Role of Chirality in Self-Assembly of Amino Acid Tryptophan on Au (111)

Proteins only use single L-enantiomer of amino acids owing to the homochirality in a living system. Interestingly, a small amount of tryptophan (Trp), which is one of the essential amino acids, is required in the proteins. We studied the self-assembly of Trp on Au (111) using scanning tunneling microscopy and density functional theory. When single Trp enantiomers were deposited on Au (111), we confirmed 1D chain structures. The deposition of racemic mixture of two opposite Trp enantiomers also built chain structures, but these structures differed from those formed by single Trp enantiomers. In the self-assembly of two opposite Trp enantiomers, the absence of the single Trp enantiomers-formed chain structures is because the heterochiral configurations were more favorable energetically than the homochiral ones. In the heterochiral chain structures, an enantiomer obstructed the access between amino and carboxyl groups in the opposite enantiomers, resulting in the hindrance of formation of peptide bonding between the opposite enantiomers. From these results, we can estimate that proteins utilize a small amount of Trp to avoid the disturbance of peptide bonding formation. In addition, our study also will contribute to the understanding for origin of homochirality in a living system.