Olga Yu. Rogozhnikova

Physiological-Temperature Distance Measurement in Nucleic Acid using Triarylmethyl-Based Spin Labels and Pulsed Dipolar EPR Spectroscopy

Resolving the nanometer-scale structure of biomolecules in natural conditions still remains a challenging task. We report the first distance measurement in nucleic acid at physiological temperature using electron paramagnetic resonance (EPR). The model 10-mer DNA duplex has been labeled with reactive forms of triarylmethyl radicals and then immobilized on a sorbent in water solution and investigated by double quantum coherence EPR. We succeeded in development of optimal triarylmethyl-based labels, approach for site-directed spin labeling and efficient immobilization procedure that, working together, allowed us to measure as long distances as ~4.6 nm with high accuracy at 310 K (37 °C).

Room-Temperature Electron Spin Relaxation of Triarylmethyl Radicals at the X- and Q-Bands.

Triarylmethyl radicals (trityls, TAMs) represent a relatively new class of spin labels. The long relaxation of trityls at room temperature in liquid solutions makes them a promising alternative for traditional nitroxides. In this work we have synthesized a series of TAMs including perdeuterated Finland trityl (D36 form), mono-, di-, and triester derivatives of Finland-D36 trityl, the deuterated form of OX63, the dodeca-n-butyl homologue of Finland trityl, and triamide derivatives of Finland trityl with primary and secondary amines attached. We have studied room-temperature relaxation properties of these TAMs in liquids using pulsed electron paramagnetic resonance (EPR) at two microwave frequency bands. We have found the clear dependence of phase memory time (Tm ∼ T2) on the magnetic field: room-temperature Tm values are ∼1.5-2.5 times smaller at the Q-band (34 GHz, 1.2 T) than at the X-band (9 GHz, 0.3 T). This trend is ascribed to the contribution from g-anisotropy that is negligible at lower magnetic fields but comes into play at the Q-band. In agreement with this, the difference between T1 and Tm becomes more pronounced at the Q-band than at the X-band due to increased contributions from incomplete motional averaging of g-anisotropy. Linear dependence of (1/Tm - 1/T1) on viscosity implies that g-anisotropy is modulated by rotational motion of the trityl radical. On the basis of the analysis of previous data and results of the present work, we conclude that, in the general situation where the spin label is at least partly mobile, the X-band is most suitable for application of trityls for room-temperature pulsed EPR distance measurements.

Molecular diffusion in porous media by PGSE ESR

Diffusion in porous media is a general subject that involves many fields of research, such as chemistry (e.g. porous catalytic pallets), biology (e.g. porous cellular organelles), and materials science (e.g. porous polymer matrixes for controlled-release and gas-storage materials). Pulsed-gradient spin-echo nuclear magnetic resonance (PGSE NMR) is a powerful technique that is often employed to characterize complex diffusion patterns inside porous media. Typically it measures the motion of at least approximately 10(15) molecules occurring in the milliseconds-to-seconds time scale, which can be used to characterize diffusion in porous media with features of approximately 2-3 mum and above (in common aqueous environments). Electron Spin Resonance (ESR), which operates in the nanoseconds-to-microseconds time scale with much better spin sensitivity, can in principle be employed to measure complex diffusion patterns in porous media with much finer features (down to approximately 10 nm). However, up to now, severe technical constraints precluded the adaptation of PGSE ESR to porous media research. In this work we demonstrate for the first time the use of PGSE ESR in the characterization of molecular restricted diffusion in common liquid solutions embedded in a model system for porous media made of sub-micron glass spheres. A unique ESR resonator, efficient gradient coils and fast gradient current drivers enable these measurements. This work can be further extended in the future to many applications that involve dynamical processes occurring in porous media with features in the deep sub-micron range down to true nanometric length scales.

A triarylmethyl spin label for long-range distance measurement at physiological temperatures using T1 relaxation enhancement

Site-directed spin labeling (SDSL) in combination with electron paramagnetic resonance (EPR) spectroscopy has become an important tool for measuring distances in proteins on the order of a few nm. For this purpose pairs of spin labels, most commonly nitroxides, are site-selectively introduced into the protein. Recent efforts to develop new spin labels are focused on tailoring the intrinsic properties of the label to either extend the upper limit of measurable distances at physiological temperature, or to provide a unique spectral lineshape so that selective pairwise distances can be measured in a protein or complex containing multiple spin label species. Triarylmethyl (TAM) radicals are the foundation for a new class of spin labels that promise to provide both capabilities. Here we report a new methanethiosulfonate derivative of a TAM radical that reacts rapidly and selectively with an engineered cysteine residue to generate a TAM containing side chain (TAM1) in high yield. With a TAM1 residue and Cu(2+) bound to an engineered Cu(2+) binding site, enhanced T1 relaxation of TAM should enable measurement of interspin distances up to 50Å at physiological temperature. To achieve favorable TAM1-labeled protein concentrations without aggregation, proteins are tethered to a solid support either site-selectively using an unnatural amino acid or via native lysine residues. The methodology is general and readily extendable to complex systems, including membrane proteins.

Triarylmethanols with Bulky Aryl Groups and the NOESY/EXSY Experimental Observation of a Two-Ring-Flip Mechanism for the Helicity Reversal of Molecular Propellers

Triarylmethanols - the direct precursors of persistent trityl radicals - are racemic mixtures of chiral three-bladed molecular propellers. Depending on bulkiness of aryl groups they exhibit various liabilities to interconversion, the half- life time of room temperature racemization varying in a range between 8.4 hours and 1.32 years. NOESY/EXSY experiment performed on two representative models strongly supports the two-ring flip mechanism for the configurational interchange.

Trityl-based alkoxyamines as NMP controllers and spin-labels

Recently, new applications of trityl-nitroxide biradicals were proposed. In the present study, attachment of a trityl radical to alkoxyamines was performed for the first time. The rate constants kd of C-ON bond homolysis in these alkoxyamines were measured and found to be equal to those for alkoxyamines without trityl. The electron paramagnetic resonance (EPR) spectra of the products of alkoxyamine homolysis (trityl-TEMPO and trityl-SG1 biradicals) were recorded, and the corresponding exchange interactions were estimated. The decomposition of trityl-alkoxyamine showed more than an 80% yield of biradicals, meaning that the C-ON bond homolysis is the main reaction. The suitability of these labelled initiators/controllers for polymerisation was exemplified by means of successful nitroxide-mediated polymerisation (NMP) of styrene. Thus, this is the first report of a spin-labelled alkoxyamine suitable for NMP.

Triarylmethyl Radicals: An EPR Study of 13C Hyperfine Coupling Constants

Abstract Triarylmethyl (TAM) radicals are widely used in electron paramagnetic resonance (EPR) spectroscopy as spin labels and in EPR imaging as spin probes for in vivo oxymetry. One of the key advantages of TAMs is the extremely narrow EPR line, especially in case of deuterated analogs (~2.5 μT). Another advantage is their slow spin relaxation even at physiological temperatures; in particular, this characteristic enables the use of pulsed dipolar EPR methods for distance measurements in biomolecules. In this study, a large series of TAM radicals and their deuterated analogs was synthesized, and the corresponding spectroscopic parameters including 13C hyperfine constants were determined for the first time. We observed negligible dependence of 13C hyperfine constants on the solvent or on the structure or number of substituents at para-C atoms of the aromatic rings. In addition, we demonstrated that 13C signals at natural abundance can be used for successful distance measurements at room temperature by pulsed electron double resonance (PELDOR or DEER).

A radical containing injectable in-situ-oleogel and emulgel for prolonged in-vivo oxygen measurements with CW EPR

ABSTRACT Molecular oxygen, reactive oxygen species and free radicals derived from oxygen play important roles in a broad spectrum of physiological and pathological processes. The quantitative measurement of molecular oxygen in tissues by electron paramagnetic resonance (EPR) has great potential for understanding and diagnosing a number of diseases, and for developing and guiding therapies. This requires improvements in the free radical probe systems that sense and report molecular oxygen levels in vivo. We report on the encapsulation of existing free radical probes in lipophilic gel implants: an in‐situ‐oleogel and an emulgel, based only on well‐known, safe excipients for the incorporation of lipophilic and hydrophilic radicals, respectively. The EPR signals of encapsulated radicals were not altered compared to dissolved radicals. The high solubility of oxygen in lipophilic solvents enhanced oxygen sensitivity. The gels extended the lifetime of the radicals in tissues from tens of minutes to many days, simplifying studies with extended series of measurements. The encapsulated radicals showed a good in vivo response to changes in oxygen supply and seem to circumvent concerns from toxicity of the radical probes. These gels simplify the development of new oxygen‐sensitive free radical probes for EPR oximetry by making their in vivo stability, persistence and toxicity a function of the encapsulating gel and not a set of additional requirements for the free radical probe. HighlightsHydrophilic and lipophilic trityl radicals were encapsulated in gels.The gel constituents were known, safe pharmaceutical excipients.The encapsulation of radicals did not alter their linewidths.The gel formulations were shown to be suitable for local oxygen measurements in vivo.EPR signals were detectable up to three weeks after injection. Graphical abstract Figure. No Caption available.

A straightforward and convenient synthesis of Cbz-protected 2-(1-aminoalkyl)oxazole-5-carboxylates

Cbz-protected 2-(l-aminoalkyl)oxazole-5-carboxylates are readily obtainable by a two-step enantioselective sequence involving reaction of benzyloxycarbonyl protected amino acids with α-chloro-β-ketoesters and subsequent cyclization of the resulting acyloxyketones to required oxazoles upon treatment with neat ammonium trifluoacetate.