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

We review work on the paramagnetic amino acid 2,2,6,6-tetramethyl-N-oxyl-4-amino-4-carboxylic acid, TOAC, and its applications in studies of peptides and peptide synthesis. TOAC was the first spin label probe incorporated in peptides by means of a peptide bond. In view of the rigid character of this cyclic molecule and its attachment to the peptide backbone via a peptide bond, TOAC incorporation has been very useful to analyze backbone dynamics and peptide secondary structure. Many of these studies were performed making use of EPR spectroscopy, but other physical techniques, such as X-ray crystallography, CD, fluorescence, NMR, and FT-IR, have been employed. The use of double-labeled synthetic peptides has allowed the investigation of their secondary structure. A large number of studies have focused on the interaction of peptides, both synthetic and biologically active, with membranes. In the latter case, work has been reported on ligands and fragments of GPCR, host defense peptides, phospholamban, and β-amyloid. EPR studies of macroscopically aligned samples have provided information on the orientation of peptides in membranes. More recent studies have focused on peptide–protein and peptide–nucleic acid interactions. Moreover, TOAC has been shown to be a valuable probe for paramagnetic relaxation enhancement NMR studies of the interaction of labeled peptides with proteins. The growth of the number of TOAC-related publications suggests that this unnatural amino acid will find increasing applications in the future

Conformational studies of TOAC-labeled bradykinin analogues in model membranes

Spin-labeled analogues of bradykinin (BK) were synthesized containing the amino acid TOAC ( 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) either before Arg(1) (TOAC(0)-BK) or replacing Pro(3) (TOAC(3)-BK). Whereas the latter is inactive, the former retains about 70% of BK's activity in isolated rat uterus. A combined electron paramagnetic resonance (EPR)-circular dichroism ( CD) approach was used to examine the conformational properties of the peptides in the presence of membrane-mimetic systems ( negatively charged sodium dodecyl sulfate, SDS, and zwitterionic N-hexadecyl-N, N-dimethyl-3-ammonio-1-propanesulfonate, HPS). While the peptides bind to both monomeric and micellar SDS, no interaction occurs with HPS, evincing the contribution of electrostatic interactions. TOAC(3)-BK's EPR spectral lineshapes are broader than those of TOAC(0)-BK, indicating a more restricted degree of motion at position 3. Moreover, the motional freedom of both peptides decreased upon binding to SDS. BK and TOAC(0)-BK solution CD spectra indicate highly flexible conformations ( possibly an equilibrium between rapidly interconverting forms), while TOAC(3)-BK's spectra correspond to a more ordered structure. SDS binding induces drastic changes in BK and TOAC(0)-BK spectra, indicating stabilization of similar folds. the micelle interface promotes a higher degree of secondary structure by favoring intramolecular hydrogen bonds. in contrast, TOAC(3)-BK spectra remain essentially unchanged. These results are interpreted as due to TOAC's ring imposing a more constrained conformation. This rigidity is very likely responsible for the inability of TOAC(3)-BK to acquire the correct receptor-bound conformation, leading to loss of biological activity. On the other hand, the greater flexibility of TOAC(0)-BK and the similarity between its conformational behavior and that of the native hormone are probably related to their similar biological activity.Universidade Federal de São Paulo, Dept Biophys, São Paulo, BrazilUniv São Paulo, Dept Biochem, Inst Chem, São Paulo, BrazilUniversidade Federal de São Paulo, Dept Biophys, São Paulo, BrazilWeb of Scienc

Peptide-lipid interaction monitored by spin labeled biologically active melanocortin peptides

The present work comparatively analyzes the interaction of alpha-MSH and its more potent and long-acting analog [Nle(4), D-Phe(7)]alpha-MSH (NDP-MSH) with lipid bilayers. the peptides were spin labeled with Toac (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) at the N-terminal, as those derivatives had been previously shown to keep their full biological activity. Due to the special rigidity of the Toac covalent binding to the peptide molecule, this spin label is highly sensitive to the peptide backbone conformation and dynamics. the peptides were investigated both by the electron spin resonance (ESR) of Toac(0) and the time resolved fluorescence of Trp(9) present in the peptides. the Toac(0) ESR of the membrane-bound peptides indicates that the two peptides are inserted into the bilayer, close to the bilayer surface, in rather similar environments. A residue titration around pK(a) 7.5, possibly that of His(6), can be clearly monitored by peptide-lipid partition. Trp(9) time resolved fluorescence indicates that the peptides, and their Toac-labeled derivatives, present rather similar conformations when membrane bound, though Trp(9) in NDP-MSH, and in its Toac-labeled derivative, goes somewhat further down into the bilayer. Yet, Toac(0) ESR signal shows that the Toac-labeled N-terminal of NDP-MSH is in a shallower position in the bilayer, as compared to the hormone. (c) 2005 Elsevier Inc. All rights reserved.Univ São Paulo, Inst Fis, BR-05315970 São Paulo, SP, BrazilUniversidade Federal de São Paulo, Dept Biofis, São Paulo, SP, BrazilUniv São Paulo, Fac Filosofia Ciencias & Letras Ribeirao Pret, BR-14049 Ribeirao Preto, SP, BrazilUniversidade Federal de São Paulo, Dept Biofis, São Paulo, SP, BrazilWeb of Scienc

Exchange students’ views on tourism: the impact of first-hand experiences on transversal skills development and loyalty to the host destination

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