A circular dichroism (CD) study of [D-Pro2]- and [D-Pro3]-bradykinin, selected peptide fragments, and the model compound. N-acetyl-L-phenylalaninamide, support our previous conclusion (Biochemistry 12, 3780, 1973) that the positive 221-nm CD band of bradykinin is a composite of bands due to two chromophores, the 217-nm band characteristic of the Phe residues overlying the 223-nm band of the N-terminal sequence, Arg-Pro-Pro. The results also indicate that the 223-nm band of Arg-Pro-Pro is associated with the configuration of the Pro-Pro sequence, Arg-D-Pro-Pro and Arg-Pro-D-Pro virtually being diastereoisomers. Accordingly, the conformation of Arg-Pro-Pro was probed in further detail. Upon increasing the temperature from about 27 to 65 degrees C, Arg-Pro-Pro undergoes a conformational transition characterized by large positive values of deltaHdegrees and deltaSdegrees, which is interpreted to mean that the structure of water and, thus, solute-solvent interactions play a dominant role in determining the conformation of the peptide 13C nuclear magnetic resonance spectroscopy indicates that the effect of lowering the pH on the CD of Arg-Pro-Pro is explicable in terms of hydrogen-bond formation between the carboxyl group and Pro2 carbonyl oxygen at acid pH with concomitant cis to trans isomerization.