Molecular dynamics simulations of Ac-3Aib-Cage-3Aib-NHMe

Solvents play a stabilising role with the more stable conformations obtained in polar solvents than in vacuo. We investigate to what extent the structural propensities of the pentacyclo-undecane (PCU) cage polypeptide chain of the type Ac-3Aib-Cage-3Aib-NHMe are influenced in implicit water and in explicit solvents: methanol (MEOH), dimethyl sulphoxide (DMSO) and TIP3P water. The sampling of the α-helical conformations of the PCU cage polypeptide was investigated using the in-house modified PARM94 force-field parameters. Analysis of 50 ns molecular dynamics (MD) simulations revealed a tendency of the PCU cage polypeptide to assume bent structures, especially in polar solvents. The choice of solvents was designed to relate the simulations to physiological conditions. The individual amino-isobutyric acid residues predominantly sampled the right-handed and left-handed 3₁₀-helical conformations, indicating that the helical conformations are preferred in all four environments (in vacuo, MEOH, water and DMSO). Additionally, the 100 ns replica exchange MD (REMD) simulations of the PCU cage polypeptide in implicit water revealed more conformational variety present than in explicit solvents, and is more consistent with previous theoretical studies on the PCU cage residue. The present theoretical results may help in rationalising experimental results on these PCU cage polypeptides, and definitely show the importance of a dynamical approach for a correct interpretation and prediction of the conformational behaviour of the PCU cage molecules in different environments.