Fig. 4. (A) Distance between the –COO⁻ and –NH₃⁺ groups of l-Trp and l-DOPA. (B) Distance between the –COO⁻ and –NH₃⁺ groups of l-Trp and d-DOPA. Energy-minimized dominant interaction models of (C) l-Trp with l-DOPA and (D) l-Trp with d-DOPA, shown as a ball-and-stick model.