Tuning the cavity of cyclodextrins: altered sugar adaptors in protein pores.

Cyclodextrins (CDs) have been widely used in host-guest molecular recognition because of their chiral and hydrophobic cavities. For example, β-cyclodextrin (βCD) lodged as a molecular adaptor in protein pores such as α-hemolysin (αHL) is used for stochastic sensing. Here, we have tuned the cavity and overall size of βCD by replacing a single oxygen atom in its ring skeleton by a disulfide unit in two different configurations to both expand our ability to detect analytes and understand the interactions of βCD with protein pores. The three-dimensional structures of the two stereoisomeric CDs have been determined by the combined application of NMR spectroscopy and molecular simulation and show distorted conformations as compared to natural βCD. The interactions of these synthetic βCD analogues with mutant αHL protein pores and guest molecules were studied by single-channel electrical recording. The dissociation rate constants for both disulfide CDs from the mutant pores show ∼1000-fold increase as compared to those of unaltered βCD, but are ∼10-fold lower than the dissociation rate constants for βCD from wild-type αHL. Both of the skeleton-modified CDs show altered selectivity toward guest molecules. Our approach expands the breadth in sensitivity and diversity of sensing with protein pores and suggests structural parameters useful for CD design, particularly in the creation of asymmetric cavities.