High-Pressure and Theoretical Studies Reveal Significant Differences in the Electronic Structure and Bonding of Magnesium, Zinc, and Nickel Ions in Metalloporphyrinoids

Abstract

High pressure in combination with optical spectroscopy was used to gain insights into the interactions between Mg(2+), Zn(2+), and Ni(2+) ions and macrocyclic ligands of porphyrinoid type. In parallel, the central metal ion-macrocycle bonding was investigated using theoretical approaches. The symmetry properties of the orbitals participating in this bonding were analyzed, and pigment geometries and pressure/ligation effects were computed within DFT. Bacteriopheophytin a was applied as both a model chelator and a highly specific spectroscopic probe. The analysis of solvent and pressure effects on the spectral properties of the model Mg(2+), Zn(2+), and Ni(2+) complexes with bacteriopheophytin a shows that various chemical bonds are formed in the central pocket, depending on the valence configuration of the central metal ion. In addition, the character of this bonding depends on symmetry of the macrocyclic system. Since in most cases it is not coordinative bonding, these results challenge the conventional view of metal ion bonding in such complexes. In (labile) complexes with the main group metals, the metal ion-macrocycle interaction is mostly electrostatic. Significantly, water molecules are not preferred as a second axial ligand in such complexes, mainly due to the entropic constraints. The metal ions with a closed d shell may form (stable) complexes with the macrocycle via classical coordination bonds, engaging their p and s orbitals. Transition metals, due to the unfilled d shell, do form much more stable complexes, because of strong bonding via both coordination and covalent interactions. These conclusions are confirmed by DFT computations and theoretical considerations, which altogether provide the basis to propose a consistent and general mechanism of how the central metal ion and its interactions with the core nitrogens govern the physicochemical properties of metalloporphyrinoids.

Intrinsic Photoprotective Mechanisms in Chlorophylls

“…Thefluorescence emission maxima of Phea, Chla, and Zn-Phea, located near 665 nm, show only aw eak solvent effect and relatively small Stokes shifts (! [16,17] TheTD-DFT approach was used to calculate the relevant characteristics of the T 1 state and the electron density distribution on the frontier molecular orbitals (MOs) of the pigments.T he results obtained for Chla compare well to the previously published data, [8,18,19] which confirms the validity of the present approach. [7,14] Also the quantum yield of fluorescence emission (F F )d epends on CMI (Table 1) and the highest values were found for Chla, as in the series of metallosubstituted BChla.…”

Factors controlling the reactivity of divalent metal ions towards pheophytin a

“…The differences in the metalation rates between Zn 2+ and Ni 2+ /Co 2+ /Mn 2+ could be due in part to the fact that the latter cations form a mixed, i.e. coordinative-covalent type of bonds with the core Ns in Pheo a , in contrast to Zn 2+ , bound via purely coordinative bonds [6]. The insertion of these cations requires their valence configuration to be rebuilt along the reaction pathway, which may elevate the energetic barrier for this reaction.…”

“…Such conditions cannot be achieved in a simple in vitro system easily. The high energy demand of this process seems to be related to the fact that Mg 2+ is bound in the central cavity of porphyrinoids not by coordinative bonds but by much weaker electrostatic forces [6]. As a consequence, in the course of Mg 2+ insertion a significant change in its coordination state must take place, from a (most likely) hexacoordinated species to a singly or doubly axially coordinated ion electrostatically bound in the central cavity.…”

“…Often their functioning relies on those photophysical and photochemical features which result from the interactions of the π-electron system with the central metal ion [2–8]. In addition, the formation of metallosubstituted chlorophylls (Chls) and bacteriochlorophylls (BChls) is relevant from the biological point of view [9] and such modified pigments are useful as model compounds [6, 8, 10]. The chemistry of metalloporphyrins has been extensively explored for over a century and the literature on synthetic approaches to their preparation is indeed vast [11–13].…”

“…Recently, we have shown that the reactivity of porphyrinoids in metalation reactions is determined by both the extended aromatic π-electronic system and the peripheral groups, whose interplay creates the energetic barrier to be overcome [29]. Moreover, the type of bonding between the central metal ion and porphyrinoids depends on the structural details of the chelating molecules [6, 8]. Therefore, the effects observed for simple unsubstituted porphyrins cannot be generalized and directly applied to Chls.…”