Large Downfield Shifts Research Articles

Comparison of the complexation of fluoroquinolone antimicrobials with metal ions by nuclear magnetic resonance spectroscopy

The complexation of fluoroquinolone antimicrobials with various metal ions have been studied in aqueous solution (pD 2.5, 37°C) by 1H and 13C-NMR spectroscopy. The compounds examined are levofloxacin, ciprofloxacin and lomefloxacin. In each drug, new signals have appeared by the addition of Al 3+, suggesting that the complexes are formed between the drug and Al 3+ and that the ligand exchange is slow on the NMR time scale. Solution structure of the major species in the presence of 2.0 mol equiv of Al 3+ has been proposed based on the large downfield shifts of some specific protons. Signals of both the coordinated and free drugs have shown slight broadening at 90°C due to the enhanced rate in ligand dissociation process, though the coalescence phenomena are not observed even at this temperature. Thus, the complexes are supposed to be stable at the physiological condition. Titration experiments have revealed that the binding ability of levofloxacin toward Al 3+ is much stronger than that of ciprofloxacin and lomefloxacin at pD 2.5. In contrast to the complexation with Al 3+, the binding of these drugs with other metal ions such as Ca 2+ and Mg 2+ is much weaker; NMR signals have shown no appreciable downfield shift by the addition of Ca 2+ and Mg 2+. Based on these results, it is concluded that the fluoroquinolone antimicrobials examined in the present study at pD 2.5 exist as stable complexes in the presence of Al 3+ and the absorptivity of the drugs on oral administration could be affected by Al 3+.

Mono- and di-nuclear complexes of (trpy)MII (M = Pd, Pt) with the model nucleobase 1-methylcytosine. Crystal structure and NMR solution studies

Reactions of (trpy)MII (M = Pd and Pt; trpy = 2,2′∶6′,2″-terpyridine) with the model nucleobase 1-methylcytosine (Hmcyt) have been performed in water and studied by 1H and 195Pt NMR spectroscopy. Mononuclear [(trpy)M(Hmcyt-N 3)]2+1 (M = Pd) and 3 (M = Pt) and dinuclear [{(trpy)M}2(mcyt-N 3, N 4)]3+2 (M = Pd) and 4 (M = Pt) complexes are formed. The two (trpy)M entities in the dinuclear species 2 and 4 are arranged syn to each other in the solid state. X-Ray crystal structures have been performed for [(trpy)Pd(Hmcyt-N 3)][NO3]2·5H2O 1, [{(trpy)Pd}2(mcyt-N 3,N 4)][ClO4]3·H2O 2b, [(trpy)Pt(Hmcyt-N 3)][NO3]2·5H2O 3 and [{(trpy)Pt}2(mcyt-N 3,N 4)][ClO4]3·H2O 4b. The effect of the M–trpy entity on the resonances of the Hmcyt nucleobase in 1 and 3 (large downfield shifts of H5 and H6) is possibly related to the π-acceptor capacity of the trpy ligand.

Evidence for unusually strong intramolecular hydrogen bonding in highly nonplanar porphyrins

The 1H NMR spectra of the 5,10,15,20-tetraalkylporphyrins 2–4 reveal a large downfield shift of the signal for the NH protons and a reduction in the activation energy for NH tautomerism as the alkyl substituents become larger; these unusual changes can be rationalized in terms of greatly enhanced intramolecular hydrogen bonding of the NH protons as the bulkier substituents distort the macrocycle from planarity and contract the porphyrin core.

Bis[( η8-cyclooctatetraene)titanium] complex with perpendicularly bridging bis(trimethylsilyl)acetylene

The reduction of [( η 8-C 8H 8)Ti(THF)( μ-Cl)] 2 by magnesium in the presence of bis(trimethylsilyl)acetylene (BTMSA) afforded the diamagnetic complex [( η 8-C 8H 8)Ti] 2[ μ- η 2: η 2-Me 3SiCCSiMe 3] ( 1) in high yield. The crystal structure of 1 revealed that BTMSA is a four-electron ligand which binds two equivalent ( η 8-C 8H 8)Ti moieties under the angle of av. 69°. The average CC distance of 1.51 Å in the acetylene ligand is close to that of sp 3 carbon atoms, however, a high thermal stability and enormously large down-field chemical shift of the acetylenic carbon atoms ( δ 292.8 ppm) points to an unusually high contribution of π-back-bonding to the Ti-acetylene bond. An analogous reduction in the presence of diphenylacetylene afforded a tetraphenylcyclobutadiene complex ( η 8-C 8H 8)Ti( η 4-C 4Ph 4) and hexaphenylbenzene.

Unequivocal location of sites ofN-oxidation using natural abundance long-range1H,15N GHNMQC two-dimensional NMR

A method is described for unequivocally establishing the site of N-oxidation which relies on the relatively large downfield shift of aliphatic nitrogen resonances following oxidation. The technique described is based on 15N at natural abundance and does not require labelling. 15N chemical shifts are established on the basis of long-range 1H–15N heteronuclear coupling pathways detected via two-dimensional NMR using the GHNMQC pulse sequence. The oxidized nitrogen in the two piperazine-containing systems studied was shifted downfield by approximately +68 ppm; the opposite, non-oxidized nitrogen in the piperazine ring was shifted upfield by an average of -6 ppm (15N chemical shifts are referenced to liquid ammonia with a chemical shift of 0 ppm). N-oxidation perturbations in 15N chemical shifts were first parameterized using an oxazolidinone antibiotic (eperezolid) and its N-oxide as a model system. After the 15N shift due to N-oxidation was determined, the method was used to establish unequivocally the site of N-oxidation in PNU-101387, a piperazine-containing anxiolytic agent. Conventional spectroscopic methods were equivocal and could not reliably establish the site of N-oxidation. © 1998 John Wiley & Sons, Ltd.

Higher Sucrose Analogs: Homologation of a Glucose Unit of Sucrose by Two Carbon Atoms

The primary hydroxyl groups (at C-6 and C-6′) in 2,3,4,3′4′-penta-O-benzyl-l′-O-methoxymethyl sucrose (2) can be reactively differentiated with tert-butyldiphenylsilyl chloride. Reaction of 2 with TBDPSCl afforded only one monosilylated product protected at C-6′ (6). The regioisomeric monoprotected sucrose 8 was prepared by selective deprotection of the double silylated derivative 7. Compound 6 was converted into 2,3,4,3′,4′-penta-O-benzyl-6-carbomethoxymethylidene-1′-O-methoxymethylsucrose 10 in three steps. Osmylation of the double bond in 10 afforded stereoisomeric homologated sucroses: 11a [6(S),7(R)] and 11b [6(R),7(S)] in the ratio 3:2. A large downfield shift of the H-1 (up to 0.5 ppm) was observed for 6′-silylated derivatives.

Synthesis and characterization of [Rh IIICl 2(η 1-C 6H 5)(PYR) 2(SbPh 3)], [Rh IIICl 2)(η 1-C 6H 5)(PY) 2(THZ)], [RH IIICl 2(η 1-C 6H 5(PY) 2(PYR)], and [Rh IIICl 2(η 1-C 6H 5)(CEP)(SbPh 3) 2] [PYR = pyrazine; PY = pyridine; THZ = thiazole; CEP = N-(2-cyanoethylpyrrole)]. Crystal and molecular structure of [Rh IIICl 2(η 1-C 6H 5)(PYR) 2(SbPh 3)]

The mononuclear compounds trans-dichloro(η 1-phenyl)cis-bis(pyrazine)(triphenylstibine) rhodium(III) ( 1), and trans-dichloro(η 1-rmphenyl) N-(2-cyanoethylpyrole)trans-bis(triphenylstibine)rhodium (III) ( 3) were prepared from trans-dichloro(η 1-phenyl)tris(triphenylstibine)rhodium(III) in ethanol. The mononuclear compounds trans-dichloro(η 1-phenyl)trans-bis(pyridine)(thiazole)rhodium(III) ( 2) and trans-dichloro(η 1-phenyl) trans-bis(pyridine)(pyrazine)rhodium(III) ( 4) were obtained from trans-dichloro(η 1-phenyl)tris(pyridine)rhodium(III) in methanol. Compounds 1 and 2 were analysed via X-ray diffraction techniques from single crystals. The four compounds were characterized by spectroscopy and elemental analysis. The coordination sphere geometry for all the complexes is octahedral; the trans influence exerted by the phenyl ligand is responsible for a long Rh N bond distance (2.263(3)Å) and of large downfield shifts for 1H NMR signals. The pyrazine ligands are monodentate in both 1 and 4. In the solid state the pyrazine ligands do not bridge the metal centres. Compounds 1 and 2 are barely stable in solution at room temperature. The thiazole molecule of 2 bonds the metal through the nitrogen atom whereas sulfur does not show any donating ability. An extended-Hückel analysis shows that the pyrazine ligands contribute significantly to the frontier unoccupied molecular orbitals. The atomic charges on the non-metal bound nitrogen atoms of pyrazine ligands is −0.175 e in agreement with a bridging ability between two metal centres for this ligand.

Change in Electron Configuration of Ferric Ion in Bis(cyanide)(meso-tetraalkylporphyrinatoiron(III)), [Fe(TRP)(CN)2]-, Caused by the Nonplanarity of the Porphyrin Ring

The synthesis and characterization of a series of bis(cyanide)(meso-tetraalkylporphyrinatoiron(III)), [Fe(TRP)(CN)2]- where R is H, Me, Et, and iPr, are reported. The 1H NMR spectrum of the unsubstituted [Fe(THP)(CN)2]- shows a pyrrole signal at δ = −23.19 ppm (−25 °C) in CD2Cl2, which is quite typical as a low spin ferric complex. As the bulkiness of the meso substituent increases, the pyrrole signal moves to lower magnetic field; 0.34, −2.26, and 11.94 ppm for [Fe(TMeP)(CN)2]-, [Fe(TEtP)(CN)2]-, and [Fe(TiPrP)(CN)2]-, respectively. Corresponding to the pyrrole proton signal, the cyanide carbon signal also exhibits a large downfield shift. The difference in chemical shifts between [Fe(THP)(CN)2]- and [Fe(TiPrP)(CN)2]- reaches as much as 1443 ppm at −25 °C. The substituent dependent phenomena are also observed in EPR spectra taken in frozen CH2Cl2 solution at 4.2 K. While the unsubstituted complex gives a so called large gmax type signal at 3.65, the alkyl substituted complexes exhibit axial type spectra; the EPR parameters for [Fe(TiPrP)(CN)2]- are g⊥ = 2.43 and g∥ = 1.73. These results clearly indicate that the electronic ground state changes from the usual (dxy)2(dxz, dyz)3 to the unusual (dxz, dyz)4(dxy)1 as the substituent becomes bulkier. Analysis of the EPR g values reveals that the orbital of the unpaired electron has more than 90% dxy character in the alkyl substituted complexes. The unusual electron configuration is ascribed to the destabilization of dxy orbital and/or stabilization of dxz and dyz orbitals caused by the S4 ruffled structure of the alkyl substituted porphyrin ring. Thus, in a strongly ruffled low spin complex such as [Fe(TiPrP)(L)2]±, electron configuration of iron is presented by (dxz, dyz)4(dxy)1 regardless of the kind and basicity of the axial ligand (L). In fact, low spin bis(pyridine) complex [Fe(TiPrP)(Py)2]+ gives a pyrrole signal at quite a low field, δ = +16.4 ppm at −87 °C, which is actually the lowest pyrrole signal ever reported for the low spin ferric porphyrin complexes. Correspondingly, the EPR spectrum taken at 77 K showed a clear axial type spectrum, g⊥ = 2.46 and g∥ = 1.59. In every case examined, (dxz,dyz)4(dxy)1 ground state is more or less stabilized by the addition of methanol as exemplified by the further downfield shift of the pyrrole proton and cyanide carbon signals together with the smaller EPR g⊥ values. The methanol effect is explained in terms of the stabilization of dxz and dyz relative to dxy due to the hydrogen bond formation between coordinated cyanide and methanol.

The vibrational dependence of the hydrogen and oxygen nuclear magnetic shielding constants in OH− and OH− · H2O

Abstract The effect of hydrogen binding and vibrational motions on the oxygen and hydrogen nuclear magnetic shielding constants in the OH − and mono-hydrated OH − and mono-hydrated OH − ion (H 3 O 2 − ) is investigated by ab initio calculations. A large down-field shift in 17 O shieldings and a small down-field shift in the 1 H shieldings is found for H 3 O 2 − relative to OH − . The dependence of the nuclear magnetic shielding constants in H 3 O 2 on the strongly anharmonic symmetric and antisymmetric O···H···O stretching motions and on the internal rotation motion of the outer hydrogens is studied with the non-rigid bender model Hamiltonian [V. Spirko, W. P. Kraemer and A. Cejchan, J. Mol. Spectrosc. 136 (1989) 340] at the level of the random phase approximation (RPA). The dependence of the shielding constants in OH − on the bond length is investigated at the level of the RPA and the second order polarization propagator approximation (SOPPA). Pertinent (analytic) nuclear magnetic shielding functions are obtained by fitting to the ab initio shielding points and these functions are used to calculate the vibrational averages using the corresponding vibrational eigenfunctions. The predicted effective shielding constants of H 3 O 2 − exhibit a sizable and non-monotonic dependence on the stretching vibrational quantum numbers, whereas the dependence on the internal rotation is practically negligible. The effective shielding constants of OH − show an even larger dependence on the vibrational quantum number. The effect of the end-over-end rotational motion, however, is small.

Use of 1H-15N heteronuclear multiple-quantum coherence NMR spectroscopy to study the active site of aspartate aminotransferase.

Aspartate aminotransferase from Escherichia coli, an 88 kDa enzyme, was uniformly and selectively enriched with 15N and was studied by heteronuclear multiple-quantum coherence NMR spectroscopy in H2O. Good resolution was obtained for the downfield region (above 9.5 ppm chemical shift in the 1H dimension) for NH protons in the amide, indole, imidazole, and guanidinium group regions and several resonances were tentatively assigned. Two downfield resonances, at 12.6 and 11.36 ppm, appear to belong to oxygen- or sulfur-bound protons. The most downfield amide resonance at 11.78 ppm was assigned to the active site cysteine 192 whose peptide proton is 2.9 A away from the negatively charged carboxyl group of aspartate 199. Large downfield shifts (up to 1.15 ppm) of the indole NH resonance of the active site tryptophan 140 were observed upon binding of dicarboxylic inhibitors to the pyridoxal 5'-phosphate (PLP) form and of inorganic dianions to the pyridoxamine 5'-phosphate (PMP) form of the enzyme. We discuss these striking differences in the light of the available crystallographic data. Active sites of proteins, as well as specific inhibitory molecules, often contain negatively charged groups. These may be able to form hydrogen-bonds to NH groups and to shift the NH resonances downfield into a less crowded and therefore more readily observable region for many large proteins. Our approach, which makes use of both HMQC spectroscopy and NOE observations, should be widely applicable.

13C NMR chemical shifts of methyl cation and anion: A relationship between chemical shift and charge?

The chemical shifts of methyl cation and anion have been calculated. The tensor components about the out-of-plane p orbitals were essentially the same, but the components about the in-plane axes differed by over 700 ppm. The experimental chemical shifts of benzene and the aromatic ions fall on the line connecting the shifts and charges of the methyl ions. These data are correlated by a simple model, that also accounts for the large downfield shifts found with carbenes. No direct connection between charge and chemical shift was found.

NMR Detection of Arginine-Ligand Interactions in Complexes of Lactobacillus casei Dihydrofolate Reductase

1H-NMR and 15N-NMR signal assignments have been made for the eight arginine residues in Lactobacillus casei dihydrofolate reductase in its binary complex with methotrexate and in its ternary complex with methotrexate and NADPH. 1H-NMR chemical shifts for the guanidino groups of two of the arginines (Arg57 and Arg43) were sensitive to different modes of binding of the guanidino groups with charged oxygen atoms of the ligands. In the complexes formed with methotrexate, Arg57 showed four non-equivalent NH eta proton signals indicating hindered rotation about the N epsilon-C zeta and C zeta-N eta bonds. The NH eta 12 and NH eta 22 protons showed large downfield shifts, which would be expected for a symmetric end-on interaction of these protons with the charged oxygen atoms of a carboxylate group in methotrexate. These effects were not observed for the complex formed with trimethoprim, which does not contain any carboxylate groups. In the complex formed with NADPH present, Arg43 showed a large downfield chemical shift for its NH epsilon proton and a retardation of its rate of exchange with water. This pattern of deshielding contrasts with that detected for Arg57 and is that expected for a side-on interaction of the guanidino group protons with charged oxygen atoms of the ribose 2'-phosphate group of NADPH.

Bond Fixation in a [14]Annulene: Synthesis, Characterization, and ab Initio Computations of Furan Adducts of Dimethyldihydropyrene

Furan adducts, 13, 15, and 16, of dimethyldihydropyrene are prepared in order to test the postulate that bicyclic annelations are generally effective at inducing bond localization in aromatic systems. A large 2-ppm down-field shift of the 1H NMR shifts of the internal methyl signals in 15 compared to 16 provides a significant indicator of bond localization in 15. X-ray diffraction analysis of 13 displays regular bond length alternation. Ab initio computations that do not include dynamic electron correlation are found to be inadequate for modeling the molecular structure of 1. Density Functional Theory models 1 well and predicts bond localization in derivatives of 1 consistent with the observed NMR spectroscopic and X-ray diffraction results.

Sequence-selective interaction between mercury(II) ions and the DNA dodecamer [d(GCCGATATCGGC)]2 studied by 1H NMR spectroscopy.

The palindromic dodecamer [d(GCCGATATCGGC)]2 has been titrated by Hg(ClO4)2 in order to study sequence dependent HgII ion interactions. The titration pattern as monitored by 1D and 2D 1H NMR is consistent with a transition to a new conformer of the dodecamer induced by HgII ions. At intermediate stages of the titration, the proton signals from the new conformer coexist with those of the original one, indicating slow exchange between the two forms on the NMR timescale. The data clearly show that there is no major alteration in the secondary structure, e.g. B-->Z-form or duplex-->hairpin transition. The intra- and inter-residue sequential walk is completed except for a break between T6 and A7. At a concentration level r = [HgII]/[nucleobase] < 0.20 all four central imino signals are present. This definitely excludes thymine N3 as a possible mercuration site. In the imino region of the spectra HgII ions induce a large upfield shift of the thymine imino resonance T8-N3H, while the other thymine resonance T6-N3H is unperturbed. The guanine imino signal G4-N1H shows a large downfield ring current shift caused by major conformational changes in the duplex. The complete titration experiment indicates that mercury, initially, binds selectively to the A5'-T8 base pair. A tentative model is proposed where mercury is cross-linking the two strands via thymine T8-O4 and the deprotonated amino group of the complementary adenine base A5'.

Molecular structure of iodo [1.1]ferrocenylruthenocenophanium + BF 4− salt

The structure of iodo[1.1]ferrocenylruthenocenophanium + BF 4 − ( 1), prepared by oxidation of [1.1]ferrocenylruthenocenophane ([1.1])FcRc) with iodoruthenocenium + BF 4 − ([RcHI] + BF 4 −) was analyzed by single-crystal X-ray diffraction. The crystal form of 1 is triclinic, space group P 1, a = 9.794( 2), b = 14.393( 4), c = 7.777(2) A ̊ , α = 101.62(4), β = 93.17(4), γ = 72.09(2)°, Z = 2 , and the final R and R w were both 0.052. The cation exists in a syn-conformation, as with other [1.1]metallocenophanes, and is given the formula [Fe(II)(C 5H 4CH 2C 5H 4) 2Ru(IV)I] + with Ru(IV)-I bond (2.751(1) Å) formation in the solid. The distance between the Ru(IV) and Fe(II) atoms is 4.719(1) Å, which is slightly shorter than the value for [1.1]FcRc, and the bond angle of IRu(IV)Fe(II) is 91.50(1)°. The cation has two kinds of twisted C 5H 4CH 2C 5H 4 systems. The cyclopentadienyl rings in the [Cp(C 5H 4)RuI] + moiety are highly slanted (the dihedral angle between the Cp ring and fulvenide ligand is 33.87°) owing to the Ru(IV)-I bond. Large downfield shifts of the 13C CP/MAS NMR signals for the ruthenocene moiety are explained by the same fact.

Structural consequences of histidine phosphorylation: NMR characterization of the phosphohistidine form of histidine-containing protein from Bacillus subtilis and Escherichia coli.

The bacterial phosphoenolpyruvate:sugar phosphotransferase system involves a series of reactions in which phosphoprotein intermediates are formed. Histidine-containing protein (HPr) is phosphorylated on the N delta 1 position of the imidazole ring of His15 by enzyme I and acts as a phosphoryl donor to the sugar-specific enzymes IIA. The structure of phosphorylated HPr from Bacillus subtilis, primarily, and from Escherichia coli has been studied by nuclear magnetic resonance (NMR) spectroscopy. Phosphorylation of His15 results in large downfield shifts in amide proton and nitrogen resonances for residues 16 and 17 but results in only modest or no shifts in other backbone resonances. The exchange rates of these two amide groups are decreased more than 10-fold upon phosphorylation. Analysis of the coupling constants 3JNH alpha revealed no significant changes throughout the protein, indicating that backbone phi dihedral angles do not change detectably. 3J alpha beta and 3JN beta patterns determined from P.E.COSY and HNHB spectra, respectively, revealed a change in one side chain, that of conserved Arg17. Analysis of NOESY spectra revealed a limited number of changes in NOEs involving protons in Ser12, His15, Arg71, and Pro18 in B. subtilis HPr. The NMR results indicate that the Arg17 side chain becomes limited in its conformational range in the phosphorylated protein, taking on a conformation that points its guanidinium group toward the phosphoryl group on His15. In addition, the tautomeric and ionization states of His15 have been determined using 15N and 31P NMR. At neutral pH, the imidazole is predominantly in the protonated form and the phosphoryl group is in the dianionic form in P-His15. Altogether, the results indicate that phosphorylation of His15 yields only a local effect on the protein's structure. The data are consistent with a small change in the disposition of the histidine side chain, allowing phosphoryl group oxygens to serve as hydrogen bond acceptors for the amide protons of residues Ala16 and Arg17, which constitute the first two residues of an alpha-helix. Thus, similar to many proteins that bind phosphoryl moieties noncovalently, the phosphoryl group in P-His15-HPr is situated to allow for a favorable electrostatic interaction at the N-terminal end of an alpha-helix.

Dirhenium(III) complexes with N, N-dimethyladenine and 7-azaindole

Re 2X 2(CH 3CO 2) 4 (X=Cl, Br) reacts with anionic N6,N6-dimethyladenine (dmad −) in ethanol to form dark purple Re 2X 2(dmad) 4, in which the acetate ligands are replaced by the N3/N9-bridging purine. Different relative orientations of the dissymmetric ligands about the ReRe axis lead to four stereoisomers producing resolved signals in the 1H NMR spectra. The compound can be reversibly protonated at N7 to yield the [ReX 2(Hdmad) 4] 4+. ion. Hdmad and (Bu 4N) 2[Re 2Cl 8] in ethanol form a red-brown dinuclear material formulated as [Re 2Cl 4(Hdmad) 2]- Cl·4H 2O·0.5C 2H 5OH, where the purine is N3/N9-bridging and N7-protonated. Heating (Bu 4N) 2[Re 2Cl 8] and 7- azaindole (Haza) without solvent leads to a brown Re 2Cl 2(aza) 4 material, in which different orientations of the N1/N7-bridging ligand again produce various stereoisomers. Magnetic anisotropy of the quadruple metal-metal bond probably contributes to the very large downfield shifts observed for the 13C signals of the carbon atoms just above the ReRe vector.

Conformational differences between complexes of elongation factor Tu studied 19F-NMR spectroscopy.

An analogue of elongation factor Tu (EF-Tu) from Escherichia coli was prepared by biosynthetic incorporation of 3-fluorotyrosine. The 19F-NMR spectra of the binary complexes of this protein with GDP, GTP and elongation factor Ts (EF-Ts) and the ternary complexes EF-Tu.GDP.aurodox and EF-Tu.GDP.EF-Ts were measured. EF-Tu contains ten tyrosine residues and all of the complexes studied gave complex 19F spectra with overlapping resonances. EF-Tu.GDP gave a spectrum in which two signals were markedly different from those shown by the other complexes, the two resonances being shifted downfield by at least 3.4 ppm and 0.9 ppm relative to their shifts in the other complexes. Such large downfield shifts can be explained by second-order electric field shielding effects resulting from these two tyrosine residues being in a sterically constrained environment in EF-Tu.GDP and with the steric restraints being released in all of the other complexes. The X-ray diffraction structure of EF-Tu.GDP shows that Tyr87 in the N-terminal domain (domain I) and Tyr309 in the C-terminal domain (domain III) are both buried within the protein and are close to each other: these residues are in regions of EF-Tu previously implicated in the structural changes between EF-Tu.GDP and EF-Tu.GTP by other workers. If these tyrosine residues correspond to the two downfield resonances of the spectra of EF-Tu.GDP, the results from the 19F-NMR would be consistent with these earlier indications that domain I interacts closely with domain III in EF-Tu.GDP and that the amino acids between Gly83 and Gly100 are an important part of this interaction. For all the other complexes studied, these tyrosines are in a less sterically crowded environment consistent with a weaker interaction between the two domains. The 19F-NMR spectrum of the trypsin-cleaved product of EF-Tu.GDP, from which the X-ray diffraction structural data have been obtained, shows no significant differences from the native protein so that trypsin cleavage causes no large changes in the protein's structure.

Solid-state distortions of linear mono-, bi-, and trimetallic bis(tri- n-butylphosphine) nickel and palladium complexes having 1,4-tetrafluorophenylene bridges as observed via 31P CP/MAS NMR spectroscopy

31P CP/MAS NMR spectra were recorded for a series of eight linear mono-, bi-, and trimetallic bis(tri- n-butylphosphine) nickel and palladium complexes having 1,4-tetrafluorophenylene bridges. A 2 type spectra were obtained with Ni(PBu 3) 2(1,4-C 6F 4H) 2 ( I), Pd(PBu 3) 2(1,4-C 6F 2) ( V) and [Pd(PBu 3) 2(1,4-C 6F 4H)] 2(1,4-C 6F 4) ( VII). AB type spectra were obtained with Ni(PBu 3) 2(1,4-C 6F 4H)Br ( II), [Ni(PBu 3) 2(1,4-C 6F 4)Br 2 ( III) and [Pd(PBu 3) 2Cl] 2(1,4-C 6F 4) ( VI). The 2 J PP scalar coupling constants fell in the region of 300 to 450 Hz as expected for P-metal-P pairs in a trans configuration. The spectra of [Ni(PBu 3) 2] 3(1,4-C 6F 4) 2Br 2 ( IV) and [Pd(PBu 3) 2] 3(1,4-C 6F 4) 2Cl 2 ( VIII) consisted of overlapping A 2 and AB spins systems. Large upfield and downfield shifts were measured in the solid samples relative to the corresponding 31P NMR spectra in CDCl 3 solution. Correlating the 31P CP/MAS NMR data with reported X-ray crystallographic measurements revealed that both the chemical inequivalence and the solid versus solution resonance frequency shifts were the result of ring current effects brought about by distortions from square-planar geometry at the metal centre in the solid state.

Synthesis and first X-ray structure analysis of a stabilized chiral chlorobismuthine: fixation of molecular geometry induced by the intramolecular coordination of a sulfonyl group

A sulfonyl-stabilized chiral chlorobismuthine 1a was synthesized by the selective fluorodearylation of compound 3a with boron trifluoride–diethyl ether, followed by halogen exchange of the resulting fluorobismuthine 4a with brine. The 1H NMR spectrum of compound 1a showed an unusually large downfield shift of a proton signal (δ 9.16), and X-ray structure analysis revealed that this unique shift was due to the anisotropic deshielding of the proton adjacent to the bismuth atom by the chlorine atom in close proximity. The bismuth centre of compound 1a was found to adopt a distorted pseudotrigonal bipyramidal geometry, probably through the formation of a hypervalent 3-centre 4-electron bond with the oxygen and chlorine atoms at apical positions. This is in marked contrast to the bismuthine 3a which has a pyramidal configuration characteristic of trivalent organobismuth compounds. A quite short Bi–O distance [2.592(5)A] of compound 1a, compared with that [2.914(6)A] of the bismuthine 3a, indicates that the introduction of an electronegative chlorine atom onto the bismuth atom enhances the Lewis acidity of the metal centre, leading to the pronounced intramolecular Bi-O interaction.