The possible effect of plastic nanoparticles of waste origin on biological systems is still unclear, and could pose a severe threat. Model studies on the molecular level are urgently needed in order to help revealing interplay between these particles biological systems, and thereby to indicate the direction further research. In the present study, simulated annealing molecular dynamics was adjusted and applied to generate an array of conformations for a sample peptide oligoalanine possibly binding to polyethylene and nylon 6,6 nanoplastics. The resulting structures, with a diameter up to 5 nm, were investigated with the aid of static quantum chemical calculations. The obtained data unequivocally show that both plastic nanoparticles influence the relative stability of α-helix, β-hairpin and other conformations strongly. The polyethylene nanoparticle increases the stability of the helical foldamer. The nylon 6,6 nanoplastic offers strong plastic-peptide interactions at its surface, which make the unfolding of the peptide thermodynamically highly favorable. These results further underscore that nanoplastics can do significant, molecular level damage to living organisms via facilitating the misfolding and denaturation of proteins. Furthermore, it is apparent that plastics can have very different effects on living matter depending on their composition, hence experiments with any single kind of plastics (e.g. polystyrene) should not be considered generally valid for all nanoplastics.