Effect of Restricted Conformational Flexibility on the Permeation of Model Hexapeptides Across Caco-2 Cell Monolayers

Abstract

Purpose. To determine how restricted conformational flexibility of hexapeptides influences their cellular permeation characteristics.

Methods. Linear (Ac-Trp-Ala-Gly-Gly-X-Ala-NH₂; X = Asp, Asn, Lys) and cyclic (cyclo[Trp-Ala-Gly-Gly-X-Ala]; X = Asp, Asn, Lys) hexapeptides were synthesized, and their transport characteristics were assessed using the Caco-2 cell culture model. The lipophilicities of the hexapeptides were determined using an immobilized artificial membrane. Diffusion coefficients used to calculate molecular radii were determined by NMR. Two-dimensional NMR spectroscopy, circular dichroism, and molecular dynamic simulations were used to elucidate the most favorable solution structure of the cyclic Asp-containing peptide.

Results. The cyclic hexapeptides used in this study were 2−3 times more able to permeate (e.g., P_app = 9.3 ± 0.3 × 10⁻⁸ cm/sec, X = Asp) the Caco-2 cell monolayer than were their linear analogs (e.g., P_app = 3.2 ± 0.3 × 10₋₈ cm/sec, X = Asp). In contrast to the linear hexapeptides, the flux of the cyclic hexapeptides was independent of charge. The cyclic hexapeptides were shown to be more lipophilic than the linear hexapeptides as determined by their retention times on an immobilized phospholipid column. Determination of molecular radii by two different techniques suggests little or no difference in size between the linear and cyclic hexapeptides. Spectroscopic data indicate that the Asp-containing linear hexapeptide exists in a dynamic equilibrium between random coil and β-turn structures while the cyclic Asp-containing hexapeptide exists in a well-defined compact amphophilic structure containing two β-turns.

Conclusions. Cyclization of the linear hexapeptides increased their lipophilicities. The increased permeation characteristics of the cyclic hexapeptides as compared to their linear analogs appears to be due to an increase in their flux via the transcellular route because of these increased lipophilicities. Structural analyses of the cyclic Asp-containing hexapeptide suggest that its well-defined solution structure and, specifically the existence of two β-turns, explain its greater lipophilicity.