Conformational basis for the biological activity of TOAC-labeled angiotensin II and bradykinin: electron paramagnetic resonance, circular dichroism, and fluorescence studies.

N-Terminally and internally labeled analogues of the hormones angiotensin (AII, DRVYIHPF) and bradykinin (BK, RPPGFSPFR) were synthesized containing the paramagnetic amino acid 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC). TOAC replaced Asp1 (TOAC1-AII) and Val3 (TOAC3-AII) in AII and was inserted prior to Arg1 (TOAC0-BK) and replacing Pro3 (TOAC3-BK) in BK. The peptide conformational properties were examined as a function of trifluoroethanol (TFE) content and pH.