CONTROLLING THE 3D NANOSCALE ORGANIZATION OF BULK HETEROJUNCTION POLYMER SOLAR CELLS

In this study,the three dimensional nanoscale organization in the photoactive layers of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM) is revealed by transmission electron tomography.After annealing treatment,either at elevated temperature or during slow solvent evaporation,nanoscale interpenetrating networks are formed with high crystalline order and favorable concentration gradients of both components through the thickness of the photoactive layer.Such a tailored morphology acco...     

Geminate carbon monoxide rebinding to a c-type haem

A chemically modified form of cytochrome c(cyt. c), termed carboxymethyl cytochrome c(cm cyt. c), possesses a vacant sixth coordination site to the haem iron that is available to bind external ligands. We present data on the rapid flash photolysis of CO from the ferrous haem iron of cm cyt. c and describe the kinetics and spectral transitions that accompany the recombination. This was achieved using 30-femtosecond laser pulses and a white light continuum to monitor spectral transitions. Whereas the photo-dissociation quantum yield is close to 1, the yield of CO escape from the protein (the apparent quantum yield, varphi) relative to myoglobin (varphi=1) is small due to rapid geminate recombination of CO. On ligand photo-dissociation the haem undergoes a spin-state transition from low-spin ferrous CO bound to penta-coordinate high-spin. Subsequently the system reverts to the CO bound form. The data were fitted with a minimum number of exponentials using global analysis. Recombination of CO with the haem iron of cm cyt. c is multiphasic (tau=16 ps, 120 ps and 1 ns), involving three spectrally distinct components. The fraction of haem (0.11) not recombining with CO within 4 ns is similar to the value of varphi(0.12) measured on the same preparation by the "pulse method" (M. Brunori, G. Giacometti, E. Antonini and J. Wyman, Proc. Natl. Acad. Sci. USA, 1973, 70, 3141-3144, ). This implies that no further geminate recombination occurs at t>4 ns. This unusually efficient CO-haem geminate recombination indicates the sterically hindered ("caged") nature of the distal haem pocket in cm cyt. c from which it is difficult for CO to escape. The large geminate phase may be contrasted with the behaviour of myoglobin in which geminate recombination is small. This is in general agreement with the well-documented extensive structural dynamics in myoglobin that allow ligand passage, and a higher structural rigidity in cyt. c imposed by the restraints of minimising reorganisation energy for electron transfer (M. Brunori, D. Bourgeois and D. Vallone, J. Struct. Biol., 2004, 147, 223-234, ). The high pH ferrous form of cm cyt. c is a low-spin species having a lysine bound to the central iron atom of the haem (M. Brunori, M. Wilson and E. Antonini, J. Biol. Chem., 1972, 247, 6076-6081; G. Silkstone, G. Stanway, P. Brzezinski and M. Wilson, Biophys. Chem., 2002, 98, 65-77, ). This high pH (pH approximately 8) form of deoxy cm cyt. c undergoes photo-dissociation of lysine (although the proximal histidine is possible) after photo-excitation. Recombination occurs with a time constant (tau) of approximately 7 ps. This is similar to that observed for the geminate rebinding of the Met80 residue in native ferrous cyt. c(tau approximately 6 ps) following its photo-dissociation (S. Cianetti, M. Negrerie, M. Vos, J.-L. Martin and S. Kruglik, J. Am. Chem. Soc., 2004, 126, 13 932-13 933; W. Wang, X. Ye, A. Demidov, F. Rosca, T. Sjodin, W. Cao, M. Sheeran and P. Champion, J. Phys. Chem., 2000, 104, 10 789-10 801, ).     

EPR and optical spectroscopic studies of Met80X mutants of yeast ferricytochrome c. Models for intermediates in the alkaline transition

The ferric forms of Met80X mutants of yeast iso-1-cytochrome c (X = Ala, Ser, Asp, and Glu) display EPR and optical spectra that are strongly pH dependent. At low pH values (pH approximately 5) the sixth coordination sites are filled by H(2)O that, on elevating the pH, is replaced by OH(-) in the cases of Met80Ala and -Ser (pK approximately 5.6 and 5.9, respectively) and by a lysine amino group in the cases of Met80Asp and -Glu (pK approximately 9.3 and 11.6, respectively). The ligand sets and the pK values of the transitions are rationalized in terms of the structure of the heme pocket, and a possible mechanism of the "trigger" in the alkaline transition of the native protein is suggested.     

Probing a complex of cytochrome c and cardiolipin by magnetic circular dichroism spectroscopy: implications for the initial events in apoptosis

Oxidation of cardiolipin (CL) by its complex with cytochrome c (cyt c) plays a crucial role in triggering apoptosis. Through a combination of magnetic circular dichroism spectroscopy and potentiometric titrations, we show that both the ferric and ferrous forms of the heme group of a CL:cyt c complex exist as multiple conformers at a physiologically relevant pH of 7.4. For the ferric state, these conformers are His/Lys- and His/OH(-)-ligated. The ferrous state is predominantly high-spin and, most likely, His/-. Interconversion of the ferric and ferrous conformers is described by a single midpoint potential of -80 ± 9 mV vs SHE. These results suggest that CL oxidation in mitochondria could occur by the reaction of molecular oxygen with the ferrous CL:cyt c complex in addition to the well-described reaction of peroxides with the ferric form.