Temperature Dependence of Photoinduced Electron Transfer within Self-Assembled Uroporphyrin−Cytochrome <i>c</i> Complexes

Croney, John C.; Helms, Michael K.; Jameson, and David M.; Larsen, Randy W.

doi:10.1021/jp991007+

Cited by 12 publications

(26 citation statements)

References 35 publications

(73 reference statements)

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“…In addition, the temperature dependence of the singlet distribution of COP/Cc complex revealed similar behavior as the URO/Cc complex. This observation verifies that multiple conformations exist that can be represented by a potential energy surface where the surface should be made up of many shallow wells within an overall larger minimum (Croney et al, 2000;Jasuja et al, 2002). At high temperatures, the barrier of the shallow wells could be overcome creating an equilibrium or an average state that would be represented as a discrete lifetime.…”

Section: Discussionsupporting

confidence: 65%

Conformational Dynamics and Temperature Dependence of Photoinduced Electron Transfer within Self-Assembled Coproporphyrin:Cytochrome c Complexes

Croney

Helms

Jameson

et al. 2003

Biophysical Journal

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The focus of the present study is to better understand the complex factors influencing intermolecular electron transfer (ET) in biological molecules using a model system involving free-base coproporphyrin (COP) complexed with horse heart cytochrome c (Cc). Coproporphyrin exhibits bathochromic shifts in both the Soret and visible absorption bands in the presence of Cc and an absorption difference titration reveals a 1:1 complex with an association constant of 2.63 +/- 0.05 x 10(5) M(-1). At 20 degrees C, analysis of time-resolved fluorescence data reveals two lifetime components consisting of a discrete lifetime at 15.0 ns (free COP) and a Gaussian distribution of lifetimes centered at 2.8 ns (representing (1)COP --> Cc ET). Temperature-dependent, time-resolved fluorescence data demonstrate a shift in singlet lifetime as well as changes in the distribution width (associated with the complex). By fitting these data to semiclassical Marcus theory, the reorganizational energy (lambda) of the singlet state electron transfer was calculated to be 0.89 eV, consistent with values for other porphyrin/Cc intermolecular ET reactions. Using nanosecond transient absorption spectroscopy the temperature dependences of the forward and thermal back ET originating from triplet state were examined ((3)COP --> Cc ET). Fits of the temperature dependence of the rate constants to semiclassical Marcus theory gave lambda of 0.39 eV and 0.11 eV for the forward and back triplet ET, respectively (k(f) = (7.6 +/- 0.3) x 10(6) s(-1), k(b) = (2.4 +/- 0.3) x 10(5) s(-1)). The differing values of lambda for the forward and back triplet ET demonstrate that these ET reactions do not occur within a static complex. Comparing these results with previous studies of the uroporphyrin:Cc and tetrakis (4-carboxyphenyl)porphyrin:Cc complexes suggests that side-chain flexibility gives rise to the conformational distributions in the (1)COP --> Cc ET whereas differences in overall porphyrin charge regulates gating of the back ET reaction (reduced Cc --> COP(+)).

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Section: Discussionsupporting

confidence: 65%

Conformational Dynamics and Temperature Dependence of Photoinduced Electron Transfer within Self-Assembled Coproporphyrin:Cytochrome c Complexes

Croney

Helms

Jameson

et al. 2003

Biophysical Journal

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“…. Similar schemes have been used to describe the photoinduced electron-transfer behavior in aqueous solutions of zinc-substituted Cyt c and cupriplastocyanin (8)(9)(10)(11) and for those of Cyt c and negatively charged zinc porphyrins (28,29).…”

Section: Resultsmentioning

confidence: 99%

Peptide−Protein Interactions: Photoinduced Electron-Transfer within the Preformed and Encounter Complexes of a Designed Metallopeptide and Cytochrome c

et al. 2003

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Photoinduced electron-transfer (ET) occurs between a negatively charged metallopeptide, [Ru(bpy)(2)(phen-am)-Cys-(Glu)(5)-Gly](3-) = RuCE(5)G, and ferricytochrome c = Cyt c. In the presence of Cyt c, the triplet state lifetime of the ruthenium metallopeptide is shortened, and the emission decays via biexponential kinetics, which indicates the existence of two excited-state populations of ruthenium peptides. The faster decay component displays concentration-independent kinetics demonstrating the presence of a preformed peptide-protein complex that undergoes intra-complex electron-transfer. Values of K(b) = (3.5 +/- 0.2) x 10(4) M(-1) and k(obs)(ET)= (2.7 +/- 0.4) x 10(6) s(-1) were observed at ambient temperatures. The magnitude of k(obs)(ET) decreases with increasing solvent viscosity, and the behavior can be fit to the expression k(obs)(ET) proportional to eta(-alpha) to give alpha = 0.59 +/- 0.05. The electron-transfer process occurring in the preformed complex is therefore gated by a rate-limiting configurational change of the complex. The slower decay component displays concentration-dependent kinetics that saturate at high concentrations of Cyt c. Analysis according to rapid equilibrium formation of an encounter complex that undergoes unimolecular electron-transfer yields K(b)' = (2.5 +/- 0.7) x 10(4) M(-1) and k(obs')(ET)= (7 +/- 3) x 10(5) s(-1). The different values of k(obs)(ET) and k(obs')(ET) suggest that the peptide lies farther from the heme when in the encounter complex. The value of k(obs')(ET) is viscosity dependent indicating that the reaction occurring within the encounter complex is also configurationally gated. A value of alpha = 0.98 +/- 0.14 is observed for k(obs')(ET), which suggests that the rate-limiting gating processes in the encounter complex is different from that in the preformed complex.

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“…The spontaneous formation of tetrapyrroles containing metals that confer rich photo-and redox-activity provide a foundation for studies to probe the origin of protophotosynthetic processes. In this regard, photochemical processes of uroporphyrins have been examined for the free base, 5,6,91,[107][108][109][110][111] Mn(III) chelate, 91 and Zn(II) 51,84,91,108,112,113 chelate as excited-state redox agents; Fe(III) chelate as a groundstate electron acceptor; 91,108 and Ni(II) chelate for excited-state uptake/release of apical ligands. 42 Two candidate proto-photosynthetic processes have been examined that rely on uroporphyrins in aqueous solution (Fig.…”

Section: Njc Papermentioning

confidence: 99%

Catalytic diversification upon metal scavenging in a prebiotic model for formation of tetrapyrrole macrocycles

et al. 2013

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A prebiotic model for the formation of tetrapyrrole macrocycles was examined in aqueous solution containing representative Earth-available metals [Mg(II), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and Pd(II)].First, a hydrophilic porphyrin (uroporphyrin I) was found to undergo metalation with all metals examined except Mg(II). Second, a competition experiment among the eight metals with uroporphyrin in limiting quantity afforded preferential metalation with Mn(II), Co(II), Cu(II) and Ni(II). A multicomponent analysis method enabled absorption spectrophotometric detection of 8 distinct uroporphyrins (7 metallo-, 1 free base) in a single mixture. Third, a dione-aminoketone reaction was performed in aqueous solution containing the metals followed by photooxidation in the presence of a quinone. The reaction proceeds through multiple stages: (1) dione-aminoketone condensation to give a pyrrole equipped for selfcondensation, (2) tetramerization of the pyrrole and cyclization to give uroporphyrinogens, (3) 6e À /6H + dehydrogenation (e.g., photooxidation) to give the uroporphyrins, and (4) metalation of the uroporphyrins. The presence versus absence of metals resulted in lower yields, yet Mn(II), Fe(II), Co(II), Cu(II) and Zn(II) each individually gave the corresponding metallouroporphyrin [with trivalent metals observed in three cases: Mn(III), Fe(III), and Co(III)]. Analogous reaction in the presence of all eight metals together gave the free base, Mn(III), and Zn(II) chelates whereas other metal chelates could not be reliably detected by absorption spectroscopy or mass spectrometry. Such metalloporphyrins greatly broaden the accessible redox levels, catalytic avenues, and photochemical features versus those of the free base porphyrins. Taken together, scavenging of metals is expected to increase the functional diversity of tetrapyrroles on early Earth.

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Temperature Dependence of Photoinduced Electron Transfer within Self-Assembled Uroporphyrin−Cytochrome c Complexes

Cited by 12 publications

References 35 publications

Conformational Dynamics and Temperature Dependence of Photoinduced Electron Transfer within Self-Assembled Coproporphyrin:Cytochrome c Complexes

Conformational Dynamics and Temperature Dependence of Photoinduced Electron Transfer within Self-Assembled Coproporphyrin:Cytochrome c Complexes

Peptide−Protein Interactions: Photoinduced Electron-Transfer within the Preformed and Encounter Complexes of a Designed Metallopeptide and Cytochrome c

Catalytic diversification upon metal scavenging in a prebiotic model for formation of tetrapyrrole macrocycles

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