Reorganization criteria and their effects on inner-sphere barriers for transition metal redox pairs M(H2O)62+/3+ (M = V, Cr, Mn, Fe and Co)

Long-Range Electron Transfer with Myoglobin Immobilized at Au/Mixed-SAM Junctions: Mechanistic Impact of the Strong Protein Confinement

“…The double-sided blue arrow shows the Fe···O bond reorganization during electron exchange in a tight complex (case a discussed above). This implies simple bond elongation/shortening coupled to a reversible ET, just as in the liquid-phase or interfacial ET processes involving the [Fe(H 2 O) 6 ] 2+/3+ couple. − Indeed, calculations for simple [Fe(H 2 O) 5 ] 3+/2+ –OH 2 systems, , comparable to those by Strickland and Harvey for a model Fe/imidazol/porphyrin/water system, showed remarkable qualitative similarities between respective spin states and the energy term sequence; yet, the quantitative difference is evident through the higher binding energies and corresponding shorter Fe···OH 2 bond distances for all of the spin states involved, for the case of hexaaquairon complexes. The previous theory-based analysis demonstrated that, for the latter model system, around room temperature, thermally activated overbarrier (classical) surmounting prevails over the pure (zero-energy) tunneling pattern.…”

“…The contemporary generalized (multidimensional) theory of charge transfer allows for a consideration of coupling of the outer-sphere ET (both long- and short-range) to different degrees of freedom of the medium, including the first ligand-sphere vibrational (quasiharmonic) ,,− and associative/dissociative − modes, environmental (solvent-related, interfacial, and macromolecular) ballistic (see refs − , , , and − ) and dissipative (see refs − , , , and − ) modes, and so on. Examples of respective atomistic analysis of the ligand association/dissociation at model hemes − and in globins − ,− and for ET (electron exchange) in “simple” iron hexaaqua complexes − are available.…”

“…The contemporary generalized (multidimensional) theory of charge transfer allows for a consideration of coupling of the outer-sphere ET (both long- and short-range) to different degrees of freedom of the medium, including the first ligand-sphere vibrational (quasiharmonic) ,,− and associative/dissociative − modes, environmental (solvent-related, interfacial, and macromolecular) ballistic (see refs − , , , and − ) and dissipative (see refs − , , , and − ) modes, and so on. Examples of respective atomistic analysis of the ligand association/dissociation at model hemes − and in globins − ,− and for ET (electron exchange) in “simple” iron hexaaqua complexes − are available. In this subsection, based on the principles of the generalized theoretical approach and in the spirit of the above-mentioned preceding work, we attempt to construct a combined model encompassing elementary processes, with the involvement of metmyoglin (MbFe 3+ H 2 O) and deoxymyoglobin (MbFe 2+ H 2 O; the associated, metastable and dissociated forms, respectively, vide infra), under diverse experimental conditions, including those of the present work.…”

“…Furthermore, advanced theoretical modeling − and molecular dynamic simulations − were performed to comprehend Mb mechanisms at the atomistic level. In particular, notions of the generalized (“multidimensional”) theory of charge transfer − (encompassing the basic Landau–Zener energy-curve-crossing paradigm) were addressed to understand coupling of the ligand attachment/detachment reaction coordinate with the heme/protein degrees of freedom. − Almost in parallel, notions of the same general theory were applied for the mechanistic analysis of ET (electron-exchange) coupling with recognizable intramolecular (inner-sphere) and solvent (outer-sphere) vibrational modes in model metal complexes such as [Fe(H 2 O) 6 ] 2+/3+ . − However, no attempts have been made so far to treat all of the elementary events involving metmyoglobin, including the water ligand dynamics and outer-sphere ET, within a unified combined model based on the generalized multidimensional theory. − …”

26 Mechanisms of reactions in solution

“…A theoretical study of selfexchange reactions of M(H 2 O) 6 2ϩ/3ϩ , M = V, Cr, Mn, Fe and Co shows that the reorganisation model used significantly affects the inner-shell barriers of the redox pairs. 28 Studies of the outer sphere oxidation of thioglycollic acid and H 2 S by [IrCl 6 ] 2Ϫ , [Mo(CN) 8 ] 3Ϫ and [Ru(NH 3 ) 5 py] 3ϩ and related complexes in aqueous solution emphasise the importance of the exclusion of traces of copper or other transition metal ions which may influence the kinetics and indeed the nature of the reaction. 29,30,31 In the [IrCl 6 ] Ϫ reaction with thioglycollic acid, in the absence of Cu, the mechanism involves stepwise oxidation of RS ؒ to RSO Studies of the mechanisms of oxidation of a variety of hydrocarbons and related compounds by tris(hexafluoroacetylacetonate)manganese() indicate that initial electron transfer is a feature for alkoxy aromatic compounds and hydrogen atom transfer for species such as 1,4-cyclohexadiene.…”