Directions for solving the remaining problems in the synthesis of large peptides

Conclusion

During the assembly of the GFP molecule, we were able to confirm the usefulness of the solid-phase method for reducing the time to synthesize protected segments having a size of 10 to 12 residues. However, here I wish to clearly emphasize that all the segment-condensation reactions must be carried out in solution; that is, every step of the segment-condensation reactions must be carefully monitored by either TLC or HPLC and progression to the next step should only be done after confirming the homogeneity of the product. If it is not done, highly homogeneous products cannot be obtained even by the solution procedures. By applying this principle, it should be possible to synthe-size 100- or 200-residue peptides without any difficulty by the Boc/Bzl-strategy or probably even by the Fmoc/t Bu-strategy. The most important factor for realizing such a segment-condensation reaction in solution is the availability of powerful solvent systems such as mixtures of CHL/TFE, CHL/phenol, or CHL/HFIP, and a combination of useful coupling reagents, WSCD and HOOBt. I am optimistic about the total synthesis of GFP as we have not yet encountered any critical problem which would overturn our original expectations. By applying these methods, I am certain in general that we are able to synthesize an ordinary 100-residue protein within several months, and a 200-residue protein within a year. This approach should be the only possible way to synthesize such a large peptide in a homogeneous form by the chemical procedures.