Nitroxide- and trityl-based polarizing agents for MAS-DNP: Synthesis, mechanistic insights, and applications

Abstract

Dynamic nuclear polarization (DNP) is a powerful technique for enhancing the sensitivity of nuclear magnetic resonance (NMR) spectroscopy, enabling the investigation of complex materials and biological systems that are otherwise difficult to analyze. In magic-angle spinning (MAS) under solid-state NMR, the cross-effect mechanism employs biradical polarizing agents to transfer electron polarization to surrounding nuclei. However, many polarizing agents remain difficult to synthesize, often suffer from limited aqueous solubility, and the factors governing polarization transfer are not entirely understood. This thesis describes the design, synthesis, and mechanistic investigation of nitroxide- and trityl-based polarizing agents for MAS-DNP. A high-yielding and step-efficient molecular scaffold, AsymPol-Ester, was synthesized as a versatile precursor for developing watersoluble AsymPol-derivatives of both ionic and non-ionic character, functionalized with carboxylate, phosphate, and glucosamine groups. These derivatives were obtained up to seven-fold higher overall yields than previous analogues while maintaining comparable DNP performance. Systematic structural modifications of AsymPols including linker geometry, conformational flexibility and deuteration revealed that DNP efficiency is not only influenced by electron-spin interactions but also by electron-electron coupling. For trityl-nitroxide biradicals, adding spirocycles on nitroxide reduced DNP sensitivity, that benefit bis-nitroxides, unexpectedly revealed a distinct polarization transfer mechanism in trityl-nitroxide biradicals. Further, nuclear-nuclear double resonance (NUDOR) experiments demonstrated that polarization transfer in trityl-nitroxide biradicals is strongly influenced by the large difference in electron-relaxation rates between the two radicals, in contrast to bisnitroxides where electron-electron coupling governs polarization transfer. In addition, tritylnitroxide derivative, TritylTEMPO-X, with six-fold higher aqueous solubility than PyrroTriPol, was synthesized. Finally, application-specific nitroxide radicals were developed for site-directed spin-labeling of biomolecules for local DNP enhancement, and mono-radicals for incorporation into mesoporous organosilica materials.

Mögnun á kjarnskautun (e. dynamic nuclear polarization, DNP) er öflug aðferð til að auka næmni kjarnsegulgreininga (e. nuclear magnetic resonance, NMR), sem gerir kleift að rannsaka flókin efni og líffræðileg kerfi með NMR. Við snúning sýnis um svokallað töfrahorn (e. magic-angle spinning, MAS) í fastefnis-NMR flyst rafeindaskautun frá tvístakeindum (skautunarefnum) til nærliggjandi kjarna fyrir tilstilli kross-áhrifanna (e. cross-effect mechanism). Mörg slík skautunarefni eru erfið í smíði og hafa takmarkaða leysni í vatni. Þessi ritgerð lýsir hönnun og efnasmíði nítroxíð- og trítýl-afleiddra skautunarefna fyrir MAS-DNP til að takast á við þann vanda, og auk þess rannsóknum á flutningi skautunar þeirra yfir á kjarna. Tvístakeindin AsymPol-Ester var smíðuð í háum heimtum og fáum skrefum, til að útbúa vatnsleysanlegar AsymPol sameindir sem innihalda bæði jónaða og hlutlausa hópa, t.a.m. karboxýlöt, fosföt og glúkósamín. Þessar afleiður fengust í allt að sjöfalt hærri heildarheimtum en fyrri skautunarefni og með sambærilegri DNP virkni. Kerfisbundnar breytingar á byggingu AsymPol, þar á meðal lögun tengils, sveigjanleika í stellingum og innleiðing á tvívetni, leiddu í ljós að DNP virkni ræðst ekki aðeins af víxlverkun rafeinda og kjarna, heldur einnig af kúplun rafeindanna. Notkun spíróhringjanítroxíðs í trítýl-nítroxíð tvístakeind leiddi til óvæntrar lækkunar á DNP virkni, ólíkt tvínítroxíðum, sem bendir til annars konar skautunarflutnings í trítýlnítroxíðum. NUDOR (e. nuclear-nuclear double resonance) tilraunir sýndu að skautunarflutningur í trítýl-nítroxíð tvístakeindum verður fyrir sterkum áhrifum þegar mikill munur er á aförvunarhraða rafeindanna, ólíkt tvínítroxíðum. Einnig var trítýl-nítroxíð tvístakeindin TritylTEMPO-X smíðuð, með sexfalt hærri leysni í vatni en PyrroTriPol. Að auki voru nítroxíðstakeindir þróaðar fyrir staðbundna spunamerkingu (e. site-directed spin-labeling) lífsameinda fyrir markvisst DNP í flóknum kerfum, ásamt einstakeindum til að innleiða í meðalholótt lífræn kísilefni.