Biochemistry Z 0250 Singlet Oxygen Microscope: From Phase-Separated Polymers to Single Biological Cells -[43 refs.]. -(SNYDER, J. W.; ZEBGER, I.; GAO, Z.; POULSEN, L.; FREDERIKSEN, P. K.; SKOVSEN, E.; MCILROY, S. P.; KLINGER, M.; ANDERSEN, L. K.; OGILBY*, P. R.; Acc. Chem. Res. 37 (2004) 11, 894-901; Dep. Chem., Univ. Aarhus, DK-8000 Aarhus, Den.; Eng.) -Lindner 06-270

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Water drops become charged after sliding on a polymer surface. The variation of the detected charge with pH and ionic strength are compatible with OH− or H+ transfer from the drop to the polymer. These changes are accounted for by a thermodynamic model.

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The Cu A center is a dinuclear copper site that serves as an optimized hub for long-range electron transfer in hemecopper terminal oxidases. Its electronic structure can be described in terms of a s u * ground-state wavefunction with an alternative, less populated ground state of p u symmetry, which is thermally accessible. It is now shown that secondsphere mutations in the Cu A containing subunit of Thermus thermophilus ba 3 oxidase perturb the electronic structure, which leads to a substantial increase in the population of the p u state, as shown by different spectroscopic methods. This perturbation does not affect the redox potential of the metal site, and despite an increase in the reorganization energy, it is not detrimental to the electron-transfer kinetics. The mutations were achieved by replacing the loops that are involved in protein-protein interactions with cytochrome c, suggesting that transient protein binding could also elicit ground-state switching in the oxidase, which enables alternative electron-transfer pathways.

CuA is a binuclear copper site acting as electron entry port in terminal heme-copper oxidases. In the oxidized form, CuA is a mixed valence pair whose electronic structure can be described using a potential energy surface with two minima: σu* and πu, that are variably populated at room temperature. We report that mutations in the first and second coordination spheres of the binuclear metallocofactor can be combined in an additive manner to tune the energy gap and, thus, the relative populations of the two lowest-lying states. A series of designed mutants span σu*/πu energy gaps ranging from 900 to 13 cm−1. The smallest gap corresponds to a variant with an effectively degenerate ground state. All engineered sites preserve the mixed-valence character of this metal center and the electron transfer functionality. An increase of the Cu-Cu distance less than 0.06 Å modifies the σu*/πu energy gap by almost two orders of magnitude, with longer distances eliciting a larger population of the πu state. This scenario offers a stark contrast to synthetic systems, as model compounds require a lengthening of 0.5 Å in the Cu-Cu distance to stabilize the πu state. These findings show that the tight control of the protein environment allows drastic perturbations in the electronic structure of CuA sites with minor geometric changes.

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