Photolabile Protecting Groups for Nitroxide Spin Labels

Abstract

Nitroxide spin labels can be protected against critical conditions of DNA/RNA or peptide synthesis by reduction and alkylation with light‐sensitive groups such as nitrobenzyl‐ or aminocoumarins. High chemical stability qualifies tetraethylisoindoline 5 and, with some restrictions, (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxy (TEMPO) precursor 4 as building blocks for spin‐labeled biopolymers. The yield of recovered nitroxides (80–95 %) is sufficient for PELDOR experiments. A protected TEMPO phosphoramidite was successfully used to synthesize a short spin‐labeled DNA with high yield and purity.

Protein Spin Labeling with a Photocaged Nitroxide Using Diels–Alder Chemistry

“…Moreover, we aimed to address nitroxide stability by using a protection strategy. So far, alkylation, silylation, acylation, and photoremovable protecting groups (PPGs) have been demonstrated to protect nitroxides and to release them as needed. In particular, photoirradiation for deprotection is interesting because it enables spatial and temporal control over the release of functional groups.…”

“…Their application for the protection of nitroxide spin labels during oligonucleotide synthesis was introduced by Seven et al. in 2014 and continued by Weinrich et al…”

“…We assessed the performance of our protection strategy in biological environments. We reasoned that reducing conditions could potentially intervene with the spontaneous oxidation step, converting the irradiation‐derived hydroxylamine into a nitroxide . To this end, we chose E. coli lysate because it contained relevant cell components and provided reducing conditions.…”

A Cytidine Phosphoramidite with Protected Nitroxide Spin Label: Synthesis of a Full‐Length TAR RNA and Investigation by In‐Line Probing and EPR Spectroscopy

“…A phosphoramidite building block related to 6 could be incorporated into DNA under standard conditions without destruction of the protected label. After photolysis of the coumarin group, the nitroxide radical was formed by spontaneous air oxidation . More recently, we have optimized the synthesis and adapted it to ribonucleotides.…”

Synthesis of a Cytidine Phosphoramidite with Protected Nitroxide Spin Label for EPR Experiments with RNA

“…Unfortunately, the amine and not the nitroxide was found as the main product of photolysis. This observation prompted us to investigate the chemical stability of protected nitroxides and their photochemical release in a more systematic way . Finally, we prepared the coumarin‐protected deoxyphosphoramidite 2 and used it for the synthesis of a short spin‐labeled DNA strand.…”