Despite progress, a fundamental understanding of the relationships between the molecular structure and self-assembly configuration of Fmoc-dipeptides is still in its infancy. In this work, we provide a combined experimental and computational approach that makes use of free energy equilibration of a number of related Fmoc-dipeptides to arrive at an atomistic model of Fmoc-threonine-phenylalanine-amide (Fmoc-TF-NH) which forms twisted fibres. By using dynamic peptide libraries where closely related dipeptide sequences are dynamically exchanged to eventually favour the formation of the thermodynamically most stable configuration, the relative importance of C-terminus modifications (amide versus methyl ester) and contributions of aliphatic versus aromatic amino acids (phenylalanine F vs. leucine L) is determined (F > L and NH > OMe). The approach enables a comparative interpretation of spectroscopic data, which can then be used to aid the construction of the atomistic model of the most stable structure (Fmoc-TF-NH). The comparison of the relative stabilities of the models using molecular dynamic simulations and the correlation with experimental data using dynamic peptide libraries and a range of spectroscopy methods (FTIR, CD, fluorescence) allow for the determination of the nanostructure with atomistic resolution. The final model obtained through this process is able to reproduce the experimentally observed formation of intertwining fibres for Fmoc-TF-NH, providing information of the interactions involved in the hierarchical supramolecular self-assembly. The developed methodology and approach should be of general use for the characterization of supramolecular structures.

An idealized model amphipathic alpha-helical decapeptide was synthesized and tested for efficacy as a totally synthetic lung surfactant in simple mixtures with dipalmitoylphosphatidylcholine (DPPC). Quasi-static lung compliance was restored to 92 +/- 3% of the unlavaged value at a pressure of 5 cm H2O in an in vitro lavaged rat lung model. A sustained improvement in gas exchange was also observed when guinea pigs were treated with the synthetic lung surfactant in an in vivo lavaged lung model. DPPC/peptide mixtures rapidly formed low surface tension films in the pulsating bubble surfactometer consistent with a mechanism in which the lipid and peptide mixture spreads rapidly in the lavaged lung to minimize the surface tension at the air/tissue interface. This decapeptide sequence is active in mixtures with DPPC whether the residues are in the all L or all D conformation. However, a peptide with identical sequence, but with alternating D and L amino acid residues, is relatively inactive. Positive charge interactions are not important since a peptide with formylated lysine residues is active. The activity of these decapeptides, with sequences unrelated to any of those in natural lung surfactants, shows that the classic amphipathic alpha-helical hypothesis may be useful in designing peptides that will be effective synthetic lung surfactants in binary mixtures with DPPC. The data demonstrate that a small water-soluble synthetic peptide containing an amphipathic alpha-helical structure combined solely with the major lipid of natural lung surfactant is effective in restoring lung compliance and gas exchange in surfactant-deficient lungs and may be useful in treatment of the respiratory distress syndromes.