We analyze the conditions of the adsorption of a flexible peptide onto a charged substrate in the 'wrong side' of the isoelectric point (WSIP), i.e. when surface and peptide charges have the same sign. As a model system, we focus on the casein macropeptide (CMP), both in the aglycosylated (aCMP) and fully glycosydated (gCMP) forms. We model the substrate as a uniformly charged plane while CMP is treated as a bead-and-spring model including electrostatic interactions, excluded volume effects and acid/base equilibria. Adsorption coverage, aminoacid charges and concentration profiles are computed by means of Monte Carlo simulations at fixed pH and salt concentration. We conclude that for different reasons the CMP can be adsorbed to both positively and negatively charged surfaces in the WSIP. For negatively charged surfaces, WSIP adsorption is due to the patchy distribution of charges: the peptide is attached to the surface by the positively charged end of the chain, while the repulsion of the surface for the negatively charged tail is screened by the small ions of the added salt. This effect increases with salt concentration. Conversely, a positively charged substrate induces strong charge regulation of the peptide: the acidic groups are deprotonated, and the peptide becomes negatively charged. This effect is stronger at low salt concentrations and it is more intense for gCMP than for aCMP, due to the presence of the additional sialic groups in gCMP.

Department of Chemistry University of Lleida Av Alcalde Rovira Roure 191 E 25198 Lleida Catalonia Spain

Department of Material Science and Physical Chemistry and Institute of Theoretical and Computational Chemistry University of Barcelona C Martí i Franquès 1 08028 Barcelona Catalonia Spain

Department of Physical and Macromolecular Chemistry Faculty of Science Charles University Hlavova 8 128 00 Prague 2 Czech Republic

Universidad de Buenos Aires and Instituto de Nanobiotecnología Facultad de Farmacia y Bioquímica Buenos Aires Argentina

Universidad Tecnología Nacional and Grupo Bionanotecnología y Sistemas Complejos Facultad Regional San Rafael Av General Urquiza 314C P 5600 San Rafael Mendoza Argentina

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