Structural isomers and tautomerism of chlorophyll a in the ground state studied by semiempirical MNDO-d method M. Nsangou a, * , A. Ben Fredj b , N. Jaı ¨dane b , M.G. Kwato Njock a , Z. Ben Lakhdar b a Centre de Physique Atomique Mole ´culaire et Optique Quantique, Faculte ´ des Sciences, Universite ´ de Douala, B.P. Douala 8580, Cameroon b Laboratoire de Spectroscopie Atomique Mole ´culaire et Applications, Faculte ´ des Sciences de Tunis, Universite ´ Tunis El Manar, Campus Universitaire, Tunis 1060, Tunisia Received 23 April 2004; revised 10 May 2004; accepted 24 May 2004 Abstract The structural isomers and proton transfer phenomena of the chlorophyll a system in the ground state has been studied using semiempirical MNDO-d molecular orbital calculations. The preferred conformations, total energies, charge densities, electronic dipole moments and bond orders have been determined for the keto, enol tautomers and the ‘a prime’ (27-S-epimeric isomer) of chlorophyll a. Models constructed allow examination of distortions in the pyrroles and porphyrin macrocycles of these magnesium complexes. They permit estimation of changes in bond lengths and angles produced by changes in hybridization, those produced by an inversion of the asymmetry of the carbometoxy group, changes in the p-electronic system of the chlorin systems and the distortions produced by closure of the five-membered isocyclic ring when either epimerization or enolization takes place. The results obtained are in reasonable agreement with the experimental findings and reproduce many of the observed tends. The Mg complex was more stable in the keto form where the distortions are lesser in magnitude and the conjugative effects are stronger. The structure of the transition state for the interconversion process between the enol and the keto forms was found highly strained and lead to high activation energy (90 kcal mol 21 ). This result indicates no evidence for fast proton transfer from the enol to the keto form, which is in quite agreement with the experimental results. A survey of several neutron diffraction crystal structure, NMR and infrared analysis provides experimental evidence in qualitative support of these theoretical predictions. q 2004 Elsevier B.V. All rights reserved. Keywords: Chlorophyll a; Structure; Tautomerism; Epimerization; MNDO-d 1. Introduction Chlorophyll a (chl.a) is the principal photosynthetic pigment in algae and higher plants. In the process of photosynthesis, this pigment serves as a collector of photoexcitation as well as an electron donor in the primary photosynthetic reaction. The intense absorptivity in the visible region is an important factor in light-harvesting by chl.a and it is the major pigment in all chlorophyllous antenna complexes of oxygenic organisms. A knowledge of the molecular structure of the photosynthetic apparatus depends upon the detailed elucidation of the chlorophyll a structure, its distribution and the organization of this chlorophyll-type in the cell. As a basis for such an elucidation, many structural studies using different techniques have been performed on chl.a and several of its related pigments. The solution structure, including chlorophyll interactions and side-chain conformations, has mainly been determined by magnetic resonance methods [1–3], circular dichroism [4–6], and vibrational spectroscopy [7–9]. All these studies indicate that the macrocycle shows a marked flexibility upon changes in substitution of the central metal. Microcrystalline chlorophyll has been known for a long time [10], but crystals suitable for X-ray analysis have only been obtained from methylpheophorbide a [11], methyl [12] and ethyl chlorophyllide a dihydrate [13] which confirms the structure of the macrocycle portion of the molecule. No crystal structure of the phytylated pigment has been obtained to date and a basis of detailed structural parameters characterizing this chlorophyll-type is still lacking. 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.05.023 Journal of Molecular Structure (Theochem) 681 (2004) 213–224 www.elsevier.com/locate/theochem * Corresponding author. Address: Faculte ´ des Sciences, Universite ´ de Ngaounde ´re ´, B.P. 454 Ngaounde ´re ´—Cameroun. E-mail addresses: mnsangou@yahoo.com (M. Nsangou), erijbf@ yahoo.fr (A. Ben Fredj), zohra.lakhdar@fst.rnu.tn (Z. Ben Lakhdar).