Preparation and properties of O-dansyltyrosine gramicidin C

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Abstract
Gramicidins A, B and C are a family of polypeptide antibiotics which facilitate the passive diffusion of alkali cations and protons through lipid bilayer membranes. Gramicidin forms a multimeric transmembrane channel and it was suggested that the channel is an ion-conducting dimer in equilibrium on the membrane with non-conducting monomer. The preparation and purification of a derivative of gramicidin C in which the phenolic hydroxyl of the tyrosine at position 11 was esterified to 8-dimethylaminonaphthalene-1-sulfonate (dansyl) is described. This derivative fluoresces strongly in the visible with an emission maximum in dioxane of 530 nm, an emission lifetime of 16 ns and a quantum yield of 0.8. Veatch et al. showed this O-dansyltyrosine gramicidin C to be a fully active analogue of gramicidin A in artificial lipid bilayer membranes. This derivative was utilized to further characterize the state of aggregation and rotational mobility of the 4 interconvertible conformation species formed by gramicidin in nonpolar organic solvents. Fluorescence energy transfer from the tryptophans of gramicidin A to the O-dansyltyrosine of this derivative supports the conclusion that all of these gramicidin isolated species are aggregates. Decay of fluorescence polarization anisotropy measurements yielded a rotational correlation time of 1 ns for the O-dansyltyrosine chromophore in ethanol in good agreement with the more detailed information previously obtained by 13C-NMR for the monomer in dimethyl sulfoxide. It is likely that the chromophore has much more rotational mobility than the rest of the gramicidin molecule in the aggregated conformational states.