Study on crystallographically inequivalent protons in RbH2AsO4 using static NMR and MAS NMR

Two inequivalent protons from ¹H NMR spectra of RbH₂AsO₄ in the paraelectric phase were distinguished using static NMR and MAS NMR. From the ¹H spin–lattice relaxation times in the laboratory frame, T₁, and rotating frame, T_1ρ, of the two crystallographically inequivalent hydrogen sites, i.e., H(1) and H(2), the temperature dependences of T₁ and T_1ρ for H(1) were related to the reorientational motion. The shorter H(1) bonds give rise to stronger H-bonds, and protons involved in stronger H-bonds have long relaxation times. Consequently, the RbH₂AsO₄ structure has two crystallographically inequivalent sites with two different hydrogen-bond lengths.     

Origin of asymmetric charge partitioning in the dissociation of gas-phase protein homodimers

The origin of asymmetric charge and mass partitioning observed for gas-phase dissociation of multiply charged macromolecular complexes has been hotly debated. These experiments hold the potential to provide detailed information about the interactions between the macromolecules within the complex. Here, this unusual phenomenon of asymmetric charge partitioning is investigated for several protein homodimers. Asymmetric charge partitioning in these ions depends on a number of factors, including the internal energy, charge state, and gas-phase conformation of the complex, as well as the conformational flexibility of the protein monomer in the complex. High charge states of both cytochrome c and disulfide-reduced alpha-lactalbumin homodimers dissociate by a symmetrical charge partitioning process in which both fragment monomers carry away roughly an equal number of charges. In contrast, highly asymmetric charge partitioning dominates for the lower charge states. Cytochrome c dimer ions with eleven charges formed by electrospray ionization from two solutions in which the solution-phase conformation differs dissociate with dramatically different charge partitioning. These results demonstrate that these gas-phase complexes retain a clear "memory" of the solution from which they are formed, and that information about their solution-phase conformation can be obtained from these gas-phase dissociation experiments. Cytochrome c dimer ions formed from solutions in which the conformation of the protein is native show greater asymmetric charge partitioning with increasing ion internal energy. Cytochrome c dimers that are conformationally constrained with intramolecular cross-linkers undergo predominantly symmetric charge partitioning under conditions where asymmetric charge partitioning is observed for cytochrome c dimers without cross-links. Similar results are observed for alpha-lactalbumin homodimers. These results provide convincing evidence that the origin of asymmetric charge partitioning in these homodimers is the result of one of the protein monomers unfolding in the dissociation transition state. A mechanism that accounts for these observations is proposed.     

Further studies on the origins of asymmetric charge partitioning in protein homodimers

Dissociation of gas-phase protonated protein dimers into their constituent monomers can result in either symmetric or asymmetric charge partitioning. Dissociation of alpha-lactalbumin homodimers with 15+ charges results in a symmetric, but broad, distribution of protein monomers with charge states centered around 8+/7+. In contrast, dissociation of the 15+ heterodimer consisting of one molecule in the oxidized form and one in the reduced form results in highly asymmetric charge partitioning in which the reduced species carries away predominantly 11+ charges, and the oxidized molecule carries away 4+ charges. This result cannot be adequately explained by differential charging occurring either in solution or in the electrospray process, but appears to be best explained by the reduced species unfolding upon activation in the gas phase with subsequent separation and proton transfer to the unfolding species in the dissociation complex to minimize Coulomb repulsion. For dimers of cytochrome c formed directly from solution, the 17+ charge state undergoes symmetric charge partitioning whereas dissociation of the 13+ is asymmetric. Reduction of the charge state of dimers with 17+ charges to 13+ via gas-phase proton transfer and subsequent dissociation of the mass selected 13+ ions results in a symmetric charge partitioning. This result clearly shows that the structure of the dimer ions with 13+ charges depends on the method of ion formation and that the structural difference is responsible for the symmetric versus asymmetric charge partitioning observed. This indicates that the asymmetry observed when these ions are formed directly from solution must come about due either to differences in the monomer conformations in the dimer that exist in solution or that occur during the electrospray ionization process. These results provide additional evidence for the origin of charge asymmetry that occurs in the dissociation of multiply charged protein complexes and indicate that some solution-phase information can be obtained from these gas-phase dissociation experiments.     

Direct evidence for symmetry control in cyclodextrin-water interactions

Abstract

The cyclodextrins, cyclic oligosaccharides possessing generally 6, 7, or 8 α, 1 –4 linked glucopyranose units, are widely used for the solubilisation and transport of organic molecules in aqueous media.³ Their solubilities 145 gL^?1, α-Cd; 20 gL^?1, β-CD; 220 gL^?1, γ-CD, are considerably lower than those of simple saccharides and in particular that of β-CD, the most widely available compound, is anomalously low. We have previously shown that the solubilities arise from symmetry determined interactions of the CDs with the dynamic hexagonal structure of water. In the case of the unmodified cyclodextrins all three compounds exist in solution as large aggregates, thus the solubility of these systems is controlled by the interactions between these hydrogen bonded aggregates and the hydrogen bonding networks present in water. In the case of α- or γ-CD there are favorable overlaps with the hexagonal water structure, however for the seven-fold symmetry of β-CD no such favorable interactions can occur between the odd symmetry element and the even symmetry elements of water. Hence the observed solubilities of α- and γ-CD are higher than that of β-CD.     

Frequency-switching inversion-recovery for severely hyperfine-shifted NMR: evidence of asymmetric electron relaxation in high-spin Co(II)

A new method for reliably measuring longitudinal relaxation rates for severely hyperfine-shifted NMR signals in aqueous solutions is presented. The method is illustrated for a well-defined cobalt tetracysteinate, with relevance to cobalt-substituted metalloproteins. The relaxation measurements are indicative of asymmetric electronic relaxation of the high-spin Co(II) ion.     

NMR evidence for planar chirality in natural porphyrins

The ¹H NMR spectra of several six-coordinate cobalt(III) porphyrins of general formula L₂Co(DPDME)CI, where DPDME=deuteroporphyrin dimethyl ester and L is an optically active ligand, have been measured at 250 or 400 MHz. In a few cases, two signals of equal intensity are observed for the L ligand protons. This magnetic non-equivalence, which never exceeds 0.04 ppm, is thought to arise from the planar chirality of the porphyrin ring, which makes a proton in the ligand L above the porphyrin ring and the corresponding proton below this ring diastereotopic.     

溶液中自扩散过程的NMR方法研究

对脉冲梯度场－核磁共振（ＰＦＧ－ＮＭＲ）中测定溶液分子自扩散系数的Ｓｔｉｍｕｌａｔｅｄ ｅｃｈｏ方法进行了改进,把测定自扩散系数的Ｓｔｉｍｕｌａｔｅｄ ｅｃｈｏ脉冲序列与测定自旋－晶格弛豫时间的脉冲序列串接起来,设计了两个新的脉冲序列。     

Conclusive evidence for a retention-retention pathway for the molybdenum-catalyzed asymmetric alkylation

Enantiomerically enriched, deuterated branched carbonates (Z)-(S)-PhCH(O2COMe)-CH=CHD (1a), (Z)-(R)-PhCH(O2COMe)CH=CHD (2a), and linear carbonate (E)-(S)-PhCH=CHCHD(O2COMe) (12) were employed as probes in the Mo-catalyzed asymmetric allylic alkylation with sodium dimethyl malonate, catalyzed by ligand complex 10 derived from the mixed benzamide/picolinamide of (S,S)-trans-diaminocyclohexane and (norbornadiene)Mo(CO)4. X-ray crystallography, along with solution NMR structural analysis of the pi-allyl intermediate and competition experiments, conclusively established the reaction proceeded via a retention-retention pathway. This mechanism contrasts with that defined for Pd-catalyzed allylic alkylations, which proceed via an inversion-inversion pathway.     

Chiral symmetry breaking in a microscopic model with asymmetric autocatalysis and inhibition

Asymmetric autocatalysis and inhibition have been proposed as key processes in the spontaneous emergence of chiral symmetry breaking in a prebiotic world. An elementary lattice model is formulated to simulate the kinetics of chiral symmetry breaking via autocatalysis and inhibition in a mixture of prochiral reactants, chiral products, and inert solvent. Starting from a chirally unbiased initial state, spontaneous symmetry breaking occurs in spite of equal a priori probability for creating either product enantiomer, and the coupled reaction-diffusion processes subsequently amplify the random early-stage symmetry breaking. The processes of reaction and diffusion are kinetically intertwined in a way leading to competition in the appearance of enantiomeric excess. An effective transition temperature can be identified below which spontaneous symmetry breaking appears. In the absence of inhibition, reactions are predominantly autocatalytic under both reaction control (fast diffusion, slow reaction) or diffusion control (fast reaction, slow diffusion) conditions. In the presence of inhibition, simulations with different system sizes converge to the same transition temperature under reaction control conditions, and in this limit the reactions are predominantly nonautocatalytic.     

Solution NMR techniques for large molecular and supramolecular structures

Transverse relaxation-optimized spectroscopy (TROSY) or generation of heteronuclear multiple quantum coherences during the frequency labeling period and TROSY during the acquisition period have been combined either with cross-correlated relaxation-induced polarization transfer (CRIPT) or cross-correlated relaxation-enhanced polarization transfer (CRINEPT) to obtain two-dimensional (2D) solution NMR correlation spectra of (15)N,(2)H-labeled homo-oligomeric macromolecules with molecular weights from 110 to 800 kDa. With the experimental conditions used, the line widths of the TROSY-components of the (1)H- and (15)N-signals were of the order of 60 Hz at 400 kDa, whereas, for structures of size 800 kDa, the line widths were about 75 Hz for (15)N and 110 Hz for (1)H. This paper describes the experimental schemes used and details of their setup for individual measurements. The performance of NMR experiments with large structures depends critically on the choice of the polarization transfer times, the relaxation delays between subsequent recordings, and the water-handling routines. Optimal transfer times for 2D [(15)N,(1)H]-CRIPT-TROSY experiments in H(2)O solutions were found to be 6 ms for a molecular weight of approximately 200 kDa, 2.8 ms for 400 kDa, and 1.4 ms for 800 kDa. These data validate theoretical predictions of inverse proportionality between optimal transfer time and size of the structure. The proton longitudinal relaxation times in H(2)O solution were found to be of the order of 0.8 s for structure sizes around 200 kDa, 0.4 s at 400 kDa, and 0.3 s at 800 kDa, which enabled the use of recycle times below 1 s. Since improper water handling results in severe signal loss, the water resonance was kept along the z-axis during the entire duration of the experiments by adjusting each water flip-back pulse individually.     

Intramolecular asymmetric Heck reactions: evidence for dynamic kinetic resolution effects

[reaction: see text] Enantioselectivity of the cyclization of 1 varies at different stages in the reaction. X-ray crystallography has shown that 1 exists as enantiomerically pure (M) and (P) chiral helical structures defined by the relative orientations of the arene, amide, and alkene. The relative rates of interconversion of the rotamers of 1 have been established, leading to mechanistic proposals to account for the variation of ee based on kinetic resolution effects.     

群多普利溶液构象的NMR研究

采用1D和2D NMR技术对血管紧张素转化酶抑制剂群多普利在CDCl₃溶液中存在的两种构象进行了结构解析, 并结合分子动力学和密度泛函理论方法对其进行了结构的几何优化和能量计算. 结果表明, 群多普利因分子中酰胺键的旋转而形成反式构象A和顺式构象B, 两种构象的能量差为6.35 kJ/mol, 且顺式构象为该药物的优势构象.     

Using symmetry to monitor geared bond rotation in aromatic amides by dynamic NMR

Dynamic NMR proves that the fastest interconversion between conformers of simple tertiary aromatic amide 3 is racemization via a geared (correlated) rotation of both the Ar-CO and N-CO bonds. The symmetry of 3 is such that correlated and noncorrelated rotations are easily distinguishable, even without assignment of the NMR spectrum, simply by observing the number of peaks taking part in the exchange.     

Direct NMR evidence for Ca2+ ion binding to G-quartets

We report the first 1H and 43Ca NMR characterization of Ca2+ ion binding to G-quartets.     

Kinetic evidence for a tetrameric transition state in the asymmetric autocatalytic alkylation of pyrimidyl aldehydes

Experimental kinetic data coupled with kinetic modeling implicates a tetrameric transition state in the Soai autocatalytic alkylation of pyrimidyl aldehydes. The kinetic model accurately predicts both the reaction rate and the amplification in enantiomeric excess observed in reactions carried out under a wide range of conditions. These studies reveal the Soai reaction to be an example of true autocatalytic efficiency in a template-directed "minimal system" for self-replication.     

Solution NMR approaches for establishing specificity of weak heterodimerization of membrane proteins

Solution NMR provides a powerful approach for detecting complex formation involving weak to moderate intermolecular affinity. However, solution NMR has only rarely been used to detect complex formation between two membrane proteins in model membranes. The impact of specific binding on the NMR spectrum of a membrane protein can be difficult to distinguish from spectral changes that are induced by nonspecific binding and/or by changes that arise from forced cohabitation of the two proteins in a single model membrane assembly. This is particularly the case when solubility limits make it impossible to complete a titration to the point of near saturation of complex formation. In this work experiments are presented that provide the basis for establishing whether specific complex formation occurs between two membrane proteins under conditions where binding is not of high avidity. Application of these methods led to the conclusion that the membrane protein CD147 (also known as EMMPRIN or basigin) forms a specific heterodimeric complex in the membrane with the 99-residue transmembrane C-terminal fragment of the amyloid precursor protein (C99 or APP-βCTF), the latter being the immediate precursor of the amyloid-β polypeptides that are closely linked to the etiology of Alzheimer's disease.     

Use of proton NMR to study low symmetry bonding effects in transition metal complexes

The 300 MHZ proton NMR chemical shifts of the mixed complex (salicylaldehyde)(acetyl-acetone)ethylenediimino nickel(II) and the two corresponding non-mixed complexes were studied. The differences in chemical shifts were very small, ∼ 0.05 ppm, but by looking at both the mixed and non-mixed species in the same solution relative values could be determined. The differences in chemical shifts were attributed to both a geometry effect and a basicity effect.