Sylvain R. A. Marque

Kinetic subtleties of nitroxide mediated polymerization

Due to the academic and industrial interest of Nitroxide Mediated Polymerization (NMP), a lot of investigations have focused on the kinetics of this process. During the last decade, although the simplified kinetic scheme--equilibrium reactions between dormant species (alkoxyamine) and active species (alkyl radicals and nitroxides), propagation reaction of the macro-alkyl radical, and termination reactions--was suitable to predict the main trends at the macromolecular level, it has become obvious that it was not sufficient to describe all the kinetic effects involved in the NMP process. Indeed, like the conventional radical polymerization, NMP should be described as a 3 stage process including initiation, propagation, and self- and cross-termination. These two types of radical polymerization differ by the occurrence during NMP of an activation/deactivation process involving the dormant species in both the initiation and propagation stages. Evidence is provided of the importance of the rate of homolysis of the initiator (alkoxyamines) and of the rate of the first alkyl radical addition onto the monomer for the success of NMP. Thus, the fundamental kinetics of the main reactions involved in NMP as well as side-reactions are also discussed in this tutorial review.

Scavenging of organic C-centered radicals by nitroxides.

Elena G. Bagryanskaya*,†,‡,⊥ and Sylvain R. A. Marque* †N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, Pr. Lavrentjeva 9, Novosibirsk 630090, Russia ‡International Tomography Center, Siberian Branch of the Russian Academy of Sciences, Institutskaya 3A, Novosibirsk 630090, Russia Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia Aix-Marseille Universite,́ CNRS, ICR UMR 7273, case 551, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France

New concepts in molecular imaging: non-invasive MRI spotting of proteolysis using an Overhauser effect switch.

Background Proteolysis, involved in many processes in living organisms, is tightly regulated in space and time under physiological conditions. However deregulation can occur with local persistent proteolytic activities, e.g. in inflammation, cystic fibrosis, tumors, or pancreatitis. Furthermore, little is known about the role of many proteases, hence there is a need of new imaging methods to visualize specifically normal or disease-related proteolysis in intact bodies. Methodology/Principal Findings In this paper, a new concept for non invasive proteolysis imaging is proposed. Overhauser-enhanced Magnetic Resonance Imaging (OMRI) at 0.2 Tesla was used to monitor the enzymatic hydrolysis of a nitroxide-labeled protein. In vitro, image intensity switched from 1 to 25 upon proteolysis due to the associated decrease in the motional correlation time of the substrate. The OMRI experimental device used in this study is consistent with protease imaging in mice at 0.2 T without significant heating. Simulations show that this enzymatic-driven OMRI signal switch can be obtained at lower frequencies suitable for larger animals or humans. Conclusions/Significance The method is highly sensitive and makes possible proteolysis imaging in three dimensions with a good spatial resolution. Any protease could be targeted specifically through the use of taylor-made cleavable macromolecules. At short term OMRI of proteolysis may be applied to basic research as well as to evaluate therapeutic treatments in small animal models of experimental diseases.

Chemically Triggered C–ON Bond Homolysis of Alkoxyamines. Quaternization of the Alkyl Fragment

The C-ON bond homolysis in alkoxyamine 2a can be chemically triggered by the protonation of the 4-pyridylalkyl fragment. The resulting 15-fold increase in k(d) (Chem. Commun. 2011, 47, 4291-4293) was investigated experimentally and theoretically by quaternization of the pyridyl moiety using methylating (MeOTs), acylating (AcCl), and benzylating (PhCH(2)Br) agents as well as by oxidation of the pyridyl moiety into N-oxide and by the formation of a dative bond with BH(3) as a Lewis acid.

Spin-Trapping Evidence for the Formation of Alkyl, Alkoxyl, and Alkylperoxyl Radicals in the Reactions of Dialkylzincs with Oxygen

The reactions of dialkylzincs (Me(2)Zn, Et(2)Zn, and nBu(2)Zn) with oxygen have been investigated by EPR spectroscopy using spin-trapping techniques. The use of 5-diethoxyphosphoryl-5-methyl-1-pyrrroline N-oxide (DEPMPO) as a spin-trap has allowed the involvement of alkyl, alkylperoxyl, and alkoxyl radicals in this process to be probed for the first time. The relative ratio of the corresponding spin-adducts depends strongly on the nature of the R group, which controls the C-Zn bond dissociation enthalpy, and on the experimental conditions (excess of spin-trap compared with R(2)Zn and vice versa). The results have demonstrated that Et(2)Zn and, to a lesser extent, nBu(2)Zn are much better traps for oxygen-centered species than Me(2)Zn. When the dialkylzincs were used in excess with respect to the spin-trap, the concentration of the oxygen-centered radical adducts of DEPMPO was much lower for Et(2)Zn and nBu(2)Zn than for Me(2)Zn. A detailed reaction mechanism is discussed and C-Zn, O-Zn, and O-O bond dissociation enthalpies for the proposed reaction intermediates were calculated at the UB3LYP/6-311++G(3df,3pd)//UB3LYP/6-31G(d,p) level of theory to support the rationale.

Synthesis of highly labile SG1-based alkoxyamines under photochemical conditions.

Highly labile SG1-based alkoxyamines were synthesized using the photodecomposition of both azo compounds and dithiocarbamates. The former method was a straightforward procedure to obtain the alkoxyamines, but a high [azo]/[nitroxide] ratio is needed. The latter method required only a stoichiometric amount of dithiocarbamate and allowed the recovery of the disulfide after irradiation. This enabled combination of the two methods in a process where only 0.75 equiv of azo compound is needed and where sulfurous compounds acted only as intermediates.

Enlarging the Panoply of Site-Directed Spin Labeling Electron Paramagnetic Resonance (SDSL-EPR): Sensitive and Selective Spin-Labeling of Tyrosine Using an Isoindoline-Based Nitroxide

Site-directed spin labeling (SDSL) combined with electron paramagnetic resonance (EPR) spectroscopy has emerged as a powerful approach to study structure and dynamics in proteins. One limitation of this approach is the fact that classical spin labels are functionalized to be grafted on natural or site-directed mutagenesis generated cysteine residues. Despite the widespread success of cysteine-based modification strategies, the technique becomes unsuitable when cysteine residues play a functional or structural role in the protein under study. To overcome this limitation, we propose an isoindoline-based nitroxide to selectively target tyrosine residues using a Mannich type reaction, the feasibility of which has been demonstrated in a previous study. This nitroxide has been synthesized and successfully grafted successively on p-cresol, a small tetrapeptide and a model protein: a small chloroplastic protein CP12 having functional cysteines and a single tyrosine. Studying the association of the labeled CP12 with its partner protein, we showed that the isoindoline-based nitroxide is a good reporter to reveal changes in its local environment contrary to the previous study where the label was poorly sensitive to probe structural changes. The successful targeting of tyrosine residues with the isoindoline-based nitroxide thus offers a highly promising approach, complementary to the classical cysteine-SDSL one, which significantly enlarges the field of applications of the technique for probing protein dynamics.

In vivo high-resolution 3D Overhauser-enhanced MRI in mice at 0.2 T

Overhauser-enhanced MRI (OMRI) offers the potentiality of detecting low-concentrated species generated by specific biological processes. However molecular imaging applications of OMRI need significant improvement in spatial localization. Here it is shown that 3D-OMRI of a free radical injected in tumor-bearing mice can be performed at high anatomical resolution at a constant field. A 30 mm cavity operating at 5.43 GHz was inserted in a C-shaped magnet for proton MRI at 0.194 T. Nude mice with or without brain-implanted C6 rat glioma were positioned in the cavity and injected with TOPCA (1-oxyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole-3-carboxylic acid). OMRI was performed in 3D within several minutes in the brain region without high overheating of the animals. Voxel size was 0.5 × 0.5 × 1 mm³ , providing good delineation of brain regions. Signal amplifications ranged from 2 in tumors to 10 in vessels several minutes after TOPCA injection. Time-course of signal enhancement could be measured by 2D OMRI at 15 s time intervals in a localized thin slice. The method opens the way for molecular imaging of biological activities able to generate OMRI-visible free radicals.

Tyrosine‐Targeted Spin Labeling and EPR Spectroscopy: An Alternative Strategy for Studying Structural Transitions in Proteins

Such a difficulty was recently encountered in the study of a small and flexible chloroplast protein CP12 from the green alga Chlamydomonas reinhardtii by spin-labeling EPR spectroscopy. In this organism, CP12 contains four cysteine residues involved in two disulfide bridges in its oxidized state. Although the introduction of spin labels at the two cysteine residues of the C-terminal disulfide bridge enabled us to identify a new role of the partner protein glyceraldehyde 3-phosphate dehydrogenase (GAPDH), it precluded the direct study of the complex formation GAPDH/CP12. [4] To overcome this difficulty, grafting of the nitroxide probe to residues other than cysteines is required. One strategy using a genetically encoded unnatural amino acid has been recently proposed. [5] The incorporation of such unnatural amino acids relies, however, on a rather complex strategy involving an orthogonal tRNA/aminoacyl-tRNA synthetase pair specific for the unnatural amino acid. As such a strategy is difficult to set up, we propose an alternative method consisting of selectively targeting residues other than cysteines with a nitroxide probe. Bioconjugation of small molecules to protein residues is very challenging, and several reactions have recently been proposed to target specific residues selectively. [6] In particular, efforts have been paid to modify the aromatic amino acid side chains of tryptophan [7] and tyrosine. [8] Among them, a threecomponent Mannich-type reaction has been developed that allows the modification of tyrosine under mild, biocompatible, and metal-free conditions. [8a] Inspired by these recent studies, we present the selective grafting of a nitroxide probe to tyrosine by using the Mannich-type reaction on CP12, a protein bearing only one natural tyrosine residue. This unique tyrosine residue, located at position 78 in the sequence of a total of 80 amino acids, makes this protein an ideal candidate for demonstrating the feasibility of tyrosine-targeted spin

Regio-Defined Amino[5]Oxa- and Thiahelicenes: A Dramatic Impact of the Nature of the Heteroatom on the Helical Shape and Racemization Barriers

The present approach to heterohelicenes provides original [5]oxa- and thiahelicenes, where both oxygen and sulfur atoms are located at the end of the inner helix. Quantum chemical calculations are carried out to determine the pathway for interconversion between two enantiomers and demonstrate that the energy barrier is strongly dependent on the nature of the heteroatom present on the helical shape.