Theoretical Study of the Dimerization of Chlorophyll (a) and Its Hydrates: Implication for Chlorophyll (a) Aggregation

Abstract

Dimeric structures chlorophyll (a) (Chla) and their mono‐ and dihydrated have been suggested to play an important role in the mechanism of photoreaction center chlorophyll special pairs PSI and PSII. Despite their functional importance, the molecular basis structures for interacting two Chla molecules and the structural stabilization role of H2O in the formation of hydrated Chla dimer complexes is poorly understood. In this article, the different coordination modes between two interacting Chla molecules and the configurational (orientation and distance) features between the dimer and bound H2O molecules are characterized by means of super molecule approach the density functional theory DFT. An estimation of the thermodynamic quantities is made for Chla dimerization and hydration processes. The results indicate that structure including ester linkages via H2O hydrogen bonding is the most favorable conformation for the dihydrated chlorophyll (a) dimer at B3LYP/6‐31G*‐DCP level of calculation. The dispersion interaction is shown to be of great significance for the Chla dimer stabilization. In aqueous nonpolar solvent, the thermodynamics show that Chla has a slightly stronger driving force for full hydration than for dimerization and that hydration of the dimers is rather weakly exergonic. The tetrahydrated dimers having a similar arrangement to that in crystals of ethyl chlorophyllide (a) dihydrate are found to be more stable than the Chla dihydrated dimer. The data underscore the key role of H‐bonding in the stability of Chla‐H2O adducts and, in particular, the great importance of the Chla monomeric dihydrated species in the hydration and dimerization of Chla in aqueous media. Clearly, the Chla dihydrates (Chla‐2 H2O) are found more stable than the monohydrates (Chla‐H2O) and the Chla dimers (Chla2), owing to a particular structure in which cooperative interactions occur between the H2O molecules and Chla. Calculations also indicate that the most thermodynamically preferred pathway for the formation of Chla dimer hydrates can be represented by two steps: the first corresponds to the formation of Chla monomeric dihydrates and the second is the dimerization of the dihydrates on to tetrahydrated Chla dimers. These results allow to obtain a new possible pathway for Chla dimer formation processes and could provide new insights to the aggregation of chlorophyll (a) in solution.

The Ability of Chlorophyll to Trap Carcinogen Aflatoxin B1: A Theoretical Approach

“…It has been reported that the Mg–N bond can be elongated to 0.02–0.04 Å by neutral ligands [ 45 ]. In this work, the resulting distance order for all complexes was N1 > N3 > N2 > N4, in agreement with previous reports [ 48 , 49 , 50 ]. The Mg 2+ –AFB 1 complex caused small but noticeable changes in the geometry of the porphyrin moiety of chl a 1 due to a change in the coordination number (4 to 5) of the Mg 2+ ion.…”

“…Moreover, in the higher-energy chl a 2 –AFB 1 complex, the Mg–N interatomic distances are longer than in uncoordinated chl a 2 ( Table 1 ) due to the saturated bond on the N1 ring, as previously reported (0.02 to 0.04 Å) [ 45 ]. In this work, the resulting distance order for all complexes was N1 > N3 > N2 > N4, in agreement with previous reports [ 48 , 49 , 50 ]. Additionally, the Mg 2+ –AFB 1 complex caused small but noticeable changes in the geometry of the porphyrin moiety of chl a 2 -α-D–AFB 1 , chl a 2 -α-E–AFB 1 , and chl a 2 -β-E–AFB 1 , which was due to a change in the coordination number (four to five) of the Mg 2+ ion.…”

“…In general, the Mg 2+ ion coordinated with the nitrogen atoms yielded a square-based pyramid geometry with the Mg 2+ located out-of-plane due to the coordination number (5), in agreement with literature data [ 35 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 ]. A displacement of the Mg 2+ ion toward the axial ligand was perceived from 0.57 to 0.68 Å; according to Heimdal et al [ 45 ] and Zucchelli et al, this displacement was 0.43–0.54 Å [ 52 ].…”

“…As in the case of chl a 1 , comparable changes in configuration for the substituents were perceived, and for chl a 2 -β-D–AFB 1 , a change was observed in the methyl substituent on N3. As previously shown for chl a 1 , the complexes also yielded a square-based pyramid geometry [ 35 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 ]. A displacement of the Mg 2+ ion toward the axial ligand was perceived from 0.57 to 0.68 Å [ 45 ]; according to Zucchelli et al, this displacement was 0.43–0.54 Å [ 52 ].…”

“…This is shown in Table 1 . Moreover, the previous order established for the distances of Mg–N, N1 > N3 > N2 > N4, is also maintained in these complexes [ 48 , 49 , 50 ]. Consequently, the Mg 2+ –2AFB 1 complex achieved small but noticeable changes in the configuration of the vinylic group on N2.…”