A fluorescence study of tryptophan-histidine interactions in the peptide anantin and in solution

Anantin is a heptadecapeptide in which the C-terminal peptide chain pierces the covalently cyclized peptide ring formed by an amide link between the alpha-NH2 end group and the beta-carboxyl group of Asp(8). It contains a tryptophan and a histidine at positions 5 and 12, respectively. Des-Phe(17)-anantin lacks the C-terminal phenylalanine. Fluorescence emission intensity as a function of pH follows the ionization of a single residue. The pKa amounts to 7.23 +/- 0.03 for anantin and is attributed to His(12). At pH 9 the quantum yield is 0.12 +/- 0.01 for anantin, whereas at pH 4.5 the quantum yield decreases more than two-fold (0.05 +/- 0.01). Practically identical parameters are observed for des-Phe(17)-anantin. This pH dependency reveals intramolecular quenching of the excited indole ring of Trp(5) by the imidazole of His(12), which results in a marked decrease of the tryptophan fluorescence at low pH. In a multifrequency phase fluorometric study the fluorescence lifetimes for both peptides at pH 4.5 and pH 9 are determined. At both, pH fluorescence decay is well described by a sum of two exponentials. For anantin at pH 4.5 the lifetimes are 0.72 +/- 0.07 ns and 1.67 +/- 0.07 ns. At pH 9 the lifetimes are 1.11 +/- 0.12 ns and 2.55 +/- 0.03 ns. In methanol we find two lifetimes for anantin: 0.68 +/- 0.01 ns and 2.57 +/- 0.01 ns. The lifetimes are found to be slightly dependent upon emission wavelength. For des-Phe(17)-anantin practically the same values are observed.(ABSTRACT TRUNCATED AT 250 WORDS)