Improved methodologies are provided to synthesize (1R,2S)-2-aminocyclobutane-1-carboxylic acid derivatives and their incorporation into beta-peptides of 2-8 residues bearing different N-protecting groups. The conformational analysis of these oligomers has been carried out by using experimental techniques along with theoretical calculations. This study shows that these oligomers adopt preferentially a strand-type conformation in solution induced by the formation of intra-residue six-membered hydrogen-bonded rings, affording cis-fused [4.2.0]octane structural units that confer high rigidity on these beta-peptides. Moreover, all of them are prone to self-assemble producing nano-sized fibres, as evidenced by TEM, AFM and SPFM, and, in some instances, they also form gels. These techniques and molecular modelling allowed us to suggest an aggregation model for the assembly structures in which a parallel molecular-arrangement is preferred and the conformation is similar to that observed in solution. According to this model, both hydrogen-bonding and hydrophobic interactions would account for formation of the assemblies.
Molecular dynamics (MD) simulations are carried out for liquid acetonitrile using a new six-site model for the solvent molecules. The recent force field of Cornell et al. ( J Am Chem Soc, 1995, 117, 5179) was used under the RESP approach to obtain the atomic charges. A new flexible all-atom solvent model was achieved whose density, heat of vaporization, and isothermal compressibility values are in good agreement with available experimental data, especially for a generic force field. Radial distribution functions are calculated and discussed to study the liquid structure in detail.
The alpha-, beta-, and gamma-cyclodextrin (CyDs) dimers were studied by molecular dynamics (MD) simulations in water as an explicit solvent. The relative stability of dimers and the involved molecular interactions were determined. Three possible starting orientations were considered for the dimers: head-to-head, head-to-tail, and tail-to-tail. MD simulations were performed over a period of 5 ns to ensure the stability of the system for both the CyD dimers and monomers. The MM-PBSA methodology was used to obtain the free binding energy of the dimers and to determine the most stable arrangement for each solvated CyD. In a vacuum, MD simulations provided the head-to-head orientation as the most stable orientation for the three CyDs, while in aqueous solution the, the head-to-tail orientation was found to be the most stable for the alpha-CyD dimer and the tail-to-tail orientation the most stable for the beta- and gamma-CyD dimers.
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