Electron transfer through a prenucleated bimetalated alanine-based peptide helix

Area of Science:

• Biophysical Chemistry
• Photochemistry
• Supramolecular Chemistry

Background:

• Peptide structure and function are critical in biological systems.
• Controlling peptide secondary structure, like alpha-helices, is key for designing functional biomolecules.
• Photoactive metal complexes can be incorporated into peptides to create novel photoresponsive systems.

Purpose of the Study:

• To synthesize a bimetalated peptide with photoinducible electron donor and acceptor sites.
• To investigate the effect of cross-linking on peptide helical content.
• To measure photoinduced electron transfer (PET) rates and analyze electronic coupling mechanisms.

Main Methods:

• Synthesis of a 22-residue alanine-based peptide incorporating a tris(bipyridyl)ruthenium(II) donor.
• Cross-linking of histidine residues with a tetraammineruthenium(III) acceptor to stabilize helical structure.
• Circular dichroism (CD) spectroscopy to assess helical content and temperature dependence.
• Fluorescence quenching methods to measure photoinduced electron transfer rates.
• Marcus equation and pathway model analysis to determine electronic coupling matrix elements.

Main Results:

• The synthesized peptide contains a photoinducible electron donor and an electron acceptor site.
• Cross-linking significantly enhanced average helix content from 67% to 84%.
• Residues between donor and acceptor were found to be 92% helical.
• Photoinduced electron transfer rate constants (k(ET)) were measured as 7 x 10^6 s^-1 in H2O and 5 x 10^6 s^-1 in D2O.
• Electronic coupling matrix element (H(ab)) was calculated to be 0.19 cm^-1.

Conclusions:

• The bimetalated peptide exhibits enhanced helical stability due to cross-linking.
• The study successfully measured photoinduced electron transfer between the incorporated donor and acceptor.
• The results support the involvement of hydrogen bonds in mediating electronic coupling within the alpha-helix.