Dynamic Nuclear Polarization with Vanadium(IV) Metal Centers

•Harnessed V4+ metal centers as polarizing agents for dynamic nuclear polarization•Determined spin-diffusion barrier size using V4+-to-1H polarization transfer•Traced pathways V4+-1H spin “rulers”•Direct of 1H nuclei located 12.6 Å from the center Dynamic (DNP) offers a strategy to amplify inherently insensitive magnetic resonance (NMR) signal by orders magnitude, dramatically enhancing NMR studies molecules and materials. To date, most DNP have relied on exogenous (PAs) with narrow electron paramagnetic (EPR) lines. Expanding catalog PAs at core active sites in enzymes functional materials will offer opportunity illuminate local structure around these during chemical transformations. This study is first demonstration broad EPR lines enhance detect discrete distances center. These results pave way toward endogenous which metals play key roles. harnesses large spins increase sensitivity. expands scope beyond its traditional focus hyperpolarizing solvent network (PAs). We introduce positioned set vanadyl complexes tunable distances. traced transfer spins, specifically differentiating between direct V4+-1Hs spin-diffusion-mediated bulk buildup illuminated effect distance processes. deepen our understanding expand broad-line transition metals. establishes crucial steps employing strategically conceptual framework hyperfine spectroscopy that merges both spatial diagnosis target spins. Nuclear spectroscopy, widely used tool elucidating fundamental chemical, structural, dynamical information materials, limited poor broadly applicable hyperpolarization method magnitudes, relying highly polarized (e) surrounding (n). In typical experiment, source unpaired spins—known agent (PA)— mixed sample 1H-rich glassing matrix. Microwave (?w) irradiation near frequency PA can drive Current state-of-the-art methodologies already transformed fields structural biology science.1Lilly Thankamony A.S. Wittmann J.J. Kaushik M. Corzilius B. sensitivity enhancement modern solid-state NMR.Prog. Nucl. Magn. Reson. Spectrosc. 2017; 102–103: 120-195Crossref PubMed Scopus (237) Google Scholar nitrogen-centered nitroxide or carbon-centered trityl radicals are nearly exclusively PAs, owing their stability, solubility, molecular geometry, relatively long relaxation time, an g factor 2.0, matching free electron.1Lilly However, exogenously introduced do not serve report specific locales A huge exists use centers, intrinsic system interest, PAs. electronically symmetric metals, such Gd3+, Mn2+, Cr3+, central bands been high-field (> 5 T) DNP.2Corzilius Smith A.A. Barnes A.B. Luchinat C. Bertini I. Griffin R.G. High-field high-spin ions.J. Am. Chem. Soc. 2011; 133: 5648-5651Crossref (94) Scholar, 3Kaushik Bahrenberg T. Can T.V. Caporini M.A. Silvers R. Heiliger J. Schwalbe H. Gd(iii) Mn(ii) polarization: small chelate applications site-directed labeling proteins.Phys. 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Kim electrode materials.J Phys Chem C Nanomater Interfaces. 2020; 124: 7082-7090Crossref (15) many ions, Ni+, Cu2+, Ni3+, V4+, present function catalytic compounds, metalloenzymes, considered inaccessible due wide values significantly shifted 2.0. capabilities utilize would provide significant step DNP-enhanced centers. Attaining necessitates rigorous pathways. Fundamentally, comprises two stages: first, PA-nuclear interactions; second, diffusion, exchange energy nearby, unpolarized propagate nuclei. processes, detected spectrum, influenced effects, (PRE)9Bloembergen N. On interaction lattice.Physica. 1949; 15: 386-426Crossref (713) 10Blumberg W.E. spin-lattice caused impurities.Phys. Rev. 1960; 119: 79-84Crossref (329) 11Takahashi Hanson van Tol Sherwin M.S. Awschalom D.D. Quenching decoherence diamond through bath polarization.Phys. Lett. 2008; 101047601Crossref (181) contact pseudo-contact shifts (CS PCS),12McConnell H.M. Chesnut D.B. 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Adv. 5eaax2743Crossref (20) The exact critical parameter determining pathway rate, it determines location nearest conduit remote Hence, rate depends closest metal-nuclear should gradient rates. Many models rely knowledge rate,19Pinon A.C. resonance. EPFL, 2018https://infoscience.epfl.ch/record/256344?ln=enGoogle 20Smith Maly Solid pathways.J. 2012; 136015101Crossref (85) 21Wittmann Eckardt Harneit W. Electron-driven supports crossing MAS DNP.Phys. 2018; 20: 11418-11429Crossref be aided experimentally validated electron-nuclear rates determine propagation modulate DNP. this study, we demonstrate viability 1H-NMR localized protons 6.9 designed series deliberately installed 1H-containing trimethylene groups varying otherwise nuclear-spin-free ligand backbone.22Graham M.J. Yu C.J. Krzyaniak M.D. Wasielewski M.R. Freedman D.E. Synthetic approach decoherence.J. 139: 3196-3201Crossref (43) “rulers” allow systematic effectiveness 1Hs conduct process wide-line broadband transitions. versatile unique (to date) instrument powered tunable-frequency (193–201 GHz) microwave waveform generation (AWG) enabled experiments. spectra curves quantified radius 4.0–6.6 revealed directly position relative barrier. Crucially, to, detection of, away via interaction. work paves cofactors. distinguish local, select order achieve global enhancement, dub novel category experiments spectroscopy. rulers, i.e., controlled average (RV-H), shown Figure 1A, RV-H = 4.0, 6.6, 9.3, 1–4, respectively. formulas (Ph4P)2[VO(C3H6S2)2] (1), (Ph4P)2[VO(C5H6S4)2] (2), (Ph4P)2[VO(C7H6S6)2] (3), (Ph4P)2[VO(C9H6S8)2] (4) tetraphenylphosphonium cations were dissolved 99.5% deuterated dimethylformamide (DMF). field-swept echo-detected recorded ?w (??w) 240 GHz sweeping field 8.4 9 T K (see S1). principle components coupling (A) ([Ar]3d1, S 1?2) 100% abundant 51V isotope (I 7?2) extracted fitting EasySpin.23Stoll Schweiger EasySpin, comprehensive software package spectral simulation analysis EPR.J. 2006; 178: 42-55Crossref (3750) 1B depicts line shapes simulated based fitted parameters. tensor found well resolved high (Table S1 S2) agreement previously reported same X-band CW analysis.22Graham span more than 3 GHz, broader compared nitroxide-based 0.6–1 4 K. spectrum inhomogenously broadened result g-anisotropy corroborated data EasySpin (Figure S1; Table While broadening spanning 100s GHz,24Barra A.-L. Gräslund Andersson K.K. very (VHF-EPR) proteins models.in: Grinberg O.Y. Berliner L.J. Very High Frequency (VHF) ESR/EPR. Springer, 2004: 145-163Crossref only narrow-line (line width < 800 MHz) utilized current range commercial instruments. context, widths exceeding 1 categorized previous complexes, was 4.0 6.6 Å. Here, explore DNP, under conditions (at detection, influence process. whether viable agents, investigated profiles 1–4. EPR-NMR pulse sequence, varied over 2A). factors calculating ratio ? (SON ? SOFF)/SOFF, where SON SOFF intensities ?w-on ?w-off equal times, 2B shows chirped trains. across asymmetric maximum positive negative positions separated ? GHz. exhibited larger enhancements 192.4 191.5 general features observed monochromatic exception lower overall (?CW-DNP, S2). insight into mechanism systems. display sharper intensity slope higher end longer tail low frequencies, reflecting dispersion inhomogeneous 1B). indicates underlying differential (SE).25Wenckebach W.T. effect.Appl. 227-235Crossref conventional SE, dipolar interactions permit forbidden double quantum zero excitations lead characteristic twice Larmor frequency. Differential SE gives superposition whose frequencies inhomogeneously line. cases, shape (width symmetry) dominated shape, case resulted profile. basic feature could replicated numerical simulations difference enhancements, according each point, calculated respective compute net profile reconstructed S3). measured (??w 191.30, 191.50, 191.35, 191.40 respectively) yielded ?Chirp-DNP 1, 19, 12, 1–4 pulses 3, line). ?CW-DNP 0.5, 1.7, 1.8 respectively dashed-lines). Corresponding collected similar S4). 2 attributed solubility DMF (77.2 mM),26We hypothesize particularly optimal balance non-polar nature ligands charge complex. With linkers 4, nonpolar those dominant inhibit DMF. ionic nature, combined 2– may render ionic, hence reducing Taken together, propose complex, thereby DMF.Google resulting sample. demonstrate, pulses. performance boost obtained transitioning chirp train has recently biradicals.27Kaminker Han Amplification 200 shaping saturation profile.J. 9: 3110-3115Crossref (14) Broadband trains access greater population constructively participate enhancement. spread out density range. As result, engage that, hence, enhancements. Thus, shaped fully saturate transition, successfully generate gain ?Chirp-DNP/?CW-DNP > samples reaching up ?10 (with ?33 ?3.5 region enhancement). By implementing pulses, traditionally believed realize need understand Specifically, know characteristics agent. Because fundamentally interactions, systematically investigate modular interactions. Essential investigation identifying distinct participating Based preparation, enhanced originate three different sources: moieties (PPh4+) counterion, 0.5% DMF-d7 solvent. begin investigation, turned 3), show common complexes. clearly indicate presence widths. signals identify 1–3, inter-pulse delay (?) 30–200 ?s S5) observe selectively suppressed increasing ?. ? 50 (cyan, line) (magenta, dashed 4A. 200-?s delay, single component. constituting component shorter transverse times (T2n) deconvolution acquired peak centered ?5 ppm upfield ?14 S7). combination T2n, upfield-shifted suggests experience (PRE PCS) signal. further reinforced solution-state wherein peaks (centered 2.22 ppm) (counterion 7.77 7.91, 2.80, 2.64 S6. temperatures also ½ vanadium complexes.28Morse Hendrickson D.N. Rauchfuss T.B. Wilson S.R. Highly oxidizing organometallics: physicochemical characterization (methylcyclopentadienyl)vanadium(IV) trichloride related vanadium(III) titanium(III) derivatives.Organometallics. 1988; 7: 496-502Crossref assigned broad-signal T2n covalently attached referred “complex protons,” “solvent protons” include counter discount possibility counterion being too close leading performed additional pulsed (known echo envelope modulation [ESEEM]) V4+-31P couplings (ESEEM sensitive 3–7 range) 31P PPh4 (data shown). no modulations observed, indicating counterions V4+. observation consistent solvation shell sufficiently ion, far enough Moreover, absent confirms absence 1–3 quenching strong PRE 4. contrast, visible identity constitute confirmed, then proceeded 2–4 recorded, 4B, including Note shown, there 3). test total concentration affected curve, concentrations 2, 77.2 mM saturation) 13 (comparable complexes) measured. All stretched exponential:I=I0[1?e?(tTDNP)n]where I0 saturation, TDNP time constant buildup, n stretch factor. value (? 1) provides about expect mono-exponential process, 1. Should contribute multi-exponential lowers 0.5.29Tse Hartmann without diffusion.Phys. 1968; 21: 511-514Crossref (157) TDNP, n, fits given 1.Table 1Fitted Parameters Proton Buildup Curves Chirped ExperimentsComplex#TDNP (s)NI04 (complex)18.2 ± 1.50.60 0.031.003 0.0204 (solvent)47.7 6.50.79 0.061.021 0.049388.0 4.00.78 0.011.176 0.0182142.7 28.30.80 0.091.053 0.0722 (77 mM)120.4 9.00.93 0.021.253 0.048 Open table new tab Between types nuclei, shortest smallest 2–4. exhibit stretching (n 0.6) we, therefore, attribute diffusion. Polarization involves single-step expected faster stochastic, multi-step nuclear-spin-diffusion