Structure and properties of metalloporphyrins and hemoproteins: the vibronic approach

Pseudo Jahn-Teller origin of instability of planar configurations of hexa-heterocycles. Application to compounds with 1,2- and 1,4-C4X2 skeletons (X = O, S, Se, Te)

“…There are two ways to suppress the PJTE instability in order to restore the high-symmetry configuration [20,21]: (1) to increase the energy gap between the ground state and the active excited state that produces the instability of the ground states, and (2) to populate that excited state with electrons. For example of the realization of the mechanism (1), the driven by the PJTE out-of-plane displacement of the iron atom from the porphyrin ring in hemoglobin was shown to return into the plane by oxygenation due to the increase of the energy gap to the active excited state, thus suppressing the PJTE [22]. Examples of realization of the mechanism (2) were demonstrated recently [25].…”

“…The phenomenon of puckering as due to the PJTE, although revealed before [22], was first formulated in a generalized way in Ref. [23], and then employed to explain the origin of this structural property of a variety of molecular systems (see, e.g., [20,21,[23][24][25][26][27][28] and references therein).…”

The photochemistry of transition metal complexes using density functional theory

“…In other words, NO and O 2 photoexcitation results in the bending of the Fe−X−O bond, rather then in their dissociation. The distinct geometry for the NO-and O 2 -haem is originated by the occupancy of FeXO π * degenerate orbitals, which cause the bending of the ligand [70]. In the case of O 2 , photolysis experiments suggest that a side-on complex is formed and might contribute to lowering the photodissociation yield in the O 2 -haem [71,72].…”

Subpicosecond oxygen trapping in the heme pocket of the oxygen sensor FixL observed by time-resolved resonance Raman spectroscopy

“…This strongly suggests that the electronic structures of the ground and lowest excited states of the heme are nearly identical and that the excitation would lead to rupture of the heme-oxygen bond within tens of femtoseconds (35) with a similar yield. Also, we note that the efficiency of mixing of the excited state with the Fe-O 2 antibonding orbital is thought to be strongly determined by the bending configuration of the initially bound heme-ligand complex (22,36 Assuming a quantum yields of Fe-O 2 bond dissociation of FixLH close to that of Mb, as discussed above, in principle, on the basis of the observed TR 3 spectra at t Ͼ0.5 ps, two possible scenarios can be considered to explain the FixLH photoproduct heme structure: (i) The oxygen-heme iron bond is broken and five-coordinate heme is formed, but the heme structure remains planar and there is no transition from low-to high-spin iron. (ii) The oxygen-heme iron bond is broken but the oxygen molecule is forced to stay in place by the environmental constraints and rebinds rapidly, preventing the creation of the deoxy-like domed heme structure.…”