Selective complexation of divalent cations by a cyclic α,β-peptoid hexamer: a spectroscopic and computational study

Area of Science:

• Supramolecular Chemistry
• Coordination Chemistry
• Peptoid Chemistry

Background:

• Cyclic peptoids are emerging as versatile scaffolds for molecular recognition.
• Understanding cation complexation is crucial for developing selective binding agents.
• Previous studies focused on smaller cyclic peptoids and their cation interactions.

Purpose of the Study:

• To investigate the cation complexation properties of a 21-membered cyclic peptoid hexamer (cP1) with chiral (S)-phenylethyl side chains.
• To determine the selectivity of cP1 towards monovalent and divalent cations.
• To elucidate the coordination modes and structural aspects of the formed complexes.

Main Methods:

• Qualitative and quantitative analysis of cation binding.
• Spectroscopic techniques: 1H NMR, circular dichroism, and fluorescence.
• Computational methods: Molecular modeling.

Main Results:

• The cyclic peptoid cP1 did not complex monovalent cations (Na+, K+, Ag+).
• cP1 demonstrated selectivity for divalent cations (Ca2+, Ba2+, Sr2+, Mg2+).
• Hexacoordinated complexes were observed for Ca2+ and Ba2+ (ionic radii ~1 Å), while 5-coordination was preferred for Mg2+ and larger Ba2+.

Conclusions:

• The cyclic peptoid cP1 exhibits distinct selectivity for divalent cations over monovalent ones.
• The coordination number and symmetry of the complexes are influenced by the ionic radii of the complexed cations.
• This research provides valuable insights into the design of peptoid-based receptors for specific ion recognition.