⁺ below 0.1 M, indicating a net monovalent cation uptake. However, at [Na⁺] above 0.1 M, the expected negative slope demonstrates a monovalent cation displacement upon complex formation. The slope of −1.43 is surprisingly small resulting in an abscissa intercept of log 6.3.