On the use of the MNDO-d semiempirical method for the structural study of chlorophyll b and anhydrous chlorophyll b dimer M. Nsangou a, * , A. Ben Fredj b , N. Jaidane b , M.G. Kwato Njock a , Z. Ben Lakhdar b a CEPAMOQ, Faculte ´ des Sciences, Universite ´ de Douala, B.P. 8580 Douala, Cameroun b Laboratoire de Spectroscopie Atomique, Mole ´culaire et Applications Faculte ´ des Sciences de Tunis, 1060 Tunis, Tunisie Received 16 December 2004; revised 10 March 2005; accepted 11 March 2005 Abstract Semiempirical MNDO-d molecular orbital calculations have been used for the structural study of chlorophyll b and anhydrous chlorophyll b dimer systems in their respective ground states. A model of chlorophyll b dimer has been considered whereby the magnesium atom is five- coordinate with an oxygen atom in the fifth coordination site. Total energies, net electric charges, and electronic dipole moments have been determined for the ground states of the above-mentioned molecules. The proposed models allow for the examination of distortions within the pyrroles and porphyrin macrocycles of these magnesium complexes. They also permit the estimate of changes in bond lengths, bond angles and dihedrals produced by dimerization via the acetyl oxygen of the carbometoxy group and the central magnesium atom, and changes in the p-electronic system of the chlorin systems. The results obtained are in good agreement with the experimental findings, and they reproduce many of the observed trends. In particular, neutron diffraction crystal structure, NMR and infrared spectra, provide experimental evidence in support of these theoretical predictions. Comparison with other theoretical results, shows that there is a good correlation between the DFT/B3LYP(6-31G*) and the MNDO-d, and also that the MNDO-d performs better than the HF/6-31G* and the PM5 in general. q 2005 Elsevier B.V. All rights reserved. Keywords: Chlorophyll b; Chlorophyll b dimer; Structure; MNDO-d; PM5; HF; DFT/B3LYP; 6-31G* 1. Introduction Chlorophyll b (chl.b) occurs as an accessory pigment of light harvesting systems in higher plants, green algae, Euglenaceae and Prochlorophyta, and comprises up to 30% of the total chlorophylls [1,2]. In these plants, chlorophylls are present in the form of chlorophyll–protein complex. Fortunately, it has been shown by Serlin et al. [3] and by Thornber [4] that these complexes contain three chl.b and three chl.a molecules per protein subunit. For years, many attempts of structural studies of several chlorophylls and chlorophyll derivatives have been under- taken using various techniques: circular dichroism and magnetic circular dichroism [5–7], nuclear magnetic reson- ance (NMR) [8–11], Infrared spectroscopy [12], vibrational spectroscopy [13], MNDO-d [14], PM5 [15] and PM3 [16] semiempirical methods, ab initio and density functional theory (DFT) [15]. All these studies showed chl.b and chl.a to exist in strongly solvent dependent monomer–dimer–oligomer equi- librium. The above-cited works also indicate a marked flexibility of the macrocycle upon substitution of the central Mg by Ni, Zn, Fe or VaO. Moreover, infrared [12] and NMR [17] studies have shown that magnesium was definitely involved in these phenomena. It now appears well established that magnesium in chlorophyll with coordination number of four is coordinately unsaturated. This coordination unsatura- tion is responsible for the chlorophyll–nucleophile interaction, and in the absence of extraneous nucleophiles, to self- interactions [9] to form dimers, using their nucleophilic functional groups. Molecular aggregation of chlorophylls and chlorophyll derivatives have been a subject of intense studies in several laboratories [18–21]. The presence of the C 26 keto and C 30 carbometoxy carbonyls in ring V and of the C 41 propionic ester carbonyl in ring IV of the chl.b molecule gives rise to many self interactions amongst chl.b molecules (see Fig. 1 for rings numbering and atoms labelling). The chl.b, as well as the chl.a, Journal of Molecular Structure: THEOCHEM 726 (2005) 245–251 www.elsevier.com/locate/theochem 0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2005.03.048 * Corresponding author. Address: Faculte ´ des Sciences, Universite ´ de Ngaounde ´re ´ B.P. 454 Ngaounde ´re ´, Cameroun. Tel.: C237 7645210. E-mail address: mnsangou@yahoo.com (M. Nsangou).