Cooperative and competitive adsorption of amino acids with Ca²⁺ on rutile (α-TiO₂)

The interactions of biomolecules such as amino acids with mineral surfaces in the near-surface environment are an important part of the short and long-term carbon cycles. Amino acid-mineral surface interactions also play an important role in biomineralization, biomedicine, and in assembling the building blocks of life in the prebiotic era. Although the pH effects during adsorption of amino acids onto mineral surfaces have been studied, little is known about the effects of environmentally important divalent cations. In this study, we investigated the adsorption of the oppositely charged amino acids glutamate and lysine with and without the addition of divalent calcium. Without calcium, glutamate shows a maximum in adsorption at a pH of ∼4 and lysine shows a maximum in adsorption at a pH of ∼9.4. In comparison, with calcium present, glutamate showed maxima in adsorption at both low and high pH, whereas lysine showed no adsorption at all. These dramatic effects can be described as cooperative adsorption between glutamate and Ca(2+) and as competitive adsorption between lysine and Ca(2+). The origin of these effects can be attributed to electrostatic phenomena. Adsorption of Ca(2+) at high pH makes the rutile surface more positive, which attracts glutamate and repels lysine. Our results indicate that the interactions of biomolecules with mineral surfaces in the environment will be strongly affected by the major dissolved species in natural waters.