Steric Control of Bacteriochlorophyll Ligation Agnieszka Kania ² and Leszek Fiedor* ,‡ Contribution from the Faculty of Chemistry, Jagiellonian UniVersity, Ingardena 3, 30-060 Cracow, Poland, and Faculty of Biotechnology, Jagiellonian UniVersity, Gronostajowa 7, 30-387 Cracow, Poland Received August 23, 2005; E-mail: lfiedor@mol.uj.edu.pl Abstract: 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, ΔH°, ∆S°, and ∆G° of binding various types of ligands (acetone, dimethylformamide, imidazole, and pyridine) to diastereoisomeric bacteriochlo- rophylls were derived from respective van’t Hoff’s plots. At ambient temperatures, only ligation by imidazole and pyridine occurs spontaneously while ∆G° becomes positive for ligation by acetone and dimethylfor- mamide, 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. ∆S° and ∆H° 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 ∆S° values, regardless of the ligand type, while the difference in ∆H° 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. The coordination of ligands to Mg 2+ ions, essential in many biological systems, can be intuitively understood in terms of interactions between hard/soft acids and hard/soft bases (HSAB model 1 ). For instance, the preferential binding of water and other oxygen-containing ligands (amides, ketones, ethers, and alco- hols, etc.) to Mg 2+ is explained as the reaction of hard bases with a hard center. 2,3 This description, though only capturing qualitative characteristics, is commonly used because it seems difficult to obtain quantitative information about the coordination interactions of the Mg 2+ ion. 2-6 These weak interactions are difficult to monitor spectroscopically because Mg is a light element with a simple electronic configuration and Mg 2+ complexes are often kinetically too labile. 1,7 Chlorophylls (Chls), the chief photosynthetic pigments, comprise a class of very important biological ligands of divalent Mg ion, which together form kinetically stable complexes. 8 With the incorporation of this metal center, Chls gain new coordi- native properties because out of six binding positions in the first coordination sphere of Mg 2+ ion only four are satisfied in Mg-tetrapyrrole complexes. The presence of two coordinatively unsaturated sites in axial positions determines Chl interactions with the environment (polypeptides and solvents) and thus is of great structural importance for virtually all photosynthetic Chl-proteins. This is why the axial ligation of Chls has long been a subject of extensive studies, 9-16 but surprisingly, little thermodynamic data on chlorophyll-ligand interactions can be ² Faculty of Chemistry. ‡ Faculty of Biotechnology. (1) Douglas, B.; McDaniel, D.; Alexander, J. Concepts and models of inorganic chemistry, 3rd ed.; Wiley & Sons: New York, 1994. (2) Bock, C. W.; Katz, A. K.; Markham, G. D.; Glusker, J. P. J. Am. Chem. Soc. 1999, 121, 7360-7372. (3) Dudev, T.; Cowan, J. A.; Lim, C. J. Am. Chem. Soc. 1999, 121, 7665- 7673. (4) Bock, C. W.; Katz, A. K.; Glusker, J. P. J. Am. Chem. Soc. 1995, 117, 3754-3765. (5) Peschke, M.; Blades, A. T.; Kebarle, P. J. Am. Chem. Soc. 2000, 122, 10440-10449. (6) Walker, N.; Dobson, M. P.; Wright, R. R.; Barran, P. E.; Murrell, J. N.; Stace, A. J. J. Am. Chem. Soc. 2000, 122, 11138-11145. (7) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.; Wiley & Sons: New York, 1988. (8) Scheer, H. Chlorophylls; CRC Press: Boca Raton, FL, 1991. (9) Vernon, L. P.; Seely, G. R. The Chlorophylls; Academic Press: New York, 1966. (10) Katz, J. J.; Strain, H. H.; Leussing, D. L.; Dougherty, R. C. J. Am. Chem. Soc. 1968, 90, 784-791. (11) Evans, T. A.; Katz, J. J. Biochim. Biophys. Acta 1975, 396, 414-426. (12) Cotton, T. M.; Loach, P. A.; Katz, J. J.; Ballschmiter, K. Photochem. Photobiol. 1978, 27, 735-749. (13) Cotton, T. M.; Van Duyne, R. P. J. Am. Chem. Soc. 1981, 103, 6020- 6026. (14) Clarke, R. H.; Hotchandani, S.; Jagannathan, S. P.; Leblanc, R. M. Chem. Phys. Lett. 1982, 89, 37-40. (15) Brereton, R. G.; Sanders, J. K. M. J. Chem. Soc., Perkin Trans. 1 1983, 423-430. (16) Krawczyk, S. Biochim. Biophys. Acta 1989, 976, 140-149. Published on Web 12/16/2005 454 9 J. AM. CHEM. SOC. 2006, 128, 454-458 10.1021/ja055537x CCC: $33.50 © 2006 American Chemical Society