Co‐oligopeptides of aromatic amino acids and glycine with a variable distance between the aromatic residues. II. Ultraviolet absorption and circular dichroism properties of co‐oligopeptides of tryptophan and glycine

Abstract

AbstractThe uv absorption and circular dichroism (CD) properties in water (pH 5.9) and trifluoroethanol of several co‐oligopeptides of glycine and tryptophan have been investigated. These compounds contain one tryptophyl residue, such as H‐Gly‐Trp‐OH, H‐Trp‐Gly‐OH, and H‐Gly‐Trp‐Gly‐OH; or two, such as H‐Trp‐Trp‐OH, H‐Gly‐Trp‐Trp‐OH, H‐Trp‐Trp‐Gly‐OH, and H‐Gly‐Trp‐(Gly)n‐Trp‐Gly‐OH (I, n = 0,1,2). Furthermore, the case of some protected derivatives, such as Z‐Trp‐Gly‐OMe, Z‐Gly‐Trp‐Gly‐OMe, and Z‐Trp‐Trp‐OMe, Z‐Trp‐Trp‐Gly‐OMe, Z‐Gly‐Trp‐Trp‐Ome were investigated. The extinction coefficients in H2O in the region of the L bands, referred to one mole of tryptophyl residue, are essentially the same for all compounds within the limit of experimental error, thus indicating that no strong chromophoric interaction takes place in the oligopeptides containing two aromatic residues. However, in the far uv region, anomalies in the absorption properties cannot be excluded. The investigated compounds show marked differences in their CD properties. Whereas in the case of lower molecular‐weight peptides (e.g., H‐Trp‐Gly‐OH and H‐Gly‐Trp‐OH), these differences can be ascribed, at least in part, to the influence of the charge end groups on the indole chromophore, in the case of the compounds containing two tryptophyl residues, the differences in CD properties are assumed to reflect the different structures and conformations of the oligopeptides. In particular, we observed a negative dichroic band at ∼225 nm in H‐Gly‐Trp‐Trp‐OH and I(n = 0) in water. In trifluoroethanol, this band has a much larger intensity, and it is present also in Z‐Gly‐Trp‐Trp‐OMe, Z‐Trp‐Trp‐Gly‐OMe, and Z‐Trp‐Trp‐OMe. It is argued that such a negative band is characteristic of the sequence ‐CO‐Trp‐Trp‐, and the possibility that it derives from intramolecular chromophoric interaction is discussed. It is also pointed out, particularly in view of the high ellipticity values found in some cases (up to 100,000 deg cm2 dmol−1), that this feature may reflect a high degree of conformational rigidity connected with the sequence ‐CO‐Trp‐Trp‐.