Conformational characteristics of luliberin. Luminescence properties at liquid-nitrogen temperature

The luminescence properties (fluorescence and phosphorescence) of luliberin have been investigated at liquid-nitrogen temperature (77 K) in 50% ethylene glycolaqueous buffer at various pH's. Calculation of the energy-transfer efficiency between Trp and Tyr residues leads to an evaluation of the distance separating these residues. In alkaline medium, when tyrosine is ionized, a 100% transfer efficiency occurs from Trp to Tyr- at the singlet level and also from Tyr- to Trp at the triplet level indicating that the TRP-Tyr distance is less than about 5A. When luliberin is at pH 7.8 (or 4.5), transfer efficiency from Tyr to Trp at the singlet level is 85-90% which should correspond to a distance between Trp and Tyr of about 10-12 A. These results are discussed with respect to the role played by aromatic amino acids both in the hormone conformation and in the hormone biological potency. Moreover, comparison with the data obtained from fluorescence or circular dichroism studies at room temperature allows us to deduce some characteristics of luliberin conformation. Structure-activity relationships in the aromatic region of luliberin are also discussed.