Analysis of interactions between amino acids and monolayers of charged side chains†

Protein–protein interactions (PPIs) are regulated by multiple interactions among amino acids. However, the contribution of individual amino acid–amino acid interactions (AAIs) in PPIs is currently unclear because it is difficult to analyze the weak and nonspecific interactions among amino acids. Therefore, we constructed a quantitative analytical model to evaluate AAIs using a device with self-assembled monolayers (SAMs). We could evaluate the μM-order dissociation constant between amino acids and the side chain of amino acids based on the electrical response. In the cationic amino acid group, concentration-dependent responses were observed on a negatively charged SAM (3-mercaptopropionic acid). These responses were modulated by the concentration and valence of the competing ions, which indicated that the strength of electrostatic interactions among amino acids is different. In contrast, nonspecific responses to all amino acids used in this study were obtained on a positively charged SAM (2-mercaptoethylamine). These results indicate that the selectivity of interaction depends on the type of side chain in the assembled state. We believe that the analytical platform constructed in this study can be adapted to evaluate various AAIs that govern PPIs.