Helical secondary structure, effect rates

We prepared three bifunctional redox protein maquettes based on 12 16-, and 20-mer three-helix bundles. In each case, the helix was capped with a Co(III) tris-bipyridyl electron acceptor and also functionalized with a C-terminal viologen (l-ethyl-V-ethyl-4,4 -bipyridinium) donor. Electron transfer (ET) was initiated by pulse radiolysis and flash photolysis and followed spectrometrically to determine average, concentration-independent, first-order rates for the 16-mer and 20-mer maquettes. For the 16-mer bundle, the a-helical content was adjusted by the addition of urea or trifluoroethanol to solutions containing the metal-loprotein. This conformational flexibility under different solvent conditions was exploited to probe the effects of helical secondary structure on ET rates. In addition to describing experimental results from these helical systems, this chapter discusses several additional metalloprotein models from the recent literature. 

The structure of the native protein can be important in influencing the rate, extent, and effects of denaturation. Proline-rich peptides and proteins capable of a secondary helical structure appear to favor partially folded structures under denaturing conditions these are simultaneously present with random-coil denatured molecules (32). The presence of these two slowly interconverting conformers then leads to increased reversed-phase band broadening as above. Structural factors which disfavor protein denaturation can also operate to reduce band width. For the HIC separation of various apolipoproteins (96), it was found that more hydrophobic proteins gave narrower bands. This was attributed to a more structure conformer in the retained state for the more hydrophobic protein. That is, a single (native ) conformer exists during the separation of more hydrophobic proteins in this system, but some denaturation of less hydrophobic proteins occurs. 

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