Terbium Luminescence from Complexes of Angiotensin II, Small Peptides, and Amino Acids

Lanthanide ions have been successfully used as luminescent probes in several biological systems. The terbium (Tb) luminescence is greatly enhanced in Tb(III)-protein systems by non-radiative energy transfer from aromatic chromophores to bound Tb ions. In this study, the luminescent properties of terbium have been used to monitor the metal-peptide interactions of angiotensin II, a linear octopeptide hormone (Asp-ArgVal-Tyr-Ile-His-Pro-Phe). The terbium luminescence was enhanced when the metal-complex was excited at either 259 nm or 280 nm (Phe and Tyr absorption bands, respectively). The results from a series of experiments with amino acids and small peptides show that the terbium luminescence from the Tb-Ang II complex is about the same as observed for a Tb-Phe complex when excited at 259 nm, but only 34% of the value of a Tb-Tyr complex excited at 280 nm. These results confirm the structural model of angiotensin II and show that the low enhancement of the terbium luminescence in peptide and amino acid complexes is due to weak binding and not poor energy transfer. OHIOJ. SCI. 86(4): 140-143, 1986