4-Amino-1-oxyl-2,2,6,6-tetramethylpiperidine-3-carboxylic acid (β-TOAC), the first spin-labelled, cyclic, chiral β-amino acid resolved in an enantiomerically pure state

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.

Conformation of peptides bound to the transporter associated with antigen processing (TAP)

The ATP-binding cassette transporter associated with antigen processing (TAP) plays a key role in the adaptive immune defense against infected or malignantly transformed cells by translocating proteasomal degradation products into the lumen of the endoplasmic reticulum for loading onto MHC class I molecules. The broad substrate spectrum of TAP, rendering peptides from 8 to 40 residues, including even branched or modified molecules, suggests an unforeseen structural flexibility of the substrate-binding pocket. Here we used EPR spectroscopy to reveal conformational details of the bound peptides. Side-chain dynamics and environmental polarity were derived from covalently attached 2,2,5,5-tetramethylpyrrolidine-1-oxyl spin probes, whereas 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid spin-labeled peptides were used to detect backbone properties. Dependent on the spin probe's position, striking differences in affinity, dynamics, and polarity were found. The side-chains' mobility was strongly restricted at the ends of the peptide, whereas the central region was flexible, suggesting a central peptide bulge. In the end, double electron electron resonance allowed the determination of intrapeptide distances in doubly labeled peptides bound to TAP. Simulations based on a rotamer library led to the conclusion that peptides bind to TAP in an extended kinked structure, analogous to those bound to MHC class I proteins.

The Bip method, based on the induced circular dichroism of a flexible biphenyl probe in terminally protected -Bip-Xaa*- dipeptides, for assignment of the absolute configuration of beta-amino acids

An induced axial chirality of the biphenyl core of the Bip (2',1':1,2;1'',2'':3,4-dibenzcyclohepta-1,3-diene-6-amino-6-carboxylic acid) residue in the terminally protected dipeptides Boc-Bip-beta-Xaa*-OMe (beta-Xaa* = L-beta(3)-HAla, L-beta(3)-HVal, L-beta(3)-HLeu, L-beta(3)-HPro, trans-(1S,2S)-ACHC, trans-(1R,2R)-ACHC, trans-(1S,2S)-ACPC, trans-(1R,2R)-ACPC) resulted in an induced circular dichroism, revealing the usefulness of the Bip method for a reliable and fast assignment of the absolute configuration of chiral beta-amino acids. Remarkably, the Bip method was also applied to the unique spin-labeled, cyclic, beta-amino acids cis/trans-beta-TOAC and trans-POAC. In particular, this study allowed the assignment of the unknown absolute configurations of the enantiomers of the latter compound.

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. Electron paramagnetic resonance spectra were sensitive to both variables and showed that internally labeled analogues yielded rotational correlation times (tauC) considerably larger than N-terminally labeled ones, evincing the greater freedom of motion of the N-terminus. In TFE, tauC increased due to viscosity effects. Calculation of tau(Cpeptide)/tau(CTOAC) ratios indicated that the peptides acquired more folded conformations. Circular dichroism spectra showed that, except for TOAC1-AII in TFE, the N-terminally labeled analogues displayed a conformational behavior similar to that of the parent peptides. In contrast, under all conditions, the TOAC3 derivatives acquired more restricted conformations. Fluorescence spectra of AII and its derivatives were especially sensitive to the ionization of Tyr4. Fluorescence quenching by the nitroxide moiety was much more pronounced for TOAC3-AII. The conformational behavior of the TOAC derivatives bears excellent correlation with their biological activity, since, while the N-terminally labeled peptides were partially active, their internally labeled counterparts were inactive [Nakaie, C. R., et al., Peptides 2002, 23, 65-70]. The data demonstrate that insertion of TOAC in the middle of the peptide chain induces conformational restrictions that lead to loss of backbone flexibility, not allowing the peptides to acquire their receptor-bound conformation.

Liquid chromatographic enantioseparation of spin-labelled β-amino acids

Direct and indirect high-performance liquid chromatographic (HPLC) methods were developed for the enantioseparation of spin-labelled, cyclic, chiral beta-amino acids containing nitroxide free radicals, trans-3-amino- 1-oxyl-2,2,5,5-tetramethylpyrrolidine-4-carboxylic acid (trans-POAC), cis-4-amino-1-oxyl-2,2,6,6-tetramethylpiperidine-3-carboxylic acid (cis-beta-TOAC) and their N-Fmoc-protected analogues, synthesized in racemic and enantiomerically pure forms. The direct method involved the use of a Chiralcel OD-RH column, while indirect separation was carried out by application of either 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate or (S)-N-(4-nitrophenoxycarbonyl)-phenylalanine methoxyethyl ester as chiral derivatizing agent. Use of 1-fluoro-2,4-dinitrophenyl-5-L-alanine amide (Marfey's reagent) as chiral derivatizing agent failed because of the low of yield of the derivatization reaction. Selection and variation of the mobile phase was restricted by the sensitivity of the spin-labelled amino acids to acidic conditions. Conditions affording the best resolution were found and the differences in separation capability of the methods were noted. The sequence of elution of the enantiomers was determined by different methods and, in the case of the beta-TOAC analogues, the absolute configurations of the enantiomers corresponding to each peak were identified.