Polypeptides and Oligonucleotides with Pendant Porphyrins : From Host-Guest Recognition to Self-Organizing Strands

In natural photosynthetic systems, the solar energy is collected by pigment molecules attached to the light harvesting complexes. In these units, the chlorophylls are held in a favored spacing and orientation by fairly short a-helical polypeptides.¹ When a photon hits one of the chlorophylls, the absorbed energy spreads extremely rapidly to the others until the reaction center is reached within the cell membrane, where the solar energy is converted into chemical energy used by the cell to grow.  In this way, the energy contained in a single photon is conducted in a very short time and with minimal loss of energy from the point where it is absorbed to where it is needed. The extraordinary efficiency of the energy migration over long distances with minimal loss of energy is ascribed to the favored spacing and orientation of the chlorophylls, which are held in an appropriate parallel conformation. Beyond the control of the structure of multi-chromophoric arrays, monitoring the spatial orientation of the chromophores in artificial light harvesting devices is a challenge of growing interest.  Indeed, multi-porphyrinic arrays attract more and more attention for the elaboration of photonic and electronic wires.²

      In order to mimic the light harvesting antennae of the photosynthetic system, we prepared porphyrin-functionalized a-polypeptides such as an octamer and an hexadecamer.³ The polypeptidic backbone confers, after a certain degree of oligomerization, the establishment of a 3₁₀ helical secondary structure. Such a conformation, favored by the natural tendency of the porphyrins to aggregate, induces an overlap of the chromophores, which thus present sufficient electronic coupling to promote a good exciton migration within the molecular wire.⁴

      The 3₁₀helical secondary structure of this polypeptides generates three stacks of porphyrins pointing in three different positions of space with an angle between these three piles of about 120°.  We recently synthesized a new family of polypeptides in which one porphyrin out of four is replaced by a porphyrin functionalized by a crown ether capable of complexing ammonium groups. All the crown ether functions thus point in one direction of space, being ready for molecular recognition with complementary molecules bearing ammonium groups.  These results pave the way towards the preparation of pH dependant new type of glues.

      The ability of these peptides to accomodate guests was investigated through ligand binding studies carried out in dichloromethane with DABCO as bidentate base. The complexation of DABCO by these peptides was monitored by UV-visible spectrophotometric titration in CH₂Cl₂. We showed that the enhanced stability of the complexe octapeptide/DABCO can be ascribed to a pre-organization of the octapeptide forming cavities, and provides convincing evidence that the bidentate base is inserted into the cavities of the octapeptide viahost/guest interactions. We thus took advantage of these kind of host/guest interactions to assemble two poplypeptidic stains, opening the way to another type of novel glues.

 

Acknowledgements

This work was supported by the CNRS and the French Ministry of Research. 

References

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