The effect of anions and pH on amino acids binding affinity to hydroxyapatite nanoparticles

Charged functional groups present on the surface of biomaterials play a crucial role in regulating the affinity and binding of biomolecules, including amino acids. These interactions are pivotal in designing biomaterials with tailored properties for applications ranging from drug delivery systems to tissue engineering constructs. In this study, we investigated the binding affinity of three types of amino acids: phenylalanine (a neutral and nonpolar amino acid), glutamic acid (an acidic and negatively charged amino acid) and lysine (a basic and positively charged amino acid). The influence of Hofmeister anions (i.e. sulfate and bromide ions) to the binding of hydroxyapatite-amino acid nanoparticles is further elucidated. FTIR analysis revealed that the binding affinity of HAP-amino acid nanoparticles exhibited a higher adsorption capacity in ammonium sulfate compared to ammonium bromide due to differences in the dielectric environment. The association constant (K) in ammonium sulfate was calculated to be 8.3 for HAP-Phe, 4.1 for HAP-Glu and 6.1 for HAP- Lys. On the other hand, the association constant values in ammonium bromide were 7.6, 5.9 and 4.1, respectively. The binding interactions of acidic amino acids with hydroxyapatite increased with decreasing pH, while the opposite trend was observed for basic amino acids. The surface structure of the hydroxyapatite nanoparticles was affected by amino acid treatment, introducing porosity into the samples, as evidenced by SEM micro-images. The study demonstrated that the binding affinity of amino acids with different charges onto hydroxyapatite nanoparticles correlates with the surface charges of the biomaterials at different pH values. These findings contribute to the fundamental understanding of biomaterial interactions, with potential inferences for the development of next-generation biocompatible materials.