Interaction of L-Valine Homopeptides with Fullerene C60

In this theoretical study, we addressed the issue of peptide interactions with fillerene C-60. The peptides of choice were zwitterionic L-valine (Val) homopeptides up to 20 residues-long, in both alpha helix and beta strand conformations, for comparison. The computational methods were the AMBER and MM+ force fields. The values of calculated total energy of peptide + C-60 complexes exhibited considerable variability depending on where exactly the C-60 molecule is positioned in the input geometry. In MM+ calculations, if fullerene was placed close to either the N or the C terminus in the input geometry, the differences in E values were rampant between the neighboring points; however, the energies generally stabilized within 1-2 kcal mol(-1) if the C-60 position turned to be close to the central Val residues of the chain. In AMBER calculations, the behavior of calculated energies was even less homogeneous. To standardize the initial geometry conditions, we placed the fullerene molecule approximately against the center of peptide chain. The shape of the Delta E - n(Val) curves obtained depends on peptide type (alpha helix vs. beta strand) and the force field employed. In MM+ calculations of beta strand Val peptides, Delta E values had a trend to decrease (that is, the absolute values of Delta E increase), coming to a minimum at n(Val) = 7 (Delta E= - 11.71 kcal mol(-1)), then to increase producing a local maximum at n(Val) = 16 (Delta E = -7.79 kcal mol(-1)), and to decrease again until the longest Val(20) studied in this work. The three other cases (MM+ for alpha helix, AMBER for both beta strand and alpha helix) exhibited common features, where the formation energy first decreased then stabilized at about 12-16 kcal mol(-1). The geometry of Val peptides was generally very rigid and stable, and only in a few cases we observed conformational changes, where peptide chains tended to acquire a better contact with fullerene molecule.