Interactions of Amino Acids and Polypeptides with Metal Oxide Nanoparticles Probed by Fluorescent Indicator Adsorption and Displacement

The adsorption of polypeptides containing an N-terminal tryptophan (Trp) residue attached to a hexa-backbone of alanine, serine, lysine, histidine, and aspartate was investigated by monitoring the fluorescence response of the Trp chromophore upon titration with metal oxide nanoparticles (MOx-NPs: CuO, Co(3)O(4), TiO(2), MgO, and CeO(2)). After correction for light-scattering effects, a strong static fluorescence quenching was observed upon addition of CuO and Co(3)O(4) to the peptides. The interaction of MOx-NPs with the peptides was assigned to an adsorption of the peptide backbone on the nanoparticle surface. The method was refined using a derivatized amino acid, 5-fluoro-Trp (5F-Trp), which resulted in a stronger fluorescence response. The use of the fluorescent amino acid labels allowed the direct assessment of the adsorption propensities of Trp-containing peptides in dependence on the backbone, which was verified by zeta-potential measurements. Moreover, upon addition of different analytes to nanoparticles with preadsorbed Trp-containing polypeptides, adsorption propensities of the analytes were assessed by an indicator displacement strategy; that is, addition of increasing amounts of analyte resulted in a continuous fluorescence enhancement/recovery. This method afforded adsorption propensities for several analytes. The relative binding constants for the MOx-NPs, obtained from the competitive titrations, varied by more than 6 orders of magnitude for CuO (5F-TrpHis(6)-NH(2) > TrpAsp(6)-NH(2), TrpSer(6)-NH(2) > TrpLys(6)-NH(2), Trp, 5F-Trp > TrpAla(6)-NH(2)) but only 4 for Co(3)O(4) (TrpHis(6)-NH(2), TrpAsp(6)-NH(2) ≫ TrpLys(6)-NH(2), TrpAla(6)-NH(2), TrpSer(6)-NH(2), Trp, 5F-Trp). The study reveals that MOx-NPs adsorb biomolecular analytes with high selectivity, which has immediate implications for their applications in protein purification, drug delivery, and, potentially, for the assessment of their toxicology.