Steric control of bacteriochlorophyll ligation

Area of Science:

• Biophysical Chemistry
• Photochemistry
• Structural Biology

Background:

• The axial coordination of magnesium (Mg2+) in chlorophyll is crucial for photosynthesis, yet thermodynamic data on ligand binding remain scarce.
• Understanding these interactions is key to deciphering the structural and functional roles of chlorophylls in photosynthetic systems.

Purpose of the Study:

• To determine ligand binding equilibria and thermodynamic parameters for bacteriochlorophyll.
• To investigate the relationships between thermodynamic data, ligand properties, and steric interactions.
• To quantify the influence of pigment structure on ligation thermodynamics and diastereoselectivity.

Main Methods:

• Spectral deconvolution of the bacteriochlorophyll Q(X) band to analyze ligand binding.
• Temperature-dependent studies to derive enthalpy (ΔH°), entropy (ΔS°), and free energy (ΔG°) of binding using van't Hoff plots.
• Comparison of binding thermodynamics between bacteriochlorophyll diastereomers.

Main Results:

• Ligation by imidazole and pyridine is spontaneous at ambient temperatures, while acetone and dimethylformamide ligation is not, driven by entropic effects.
• Axial coordination modulates the Mg2+ ion's properties, transitioning it from 'hard' to 'softer'.
• Steric crowding in the S epimer leads to consistently lower ΔS° and ΔH° values, demonstrating structural control over ligation thermodynamics.

Conclusions:

• Pigment structural features significantly impact ligation energetics and entropy, influencing binding free energy differences between diastereomers.
• Steric control in ligation is evident, with consistent differences in ΔS° and ΔH° between epimers.
• These findings explain the diastereoselectivity observed in chlorophyll self-assembly and protein ligation during photosynthesis.