Synthesis and study of a gramicidin B mutant possessing a ditryptophan crosslink

Recent studies of peptide dimers linked by Trp-Trp (ditryptophan) crosslinks suggest that the crosslinks can reinforce antiparallel beta-structure. Depending on environment, gramicidins A, B and C form either helical ion channels with parallel beta-structure or non-functional pores with antiparallel beta-structure. In the channel conformation of the gramicidins Trp9 and Trp15 are close in space, but in the pore conformation Trp9 and Trp15 are far apart. We hypothesized that a ditryptophan crosslink between Trp9 and Trp15 could pre-organize gramicidin in an active conformation. To test the potential for preorganization, an intramolecular ditryptophan crosslink was formed between Trp9 and Trp15 in a W13F mutant of gramicidin B. Photooxidative conditions were shown to generate ditryptophan crosslinks in low yields. While not preparatively useful, photooxidative tryptophan crosslinking may have implications for protein aging processes like cataract formation. The ditryptophan crosslink in the gramicidin B mutant substantially lowered the antibiotic activity of the gramicidin B mutant, unlike the ditryptophan crosslink in the antibiotic X-indolicidin. The biaryl chromophore generated diagnostic Cotton effects in the CD spectrum that revealed the absolute stereochemistry of the biaryl chromophore, but the biaryl chromophore obscured diagnostic features below 220 nm. However, changes in peptide conformation were reflected in changes in the biaryl region of the CD spectrum above 240 nm.