Control of ditryptophan cross-linking: dihydrotryptophan as a tryptophan precursor in peptide synthesis.
In neat trifluoroacetic acid, tryptophan side chains cross-link to form a diastereomeric mixture of tryptophan dimers. Convergent oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) converts tryptophan dimers to ditryptophan. Since cross-link formation is under thermodynamic control, there has been no simple way of controlling the regiochemistry of the cross-linking process when more than one tryptophan side chain is present. Here, we show that dihydrotryptophan (Dht) can be incorporated into peptides as a tryptophan precursor, which reforms tryptophan upon treatment with DDQ. Dihydrotryptophan was prepared as a mixture of gammaS and gammaR diastereomers and the indoline nitrogen was protected with a Cbz group. The resulting amino acid, Nalpha-BOC-Dht(Cbz)-OH, was then incorporated into peptides as a mixture of diastereomers. Dht was resistant to tryptophan cross-linking in neat trifluoroacetic acid and was converted back to tryptophan during convergent oxidation of tryptophan dimers. While Dht is useful for control of ditryptophan regiochemistry and as a potential tryptophan analog, it is not a general strategy for Trp protection since DDQ is unlikely to be compatible with easily oxidized amino acids such as cysteine.
Dinh
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Department of Chemistry, University of California, Irvine 92697-2025, USA.
Van Vranken
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D000215:Acylation; D003432:Cross-Linking Reagents / Q000737:chemistry*; D010084:Oxidation-Reduction; D010455:Peptides / Q000138:chemical synthesis*; D014364:Tryptophan / Q000031:analogs & derivatives* / Q000737:chemistry*
D003432:Cross-Linking Reagents; C121500:N(epsilon)-carbobenzyloxy-gamma-delta(1)-dihydrotryptophan; D010455:Peptides; D014364:Tryptophan