Asp and His-rich model peptides as a basis for elucidating Mn(II) and Fe(II) peptide coordination chemistry

Although Mn(II) and Fe(II) ions are essential for all living organisms, particularly pathogenic bacteria, their coordination preferences with peptide ligands remain insufficiently understood. Building on our previous findings and inspired by naturally occurring Mn(II)- and Fe(II)-binding motifs, we designed six model peptides (L1: Ac-HDHDHDHHH-NH₂, L2: Ac-HDHDHHHHH-NH₂, L3: Ac-HDDHDDHDH-NH₂, L4: Ac-HHDDDDHHHH-NH₂, L5: Ac-HHDDDHHHH-NH₂, and L6: Ac-DDDDDD-NH₂) to explore the fundamental aspects of their metal coordination. Spectrometric, spectroscopic (electron paramagnetic resonance), and pH-potentiometric techniques were employed to determine the thermodynamic and structural properties of the resulting complexes. All studied peptides were found to form chelate complexes with Mn(II) and Fe(II) ions, although the stability of the complexes varies. Even though the protonation states of various species differ all investigated complexes remain stable in pH = 7.4. The thermodynamic stability of these complexes is strongly influenced by the peptide architecture, the number and type of potential binding residues, and the overall charge of the system. These findings provide new insights into the coordination behavior of Mn(II) and Fe(II) with histidine- and aspartic acid-rich sequences, contributing to a deeper understanding of metal ion binding in biological systems.