Synthesis and EPR-spectroscopic characterization of the perchlorotriarylmethyl tricarboxylic acid radical (PTMTC) and its ¹³C labelled analogue (13C-PTMTC)

Abstract

A hydrophilic tris(tetrachlorotriaryl)methyl (tetrachloro-TAM) radical labelled 50% with C at the central carbon atom was prepared. The mixture of isotopologue radicals was characterised by continuous wave and pulsed X-band electron paramagnetic spectroscopy (EPS). For the pharmaceutical and medical applications planned, the quantitative influence of oxygen, viscosity, temperature and pH on EPR line widths was studied in aqueous buffer, DMSO, water-methanol and water-glycerol mixtures. Under in vivo conditions, pH can be disregarded. There is a clear oxygen dependence of the width of theC isotopologue single EPR line in aqueous solutions while changes in rotational motion (viscosity) are observable only in the doublet lines of the central carbon of the C isotopologue. The tetrachloro-TAM proved to be very stable as a solid. Its thermal decay was determined quantitatively by thermal annealing. Towards ascorbic acid as a reducing agent and towards an oocyte cell extract it had a half-life of approx. 60 and 10 min. Thus for in vivo applications, 50%C tetrachloro-TAMs are suitable for selective and simultaneous oxygen and macroviscosity measurements in a formulation, e.g. nanocapsules.

Perchlorinated Triarylmethyl Radical 99% Enriched 13C at the Central Carbon as EPR Spin Probe Highly Sensitive to Molecular Tumbling

“…Elewa et al reported a 50% enriched 13 C 1 -PTMTC which exhibits a doublet EPR spectrum in solution with an isotropic hyperfine coupling to the 13 C 1 of A iso = 83.5 MHz (29.82 G) 22 . This value is significantly higher than for the 13 C 1 -dFT analog (A iso =65.38 MHz, 23.35G) [10][11] which indicates a higher spin density at the central carbon for PTMTC than for the Finland trityl derivative.…”

“…However, the anisotropic values of the hyperfine interaction were not reported. 22 To get insight into the anisotropy of the 13 C 1 hyperfine interaction , we performed a density functional theory calculation at the B3LYP level using the IGLO-III basis sets, specially tailored for the calculation of EPR properties (Table 1 and SI). The calculated isotropic hyperfine splitting of A iso (C 1 ) = 77.6 MHz is in good agreement with the experimental value of 83.5 MHz.…”

“…Therefore, we synthesized 99% enriched 13 C 1 -PTMTC in a four steps sequence as depicted in Scheme 1. The synthetic strategy uses the procedures reported for PTMTC with minimal modifications and the use of a commercially available chloroform- 13 C, 99 atom% 13 C to label the central carbon with 99% 13 C. 22,[25][26] The sodium salt 13 C 1 -PTMTC was used throughout the study to ensure high aqueous solubility. Since ion pairing is expected to be weak in aqueous solution and the pKa for chlorinated benzoic acids are low, the dominant species in solution is the carboxylate anion.…”

“…12 Perchlorinated triarylmethyl radicals constitute another class of trityls used in biomedical EPR to measure oxygen in vivo [13][14] , detect superoxide [15][16] or hydroxyl radical 17 , and as hyperpolarizing agents for dynamic nuclear polarization (DNP) [18][19] or in material sciences. [20][21] A perchlorotriarylmethyl tricarboxylic acid radical (PTMTC) 50% enriched 13 C 1 has been reported recently by Elewa et al 22 The authors showed that the linewidths of the doublet arising from the hyperfine interaction with the 13 C 1 were also strongly sensitive to viscosity. Moreover, the viscosity-induced line broadening did not affect the two lines equally at X-Band, with a more substantial effect on the low-field peak.…”

A 13C‐Labeled Triarylmethyl Radical as an EPR Spin Probe Highly Sensitive to Molecular Tumbling

“…However, the anisotropy of the hyperfine splitting and the effect of viscosity on t R were not provided. [21] Dissolved oxygen is also known to broaden the linewidth of TAM radicals through the Heisenberg spin exchange. However, the sensitivity of TAMs to oxygen is much smaller.…”