Determination of the intramolecular tyrosine-tryptophan distance in a 7-peptide related to the C-terminal sequence of cholecystokinin.

The solution conformation of a 7-peptide with the C-terminal sequence of cholecystokinin was investigated by evaluation of intramolecular resonance energy transfer between tyrosine (donor) in position 1 and tryptophan (acceptor) in position 4. From the relative enhancement of acceptor fluorescence a transfer efficiency of 0.70 +/- 0.04 was determined. The use of this parameter in Förster's equation permitted the calculation of the average intramolecular tyrosine-tryptophan separation, whereby the assumption of random donor-acceptor orientation was made. The resulting average distance of 10.0 +/- 0.3 A suggests some type of a folded conformation and excludes the existence of a fully extended chain in the N-terminal part of the peptide. A comparison with tyrosine-tryptophan distances observed in other biologically active polypeptides is made.