An NMR study of conformations of substituted dipeptides in dodecylphosphocholine micelles: Implications for drug transport

: Efficient transport of intact drug (solute) across the intestinal epithelium is typically a requirement for good oral activity. In general, the membrane permeability of a solute is a complex function of its size, lipophilicity, hydrogen bond potential, charge, and conformation. In conjunction with theoretical/computational and in vitro drug transport studies, seven dipeptide (R(1)-D-Xaa-D-Phe-NHMe) homologues were each dissolved in a micellar d(38)-dodecylphosphocholine solvent system. In this homologous dipeptide series, factors such as size, lipophilicity, hydrogen-bond potential, and charge were either tightly controlled or well-characterized by other methods in order to investigate by nmr how conformational factors relate to transport. Nuclear Overhauser effect spectroscopy experiments and amide-NH-H(2)O chemical exchange rates showed that the five more lipophilic dipeptides were predominately associated with micelle, whereas the two less lipophilic analogues were not. Rotating frame nuclear Overhauser effect spectroscopy derived interproton distance restraints for each analogue, along with (3)J(HH)-derived dihedral restraints, were used in molecular dynamics/simulated annealing computations. Our results suggest that-other factors being equal-flexible dipeptides having a propensity to fold together nonpolar N- and C-terminal moieties allow greater segregation of polar and nonpolar domains and may possess enhanced transport characteristics. Dipeptides that were less flexible or that retained a less amphiphilic conformation did not have comparably enhanced transport characteristics. We suggest that these conformational/transport correlations may hold true for small, highly functionalized solutes (drugs) in general.