Steric control of bacteriochlorophyll ligation.

The axial coordination of central Mg(2+) ion in chlorophylls is of great structural and functional importance for virtually all photosynthetic chlorophyll proteins; however, little thermodynamic data are available on the ligand binding to these pigments. In the present study, spectral deconvolution of the bacteriochlorophyll Q(X) band serves to determine the ligand binding equilibria and relationships between thermodynamic parameters of ligand binding, ligand properties, and steric interactions occurring within the pigment. On the basis of the temperature effects on coordination, DeltaH degrees, DeltaS degrees, and DeltaG degrees of binding various types of ligands (acetone, dimethylformamide, imidazole, and pyridine) to diastereoisomeric bacteriochlorophylls were derived from respective van't Hoff's plots. At ambient temperatures, only ligation by imidazole and pyridine occurs spontaneously while DeltaG degrees becomes positive for ligation by acetone and dimethylformamide, due to a relatively large entropic effect, which is dominating when the energetic effects of ligation are small. It reflects, in quantitative terms, the control of the equatorial coordination of the Mg(2+) ion via the axial coordination: a "hard" free Mg(2+) ion is made into a softer center through the coordination of tetrapyrrole. Pigment structural features have comparable effects on the energetic and entropic contributions to the difference of ligation free energy between the diastereoisomers of bacteriochlorophyll. DeltaS degrees and DeltaH degrees values are consistently lower for the S epimer, most likely due to the steric crowding between bulky substituents. The two epimers show a 5 J.mol(-1).K(-1) difference in DeltaS degrees values, regardless of the ligand type, while the difference in DeltaH degrees amounts to 1.7 kJ.mol(-1), depending on the ligand. Such steric control of ligation would relate to the partial diastereoselectivity of chlorophyll self-assembly and, in particular, the very high diastereoselectivity of the ligation of chlorophylls in photosynthetic proteins.