Structure investigation of amphiphilic cyclopeptides in isotropic and anisotropic environments - A model study simulating peptide-membrane interactions

It has been proposed that the membrane allows a much more efficient binding of certain small or medium-sized amphiphilic messenger molecules to their receptor, not only by accumulation of the drug, but also by induction of orientations and conformations that are much more favorable for receptor docking than structures adopted in isotropic phases. A series of eight amphiphilic cyclic peptides containing lipophilic (L-α-aminodecanoic acid = Ada, L-α-aminohexadecanoic acid = Ahd, Nhdg = N-hexadecylglycine) and hydrophilic (Lys, Asp) amino acids were synthesized and examined by means of NMR spectroscopy and molecular dynamics (MD) simulations in isotropic (CDC1₃) and membrane-mimicking anisotropic (SDS/H₂O) solvents to study the influence of the environment on their individual conformations. NMR data of cycle (-Gly¹-D-Asp²-Ahd³-Ahd⁴-Asp⁵-Gly⁶-) (C4), cyclo(-Lys¹-D-Pro²-Lys³-Ada⁴-Pro^-Ada^-) (C5) and cyclo-[-Lys¹-Pro²-Lys³-Ada⁴-D-Pro⁵-Ada⁶-) (C6) clearly indicate that those compounds are too rigid to perform a conformational change upon transition from an isotropic to an anisotropic environment. On the other hand, the experimental data of cycloe (-Gly¹-Asp²-Ahd³ -Ahd⁴-Asp⁵-Gly⁶-) (C1), cycle(-Asp¹-Ala²-Nhdg³-Ala⁴-D-Asp⁵-) (C7), and cyclo(-D-Asp¹-Ala²-Nhdg³-Ala⁴-Asp⁵-) (C8) suggest highly flexible unstructured molecules in both environments. However, for cyclo(-Asp¹-Asp²-Gly³-Ahd⁴-Ahd⁵-Gly⁶-) (C2) we observed a structure inducing effect of a membrane-like environment. The compound populates three different conformations in SDS/H₂O, whereas in CDCl₃ no preferred conformation can be detected. cyclo(-D-Asp¹ -Asp²-Gly³-Ahd⁴-Ahd⁵-Gly⁶-) (C3) clearly exhibits two different conformations with a shifted β,β-turn motif in CDCl₃ and SDS/H₂O solutions. The conformational change could be reproduced in a restraint-free MD simulation using the biphasic membrane mimetic CCl₄/H₂O. Our results give clear evidence that membrane interactions may not only lead to structure inductions, but can also induce major conformational changes in compounds already exhibiting a defined structure in isotropic solution.