Adsorption of flexible proteins in the ?wrong side? of the isoelectric point: Casein macropeptide as a model system

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.

Automated summary

This study analyzes the conditions for the adsorption of a flexible peptide onto a charged substrate at the 'wrong side' of the isoelectric point. The results show that the peptide can be adsorbed to both positively and negatively charged surfaces in this condition. For negatively charged surfaces, the peptide is attached to the surface by the positively charged end of the chain, while the repulsion between the surface and the negatively charged tail is screened by the small ions in the added salt. On the other hand, a positively charged substrate leads to charge regulation of the peptide, where the acidic groups become deprotonated and the peptide becomes negatively charged. This effect is more pronounced at low salt concentrations and is stronger for the fully glycosylated form of the peptide.