Extracavity Effect in Cyclodextrin/Surfactant Complexation

Cyclodextrin (CD) complexation is a convenient method to sequester surfactants in a controllable way, for example, during membrane-protein reconstitution. Interestingly, the equilibrium stability of CD/surfactant inclusion complexes increases with the length of the nonpolar surfactant chain even beyond the point where all hydrophobic contacts within the canonical CD cavity are saturated. To rationalize this observation, we have dissected the inclusion complexation equilibria of a structurally well-defined CD, that is, heptakis(2,6-di- O-methyl)-β-CD (DIMEB), and a homologous series of surfactants, namely, n-alkyl- N, N-dimethyl-3-ammonio-1-propanesulfonates (SB3- x) with chain lengths ranging from x = 8 to 14. Thermodynamic parameters obtained by isothermal titration calorimetry and structural insights derived from nuclear magnetic resonance spectroscopy and molecular dynamics simulations revealed that, upon inclusion, long-chain surfactants with x = ≥10 extend beyond the canonical CD cavity. This enables the formation of hydrophobic contacts between long surfactant chains and the extracavity parts of DIMEB, which make additional favorable contributions to the stability of the inclusion complex. These results explain the finding that the stability of CD/surfactant inclusion complexes monotonously increases with the surfactant chain length even for long chains that completely fill the canonical CD cavity.