A conformational two-state peptide model system containing an ultrafast, but soft light switch
Markus Loeweneck 1, Alexander G Milbradt 1, Christopher Root 2, Helmut Satzger 2, Wolfgang Zinth 2, Luis Moroder 1 and Christian Renner 1*
1 MPI of Biochemistry, Martinsried, Germany
2 Ludwig-Maximilians Universität, Munich, Germany
* To whom correspondence should be addressed. E-mail: christian.renner{at}ntu.ac.uk.
Submitted on May 29, 2005
Revised on July 6, 2005
Accepted on 5 December 2005
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Abstract |
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Combining an azobenzene chromophore with the bis-cysteinyl active-site sequence of the protein disulfide isomerase (PDI) we constructed a simple, but promising model for allosteric conformational rearrangements. Paralleling cellular signaling events, an external trigger, here absorption of a photon, leads to a structural change in one part of the molecule, namely the azobenzene-based chromophore. The change in geometry translates to the effector site, in our case the peptide sequence, where it modifies covalent and non-bonded interactions, and thus leads to a conformational rearrangement. NMR spectroscopy showed that the trans-azo and cis-azo isomer of the cyclic PDI peptide exhibit different, but well-defined structures when the two cystine residues form a disulfide bridge. Without this intramolecular crosslink conformationally more variable structural ensembles are obtained, that again differ for the two isomeric states. Ultrafast uv/vis spectroscopy confirmed that the rapid isomerization of azobenzene is not significantly slowed down when incorporated into the cyclic peptides, although the amplitudes of ballistic and diffusive pathways are changed. Important was the observation that most of the energy of an absorbed photon is dissipated to the solvent in the first few picoseconds when the actual azo-isomerization takes places. The conformational rearrangement is weakly driven due to the absence of appreciable excess energy and can be described as biased diffusion similar to natural processes.
Key Words:
azobenzene, nmr, peptides, photo isomerization, protein folding, ultrafast spectroscopy
