Purpose: To determine the different conformations of the acyloxyalkoxy-linked cyclic prodrug 1 of the model hexapeptide 2 in solution and to investigate the relationship between these solution conformations and the cellular permeability characteristics of this prodrug.
Methods: Two-dimensional Homonuclear Hartmann-Hahn spectroscopy, Rotating-Frame Overhouser effect spectroscopy, circular dichroism and molecular dynamics simulations were used to find the solution conformers of cyclic prodrug 1.
Results: Our spectroscopic findings suggest that cyclic prodrug 1 exhibits a major and a minor conformer in solution. The major conformer appears to have a well-defined secondary structure, which involves a beta-turn and 4-->1 intramolecular hydrogen bond, creating a compact structure with a reduced average hydrodynamic radius compared to the model hexapeptide 2.
Conclusions: The increased ability of cyclic prodrug 1 to permeate membranes compared to the model hexapeptide 2 could be due to reduction in the average hydrodynamic radius of the molecule facilitating paracellular flux and/or the reduction in the hydrogen bonding potential facilitating transcellular flux.