Zn NMR Research Articles

Overcoming challenges in 67Zn NMR: a new strategy of signal enhancement for MOF characterization.

67 Zn solid-state NMR suffers from low sensitivity, limiting its ability to probe the Zn2+ surroundings in MOFs. We report a breakthrough in overcoming challenges in 67Zn NMR. Combining new cryogenic MAS probe technology and performing NMR experiments at a high magnetic field results in remarkable signal enhancement, yielding enhanced information for MOF characterization.

Multi-technique structural analysis of zinc carboxylates (soaps).

A series of medium- and long-chain zinc carboxylates (zinc octanoate, zinc nonanoate, zinc decanoate, zinc undecanoate, zinc dodecanoate, zinc pivalate, zinc stearate, zinc palmitate, zinc oleate, and zinc azelate) was analyzed by ultra-high-field 67Zn NMR spectroscopy up to 35.2 T, as well as 13C NMR and FTIR spectroscopy. We also report the single-crystal X-ray diffraction structures of zinc nonanoate, zinc decanoate, and zinc oleate-the first long-chain carboxylate single-crystals to be reported for zinc. The NMR and X-ray diffraction data suggest that the carboxylates exist in three distinct geometric groups, based on structural and spectroscopic parameters. The ssNMR results presented here present a future for dynamic nuclear polarization (DNP)-NMR-based minimally invasive methods for testing artwork for the presence of zinc carboxylates.

Understanding the Solvation-Dependent Properties of Cyclic Ether Multivalent Electrolytes Using High-Field NMR and Quantum Chemistry.

Efforts to expand the technological capability of batteries have generated increased interest in divalent cationic systems. Electrolytes used for these electrochemical applications often incorporate cyclic ethers as electrolyte solvents; however, the detailed solvation environments within such systems are not well-understood. To foster insights into the solvation structures of such electrolytes, Ca(TFSI)2 and Zn(TFSI)2 dissolved in tetrahydrofuran (THF) and 2-methyl-tetrahydrofuran were investigated through multi-nuclear magnetic resonance spectroscopy (17O, 43Ca, and 67Zn NMR) combined with quantum chemistry modeling of NMR chemical shifts. NMR provides spectroscopic fingerprints that readily couple with quantum chemistry to identify a set of most probable solvation structures based on the best agreement between the theoretically predicted and experimentally measured values of chemical shifts. The multi-nuclear approach significantly enhances confidence that the correct solvation structures are identified due to the required simultaneous agreement between theory and experiment for multiple nuclear spins. Furthermore, quantum chemistry modeling provides a comparison of the solvation cluster formation energetics, allowing further refinement of the preferred solvation structures. It is shown that a range of solvation structures coexist in most of these electrolytes, with significant molecular motion and dynamic exchange among the structures. This level of solvation diversity correlates with the solubility of the electrolyte, with Zn(TFSI)2/THF exhibiting the lowest degree of each. Comparisons of analogous Ca2+ and Zn2+ solvation structures reveal a significant cation size effect that is manifested in significantly reduced cation–solvent bond lengths and thus stronger solvent bonding for Zn2+ relative to Ca2+. The strength of this bonding is further reduced by methylation of the cyclic ether ring. Solvation shells containing anions are energetically preferred in all the studied electrolytes, leading to significant quantities of contact ion pairs and consequently neutrally charged clusters. It is likely that the transport and interfacial de-solvation/re-solvation properties of these electrolytes are directed by these anion interactions. These insights into the detailed solvation structures, cation size, and solvent effects, including the molecular dynamics, are fundamentally important for the rational design of electrolytes in multivalent battery electrolyte systems.

Structure and Fragility of Zn-phosphate glasses: Results from multinuclear NMR spectroscopy and calorimetry

The structure of (ZnO)x(P2O5)100-x glasses with 35≤x ≤ 67 is investigated using 31P and ultra-high field 67Zn NMR spectroscopy. The 31P NMR results confirm a nearly binary Q-species distribution. The progressive decrease in the connectivity of the phosphate network upon addition of ZnO is manifested in a monotonic drop in Tg in the composition range 35≤x ≤ 55. The 67Zn NMR spectra indicate that the average Zn-O coordination number varies between 4 and 6 and increases progressively with increasing Zn content. The resulting increase in the connectivity of the Zn-O-P network compensates for the depolymerization of the P-O-P network, which is reflected in a reversal in the compositional trend in Tg for x > 55. In contrast to Tg, the fragility index m shows negligible composition dependence until x∼50, beyond which it increases rapidly with increasing Zn content in response to the progressive shortening of the Q2 chains.

Mere Anarchy is Loosed: Structural Disorder in Cu2Zn1–xCdxSnS4

Substitution of Cd into the photovoltaic material Cu2ZnSnS4 has been previously shown to improve power conversion efficiencies, and although various models for the structural disorder have been proposed, full characterization of Cu2Zn1–xCdxSnS4 has been hampered by a lack of understanding about the local structure around individual atoms. Through a combination of powder X-ray diffraction and solid-state NMR spectroscopy (119Sn, 113Cd, 63Cu, and 67Zn), the detailed structures of the entire series Cu2Zn1–xCdxSnS4 (x = 0–1), prepared as bulk samples by reaction of the elements at 800 °C, have been examined. The ordered kesterite structure in space group I4̅ of the end-member Cu2ZnSnS4 (x = 0) was retained up to Cu2Zn0.7Cd0.3SnS4 (x = 0.3) and the stannite structure in space group I4̅2m from Cu2Zn0.6Cd0.4SnS4 (x = 0.4) to the end-member Cu2CdSnS4 (x = 1). Although the structural transition at x = 0.4 was confirmed, 113Cd NMR spectroscopy reveals the coexistence of phases with I4̅ and I4̅2m structures in the intermediate region (x = 0.3–0.4), evidence for which was previously unattainable from XRD data alone. The 119Sn NMR spectra indicated the presence of Sn sites surrounded within distinct local chemical environments imparted by the distribution of Zn and Cd atoms disordered within the second coordination sphere, which could be resolved by their isotropic chemical shifts and quantified by their peak areas. Ultrahigh-field (21.1 T) 63Cu NMR spectroscopy revealed the two sets of Cu sites in the I4̅ structure expected for the lightly Cd-substituted members (up to Cu2Zn0.8Cd0.2SnS4) and one set of Cu sites in the I4̅2m structure for the more heavily substituted members. The balance of evidence from the comparison of experimental and DFT-computed 63Cu and 67Zn NMR spectra suggests that Cu/Zn disorder does not occur. Instead, Cd substitution in Cu2Zn1–xCdxSnS4 proceeds exclusively through disorder of Zn and Cd atoms, at least in the bulk samples examined.

Structural and functional characterization of a putative carbonic anhydrase from Geobacillus kaustophilus reveals its cambialistic function

Carbonic anhydrases (CA) are the most ubiquitous ancient zinc metalloenzymes known. Here we report the structural and functional analysis of a hypothetical protein GK2848 from Geobacillus kaustophilus. The analysis revealed that it belongs to the γ-class of CA (termed as Cag). Only a limited number of γ-class CA’s have been characterized till date. Interestingly Cag contains magnesium at its active site instead of a traditional zinc ion. Based on the structural and sequence comparison with similar γ-CA’s the putative active site residues of Cag were identified. This analysis revealed that an important catalytic residue and a proton shuttle residue (Glu62 and Glu84 respectively) of Cam (previously characterized γ-CA from Methanosarcina thermophila) are absent in Cag, however certain other active site residues are conserved both in Cag and Cam. This suggests that Cag uses a different set of residues for the reversible hydration of CO2 to HCO3− when compared with Cam. Inductively Coupled Plasma – Optical Emission Spectrometry (ICP-OES) and 25Mg and 67Zn NMR studies on Cag and its mutants revealed that either Mg or Zn can occupy the active site which suggests the cambialistic nature of the enzyme.

Ultrahigh-field 67Zn NMR reveals short-range disorder in zeolitic imidazolate framework glasses.

The structure of melt-quenched zeolitic imidazole framework (ZIF) glasses can provide insights into their glass-formation mechanism. We directly detected short-range disorder in ZIF glasses using ultrahigh-field zinc-67 solid-state nuclear magnetic resonance spectroscopy. Two distinct Zn sites characteristic of the parent crystals transformed upon melting into a single tetrahedral site with a broad distribution of structural parameters. Moreover, the ligand chemistry in ZIFs appeared to have no controlling effect on the short-range disorder, although the former affected their phase-transition behavior. These findings reveal structure-property relations and could help design metal-organic framework glasses.

How the Method of Synthesis Governs the Local and Global Structure of Zinc Aluminum Layered Double Hydroxides

Seven zinc aluminum layered double hydroxides (ZnAl LDHs), [Zn1–xAlx(OH)2Ax·nH2O], A = NO3–, Cl–, or CO32–, prepared by the urea and coprecipitation synthesis methods were investigated to determine how the synthesis parameters (pH, metal ion concentration, and postsynthesis treatment) affect the local (atomic) and long (global) range structure of the LDH product. Sample composition, purity, crystal defects, and other structural aspects of the LDH products were obtained from powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), micro-Raman, elemental analysis, and solid state 1H, 27Al, and 67Zn NMR spectroscopy. The urea method results in LDHs, which on the global scale are highly crystalline LDHs but disordered on the local scale. The disorder is correlated with the presence of Al-rich phases, which are undetected by bulk techniques (TEM, PXRD), as they are either defects within the LDH particles or separate phase(s) associated with LDHs. In contrast, samples prepared by coprecipitation with careful pH control and hydrothermal treatment have high local order and good crystallinity (large particle size). Our results show that both local (NMR) and bulk techniques are needed to assess the composition of LDHs, as the conventional PXRD and TEM analysis of LDHs failed to identify the many structural defects and/or amorphous phases present.

Monitoring and understanding the paraelectric-ferroelectric phase transition in the metal-organic framework [NH4 ][M(HCOO)3 ] by solid-state NMR spectroscopy.

The paraelectric-ferroelectric phase transition in two isostructural metal-organic frameworks (MOFs) [NH4 ][M(HCOO)3 ] (M=Mg, Zn) was investigated by in situ variable-temperature (25) Mg, (67) Zn, (14) N, and (13) C solid-state NMR (SSNMR) spectroscopy. With decreasing temperature, a disorder-order transition of NH4 (+) cations causes a change in dielectric properties. It is thought that [NH4 ][Mg(HCOO)3 ] exhibits a higher transition temperature than [NH4 ][Zn(HCOO)3 ] due to stronger hydrogen-bonding interactions between NH4 (+) ions and framework oxygen atoms. (25) Mg and (67) Zn NMR parameters are very sensitive to temperature-induced changes in structure, dynamics, and dielectric behavior; stark spectral differences across the paraelectric-ferroelectric phase transition are intimately related to subtle changes in the local environment of the metal center. Although (25) Mg and (67) Zn are challenging nuclei for SSNMR experiments, the highly spherically symmetric metal-atom environments in [NH4 ][M(HCOO)3 ] give rise to relatively narrow spectra that can be acquired in 30-60 min at a low magnetic field of 9.4 T. Complementary (14) N and (13) C SSNMR experiments were performed to probe the role of NH4 (+) -framework hydrogen bonding in the paraelectric-ferroelectric phase transition. This multinuclear SSNMR approach yields new physical insights into the [NH4 ][M(HCOO)3 ] system and shows great potential for molecular-level studies on electric phenomena in a wide variety of MOFs.

Theoretical insights into the functioning of metallopeptidases and their synthetic analogues.

CONSPECTUS: The selective hydrolysis of a peptide or amide bond (-(O═)C-NH-) by a synthetic metallopeptidase is required in a wide range of biological, biotechnological, and industrial applications. In nature, highly specialized enzymes known as proteases and peptidases are used to accomplish this daunting task. Currently, many peptide bond cleaving enzymes and synthetic reagents have been utilized to achieve efficient peptide hydrolysis. However, they possess some serious limitations. To overcome these inadequacies, a variety of metal complexes have been developed that mimic the activities of natural enzymes (metallopeptidases). However, in comparison to metallopeptidases, the hydrolytic reactions facilitated by their existing synthetic analogues are considerably slower and occur with lower catalytic turnover. This could be due to the following reasons: (1) they lack chemical properties of amino acid residues found within enzyme active sites; (2) they contain a higher metal coordination number compared with naturally occurring enzymes; and (3) they do not have access to second coordination shell residues that provide substantial rate enhancements in enzymes. Additionally, the critical structural and mechanistic information required for the development of the next generation of synthetic metallopeptidases cannot be readily obtained through existing experimental techniques. This is because most experimental techniques cannot follow the individual chemical steps in the catalytic cycle due to the fast rate of enzymes. They are also limited by the fact that the diamagnetic d(10) Zn(II) center is silent to electronic, electron spin resonance, and (67)Zn NMR spectroscopies. Therefore, we have employed molecular dynamics (MD), quantum mechanics (QM), and hybrid quantum mechanics/molecular mechanics (QM/MM) techniques to derive this information. In particular, the role of the metal ions, ligands, and microenvironment in the functioning of mono- and binuclear metal center containing enzymes such as insulin degrading enzyme (IDE) and bovine lens leucine aminopeptidase (BILAP), respectively, and their synthetic analogues have been investigated. Our results suggested that in the functioning of IDE, the chemical nature of the peptide bond played a role in the energetics of the reaction and the peptide bond cleavage occurred in the rate-limiting step of the mechanism. In the cocatalytic mechanism used by BILAP, one metal center polarized the scissile peptide bond through the formation of a bond between the metal and the carbonyl group of the substrate, while the second metal center delivered the hydroxyl nucleophile. The Zn(N3) [Zn(His, His, His)] core of matrix metalloproteinase was better than the Zn(N2O) [Zn(His, His, Glu)] core of IDE for peptide hydrolysis. Due to the synergistic interaction between the two metal centers, the binuclear metal center containing Pd2(μ-OH)([18]aneN6)](4+) complex was found to be ∼100 times faster than the mononuclear [Pd(H2O)4](2+) complex. A successful small-molecule synthetic analogue of a mononuclear metallopeptidase must contain a metal with a strong Lewis acidity capable of reducing the pKa of its water ligand to less than 7. Ideally, the metal center should include three ligands with low basicity. The steric effects or strain exerted by the microenvironment could be used to weaken the metal-ligand interactions and increase the activity of the metallopeptidase.

Local and Average Structure in Zinc Cyanide: Toward an Understanding of the Atomistic Origin of Negative Thermal Expansion

Neutron diffraction at 11.4 and 295 K and solid-state (67)Zn NMR are used to determine both the local and the average structures in the disordered, negative thermal expansion (NTE) material, Zn(CN)2. Solid-state NMR not only confirms that there is head-to-tail disorder of the C≡N groups present in the solid, but yields information about the relative abundances of the different Zn(CN)4–n(NC)n tetrahedral species, which do not follow a simple binomial distribution. The Zn(CN)4 and Zn(NC)4 species occur with much lower probabilities than are predicted by binomial theory, supporting the conclusion that they are of higher energy than the other local arrangements. The lowest energy arrangement is Zn(CN)2(NC)2. The use of total neutron diffraction at 11.4 K, with analysis of both the Bragg diffraction and the derived total correlation function, yields the first experimental determination of the individual Zn–N and Zn–C bond lengths as 1.969(2) and 2.030(2) Å, respectively. The very small difference in bond lengths, of ~0.06 Å, means that it is impossible to obtain these bond lengths using Bragg diffraction in isolation. Total neutron diffraction also provides information on both the average and the local atomic displacements responsible for NTE in Zn(CN)2. The principal motions giving rise to NTE are shown to be those in which the carbon and nitrogen atoms within individual Zn–C≡N–Zn linkages are displaced to the same side of the Zn···Zn axis. Displacements of the carbon and nitrogen atoms to opposite sides of the Zn···Zn axis, suggested previously in X-ray studies as being responsible for NTE behavior, in fact make negligible contributions at temperatures up to 295 K.

Multinuclear (67Zn, 119Sn and 65Cu) NMR spectroscopy – an ideal technique to probe the cationic ordering in Cu2ZnSnS4 photovoltaic materials

For the very first time, (67)Zn, (119)Sn and (65)Cu NMR investigations have been carried out on Cu2ZnSnS4 derivatives (CZTS) for photovoltaic applications. NMR spectroscopy is shown to be sensitive enough to probe the Cu/Zn disorder within the kesterite structure of the studied compounds. In addition, reference spectra of Cu2ZnSnS4 are provided, and experimental (67)Zn and (65)Cu parameters are compared with ab initio calculations.

Multinuclear NMR Study of Zinc Dicyanide

Abstract The isotropic negative expansion of Zn(CN)2 has been linked to a temperature induced increase in off-axis tilting of the C–N bond direction and an increase in CN-bond length. However, the bond length could be determined only indirectly based on pair-distribution function analysis and was found to be surprisingly large. Here we study Zn(CN)2 by nuclear magnetic resonance spectroscopy and first principles calculations. By using samples enriched in 13 C and 15 N the dipole coupling between carbon and nitrogen is determined, and from this an upper bound on the C–N bond length of 1.19 ± 0.01 Å is derived. This quantity agrees with earlier determinations based on diffraction but is shorter than estimates based on pair distribution function analysis. The relation of this estimate to possible dynamics in the sample is discussed. Finally, 67 Zn NMR is used together with first principles calculations to assess disorder in the material.

Characterization of Zn‐Containing Metal–Organic Frameworks by Solid‐State 67Zn NMR Spectroscopy and Computational Modeling

Metal-organic frameworks (MOFs) are an extremely important class of porous materials with many applications. The metal centers in many important MOFs are zinc cations. However, their Zn environments have not been characterized directly by (67)Zn solid-state NMR (SSNMR) spectroscopy. This is because (67)Zn (I=5/2) is unreceptive with many unfavorable NMR characteristics, leading to very low sensitivity. In this work, we report, for the first time, a (67)Zn natural abundance SSNMR spectroscopic study of several representative zeolitic imidazolate frameworks (ZIFs) and MOFs at an ultrahigh magnetic field of 21.1 T. Our work demonstrates that (67)Zn magic-angle spinning (MAS) NMR spectra are highly sensitive to the local Zn environment and can differentiate non-equivalent Zn sites. The (67)Zn NMR parameters can be predicted by theoretical calculations. Through the study of MOF-5 desolvation, we show that with the aid of computational modeling, (67)Zn NMR spectroscopy can provide valuable structural information on the MOF systems with structures that are not well described. Using ZIF-8 as an example, we further demonstrate that (67)Zn NMR spectroscopy is highly sensitive to the guest molecules present inside the cavities. Our work also shows that a combination of (67)Zn NMR data and molecular dynamics simulation can reveal detailed information on the distribution and the dynamics of the guest species. The present work establishes (67)Zn SSNMR spectroscopy as a new tool complementary to X-ray diffraction for solving outstanding structural problems and for determining the structures of many new MOFs yet to come.

SMARTER crystallography of the fluorinated inorganic–organic compound Zn3Al2F12·[HAmTAZ

We present in this paper the structure resolution of a fluorinated inorganic-organic compound--Zn(3)Al(2)F(12)·[HAmTAZ](6)--by SMARTER crystallography, i.e. by combining powder X-ray diffraction crystallography, NMR crystallography and chemical modelling of crystal (structure optimization and NMR parameter calculations). Such an approach is of particular interest for this class of fluorinated inorganic-organic compound materials since all the atoms have NMR accessible isotopes ((1)H, (13)C, (15)N, (19)F, (27)Al, (67)Zn). In Zn(3)Al(2)F(12)·[HAmTAZ](6), (27)Al and high-field (19)F and (67)Zn NMR give access to the inorganic framework while (1)H, (13)C and (15)N NMR yield insights into the organic linkers. From these NMR experiments, parts of the integrant unit are determined and used as input data for the search of a structural model from the powder diffraction data. The optimization of the atomic positions and the calculations of NMR parameters ((27)Al and (67)Zn quadrupolar parameters and (19)F, (1)H, (13)C and (15)N isotropic chemical shifts) are then performed using a density functional theory (DFT) based code. The good agreement between experimental and DFT-calculated NMR parameters validates the proposed optimized structure. The example of Zn(3)Al(2)F(12)·[HAmTAZ](6) shows that structural models can be obtained in fluorinated hybrids by SMARTER crystallography on a polycrystalline powder with an accuracy similar to those obtained from single-crystal X-ray diffraction data.

Natural abundance solid-state 67Zn NMR characterization of microporous zinc phosphites and zinc phosphates at ultrahigh magnetic field

Zinc-phosphite and -phosphate based microporous materials are crystalline open framework materials with potential industrial applications. Although (31)P MAS NMR has been used for characterization of these materials, the local environments around zinc centres have never been directly probed by solid-state NMR due to the many unfavourable NMR characteristics of (67)Zn. In this work, we have characterized the local structure around the Zn centres in several representative microporous zinc phosphites and zinc phosphates by acquiring natural abundance (67)Zn solid-state NMR spectra at ultrahigh magnetic field of 21.1 T. The observed line-shapes are mainly determined by the second order quadrupolar interaction. The NMR tensor parameters were extracted from the spectra and are related to the local geometry around the Zn centre. Computational study of the electric field gradient (EFG) tensor at Zn was performed using hybrid density functional theory (DFT) calculations at B3LYP level of theory on model clusters. The calculations using Projector Augmented-Wave (PAW) method were also carried out with the CASTEP code wherever it was possible. The work has shown that it is possible to study Zn environments in porous materials which often have very low Zn concentration by natural abundance (67)Zn SSNMR at very high magnetic fields.

High-Field Solid-State 67Zn NMR Spectroscopy of Several Zinc−Amino Acid Complexes

We report the results of our solid-state (67)Zn NMR study of the various zinc sites in four zinc-amino acid coordination complexes: bis(glycinato)zinc(II) monohydrate; bis(l-alaninato)zinc(II); bis(l-histidinato)zinc(II) dihydrate; and sodium bis(l-cysteinato)zincate(II) hexahydrate; as well as a related complex, bis(imidazole)zinc(II) chloride. We demonstrate the advantages of using high (21.1 T) applied magnetic fields for detecting (67)Zn directly at ambient temperatures using the quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence. The stepped-frequency technique was employed in cases where the central-transition (CT) (67)Zn NMR spectra were too broad to be uniformly excited. The parameters of the anisotropic zinc tensors were extracted by iterative simulations of the experimental spectra. In all cases, the quadrupolar interaction is found to dominate the central-transition (67)Zn NMR spectra; no convincing effects from chemical shift anisotropy (CSA) on the NMR spectra of the five complexes could be reliably detected at this field strength. Analyses of the experimental NMR spectra reveal that the (67)Zn quadrupolar coupling constants (C(Q)) range from 7.05 to 26.4 MHz, the isotropic chemical shifts (delta(iso)) range from 140 to 265 ppm, and the quadrupolar asymmetry parameters (eta(Q)) range from 0.20 to 0.95. The first report of the NMR spectral features of pentacoordinated zinc sites is included for two complexes. Quantum chemical calculations of the electric field gradient (EFG) and magnetic shielding tensors reproduced the experimental results to a reasonable extent. Moreover, the computationally determined orientations of both tensors permit correlations between NMR tensor properties and zinc local environments to be understood.

Atomic Positional Versus Electronic Order in Semiconducting ZnSe Nanoparticles

Size-controlled ZnSe nanoparticles with high extents of atomic positional order are shown to exhibit large size-dependent variations in their local electronic environments. Solid-state ;{77}Se and ;{67}Zn NMR spectra reveal increasingly broad distributions of ;{77}Se and ;{67}Zn environments with decreasing nanoparticle sizes, in contrast with high degrees of atomic positional order established by transmission electron microscopy and x-ray diffraction. First-principles calculations of NMR parameters distinguish between atomic positional and electronic disorder that propagate from the nanoparticle surfaces and yield insights on the order and disorder present.

Residue Ionization in LpxC Directly Observed by 67Zn NMR Spectroscopy

The pH dependence of the solid-state (67)Zn NMR lineshapes has been measured for both the wild type (WT) and the H265A mutant of Aquifex aeolicus LpxC, each in the absence of substrate (resting state). The (67)Zn NMR spectrum of WT LpxC at pH 6 (prepared at 0 degrees C) contains two overlapping quadrupole lineshapes with C q values of 10 and 12.9 MHz, while the spectrum measured for the sample prepared at a pH near 9 (at 0 degrees C) is dominated by the appearance of a third species with a C q of 14.3 MHz. These findings are consistent with the two p K a values previously observed by the bell-shaped dependence of the LpxC-catalyzed reaction. On the basis of comparison of the experimental results with predictions from quantum mechanical/molecular mechanical (QM/MM) modeling, we suggest that p K a1 (low pH) represents the ionization of Glu78 and p K a2 (high pH) reflects the ionization of another active site residue located near the zinc ion, such as His265. These results are also consistent with water being bound to the Zn (2+) ion throughout this pH range. The (67)Zn NMR spectra of the H265A mutant appear to be pH independent, with a C q of 9.55 MHz being sufficient to describe both low- and high-pH data. The QM/MM models of the H265A mutant suggest that over this pH range water is bound to the zinc ion while Glu78 is protonated.

67Zn Solid-State NMR Spectroscopy of {[TpBut,Me]Zn(OH2)}[HOB(C6F5)3]. The Importance of the Anion [HOB(C6F5)3]−

One of the paradigms of Zn2+ metallobiochemistry is that coordination of water to Zn2+ provides a mechanism of activation that involves lowering the pKa by approximately 7 pH units. This idea has become central to the development of mechanisms of action for zinc metalloproteins. However, the direct measurement of the pKa of water bound to Zn2+ in a metalloprotein has yet to be accomplished. Developing models for Zn2+-OH2 species has been a significant challenge, but we have utilized solid-state 67Zn NMR spectroscopy as a means to characterize one of the few examples of water bound to mononuclear tetrahedral Zn2+: ([Tp(But,Me)]Zn(OH2))[HOB(C6F5)3]. The measured quadrupole coupling (Cq) constant is 4.3 MHz with an asymmetry parameter of etaq of 0.6. Likewise, due to the small value of Cq, anisotropic shielding also contributed to the observed 67Zn NMR lineshape. As expected, the computed values of the magnetic resonance parameters depend critically on the nature of the anion. The predicted value of Cq for ([Tp(But,Me)]Zn(OH2))[HOB(C6F5)3] is -4.88 MHz. We discuss the results of these calculations in terms of the nature of the anion, the local electrostatics, and its subsequent hydrogen bonding to [Tp(But,Me)]Zn(OH2)+.