Conformational properties of angiotensin II and its active and inactive TOAC‐labeled analogs in the presence of micelles. Electron paramagnetic resonance, fluorescence, and circular dichroism studies

Abstract

The interaction between angiotensin II (AII, DRVYIHPF) and its analogs carrying 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) and detergents--negatively charged sodium dodecyl sulfate (SDS) and zwitterionic N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS)--was examined by means of EPR, CD, and fluorescence. EPR spectra of partially active TOAC1-AII and inactive TOAC3-AII in aqueous solution indicated fast tumbling, the freedom of motion being greater at the N-terminus. Line broadening occurred upon interaction with micelles. Below SDS critical micelle concentration, broader lines indicated complex formation with tighter molecular packing than in micelles. Small changes in hyperfine splittings evinced TOAC location at the micelle-water interface. The interaction with anionic micelles was more effective than with zwitterionic micelles. Peptide-micelle interaction caused fluorescence increase. The TOAC-promoted intramolecular fluorescence quenching was more pronounced for TOAC3-AII because of the proximity between the nitroxide and Tyr4. CD spectra showed that although both AII and TOAC1-AII presented flexible conformations in water, TOAC3-AII displayed conformational restriction because of the TOAC-imposed bend (Schreier et al., Biopolymers 2004, 74, 389). In HPS, conformational changes were observed for the labeled peptides at neutral and basic pH. In SDS, all peptides underwent pH-dependent conformational changes. Although the spectra suggested similar folds for AII and TOAC1-AII, different conformations were acquired by TOAC3-AII. The membrane environment has been hypothesized to shift conformational equilibria so as to stabilize the receptor-bound conformation of ligands. The fact that TOAC3-AII is unable to acquire conformations similar to those of native AII and partially active TOAC1-AII is probably the explanation for its lack of biological activity.

Interaction of cyclic and linear Labaditin peptides with anionic and zwitterionic micelles

“…The small k max blue shift observed when Labaditin is in contact with LPC points out that, probably, both forms of the peptide are not so embedded in the acyl-chain region of LPC micelles. The values obtained were quite low when compared with others peptides as Hylin-a1, for instance, that showed about 25 nm of shift [ [50][51][52]. Such finding suggests that the peptide must be located near the LPC micelle surface.…”

The spin label amino acid TOAC and its uses in studies of peptides: chemical, physicochemical, spectroscopic, and conformational aspects

“…Combined EPR and CD studies were also performed on the interaction of BK (Vieira et al 2002) and AII (Vieira et al 2009) and their TOAC-labeled analogues with detergent micelles—negatively charged SDS and zwitterionic HPS. Line broadening in the EPR spectra indicated micelle binding (Fig.…”

“…The arrows in the spectrum of TOAC 3 -AII in the presence of HPS at pH 10.0 indicate an immobilized component due to HPS-bound TOAC 3 -AII. Reprinted from Vieira et al (2009) with permission of John Wiley & Sons…”

Structure and reorientational dynamics of angiotensin I and II: a microscopic physical insight

“…Small linear peptides usually exist in solution as a range of conformations in rapid equilibrium (Matsoukas et al, 1990). Accordingly, interpretation of the conformational studies of AngII are conflicting, which has led to various structural models in the solution including β pleated sheet, β and γ turns, and other structures (Carpenter, Wilkes, & Schiller, 1998;Greff et al, 1976;Lintner et al, 1977;Spyroulias et al, 2003;Tzakos et al, 2004;Vieira et al, 2009). One reason for contradicting results could possibly be due to solvent effects.…”