Chemical Shifts Of Aromatic Protons Research Articles

Catalytic amination of benzyl alcohol using ruthenium cymene compounds containing bidentate N,O-donor ancillary ligands

A series of ruthenium-cymene derivatives subjecting [Ru(η6-cymene)Cl2]2 and substituted bidentate N,O donor precursors have been synthesized and characterized. Reacting [Ru(η6-cymene)Cl2]2 with two equivalents the lithium salt of phenyl keto-amine ligands LiL1∼LiL3 resulted in the formation of [Ru(η6-cymene)(L)Cl] (1–3). Similarly, the reaction between [Ru(η6-cymene)Cl2]2 and two equivalents of LiL4 or NaL5 afforded compounds {Ru(η6-cymene)[L4)]Cl} (4) and [Ru(η6-cymene)(L5)Cl] (5). The 1H NMR chemical shifts of aromatic protons of the cymene fragment for compounds 1–5 fell in the range δ 5.54–3.58, showing chemical shifts farther upfield than the corresponding protons of [Ru(η6-cymene)Cl2]2 due to the intramolecular ring current effect. The 2,6-diisopropylphenyl substituted pheny lketo-amine ligand of compound 3 shows a slow C-N bond rotation with the rotation rate constant at ca. 4.62 s−1. The catalytic aminations of benzyl alcohol with benzylamine using compounds 1–3 as catalysts resulted in higher conversion than using compounds 4 and 5. Two products, PhCH = NCH2Ph and HN(CH2Ph)2 were detected, with PhCH = NCH2Ph being the major product. When benzyl alcohol was reacted with phenylamine in the presence of compounds 1–5, the conversions are all greater than 60% and PhCH = NPh was the major product. While sterically hindered 2,6-diisopropylaniline appeared in alcohol amination reactions in the presence of catalysts, no imine or amine products were detected.

Abstract 3093: 3D structural report of NRAS isoform 5

Abstract The protein NRAS is part of the NRAS-BRAF-MEK-ERK signaling cascade. Recently, 5 NRAS isoforms were discovered by our group, and it was observed that isoform 5 overexpression in vitro resulted in a more aggressive cell phenotype. NRAS isoform 5 increases phosphorylation of downstream targets (AKT, MEK, and ERK). The structural report is unavailable for NRAS isoform 5. In addition, it was found in the current study that the NRAS isoform 5 does not have GTPase activity due to lack the canonical GTP binding region. Given the importance of this pathway in driving melanoma progression, we studied the structural details of this novel NRAS isoform 5 by two biophysical techniques, NMR and CD spectroscopy. The circular dichroism spectra of NRAS isoform 5 that was C-acetylated and N-amidated was measured using 11%, 14%, 21%, 45%, 56% and 85% trifluoroethanol (TFE) in a phosphate buffer. The secondary structural elements were induced at ∼14-15% TFE. There were no qualitative differences in the CD spectra between 56% and 85% TFE. Therefore 56% deuterated TFE was utilized in the NMR structural studies. Two dimensional (2D) homonuclear and heteronuclear experiments were acquired at 25°C with Bruker Avance III HD 600 and 800 MHz NMR spectrometers (Campus Chemical Instrument Center NMR facility), each equipped with a triple resonance z axis gradient TXI cryoprobe for amino acid chemical shift assignments and structural determination of NRAS isoform 5. The NMR chemical shifts for the protons were assigned using traditional 2D TOCSY, COSY, and NOESY experiments. The 2D proton-carbon HSQC was utilized to facilitate the assignment of the alpha and side chain aliphatic protons. In the absence of TFE, there were no observable NOEs that could represent ordered secondary structures. Other than intra residue and short range sequential NOEs (i-j ≤ 2), only a weak medium range NOE was observed from the aromatic protons of Tyr 4 to the gamma proton of Val 7/8. The secondary structure of NRAS isoform 5 was determined via the same NMR techniques in 56% TFE. The secondary structure was defined by 69 sequential NOEs and 39 medium range NOEs. NOEs characteristic of helical structure were observed in the NOESY spectra. For example, NOEs were measured from H(alpha, i) to H(beta, i+3) for residues Tyr 4, Lys 5, and Leu 6. The tertiary fold was determined by 10 key long-range NOEs. The flexible C terminal is brought in close proximity to the N-terminal helix by 4 NOEs between Val 8/9 and Val 14, and 5 NOEs between Tyr 4 and Try 20 to form a helix-turn-coil structure in presence of TFE. In addition, the proton chemical shifts for the aromatic proton chemical shifts for the tyrosine aromatic ring became distinct. NRAS isoform 5 is highly flexible in aqueous solution, but forms a helix-turn-coil structure in the presence of trifluoroethanol as determined by NMR and CD spectroscopy. This data may be utilized as a starting point to understand the biophysical interactions of this novel NRAS isoform. Citation Format: Joseph Markowitz, Tapas K. Mal, Chunhua Yuan, Nicholas B. Courtney, Mitra Patel, Andrew R. Stiff, James Blachly, Christopher Walker, Ann-Kathrin Eisfeld, Albert de la Chapelle, William E. Carson. 3D structural report of NRAS isoform 5. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3093.

Directing peptide conformation with centrally positioned pre-organized dipeptide segments: studies of a 12-residue helix and β-hairpin.

Secondary structure formation in oligopeptides can be induced by short nucleating segments with a high propensity to form hydrogen bonded turn conformations. Type I/III turns facilitate helical folding while type II'/I' turns favour hairpin formation. This principle is experimentally verified by studies of two designed dodecapeptides, Boc-Val-Phe-Leu-Phe-Val-Aib-Aib-Val-Phe-Leu-Phe-Val-OMe 1 and Boc-Val-Phe-Leu-Phe-Val-(D)Pro-(L)Pro-Val-Phe-Leu-Phe-Val-OMe 2. The N- and C-terminal flanking pentapeptide sequences in both cases are identical. Peptide 1 adopts a largely α-helical conformation in crystals, with a small 310 helical segment at the N-terminus. The overall helical fold is maintained in methanol solution as evidenced by NMR studies. Peptide 2 adopts an antiparallel β-hairpin conformation stabilized by 6 interstrand hydrogen bonds. Key nuclear Overhauser effects (NOEs) provide evidence for the antiparallel β-hairpin structure. Aromatic proton chemical shifts provide a clear distinction between the conformation of peptides 1 (helical) and 2 (β-hairpin). The proximity of facing aromatic residues positioned at non-hydrogen bonding positions in the hairpin results in extensively ring current shifted proton resonances in peptide 2.

Synthesis and physicochemical properties of fac-[Re(CO)3(κ2-N,N-dpkfah)Cl], dpkfah=di-2-pyridyl ketone 2-furoic acid hydrazone: the molecular structure of fac-[Re(CO)3(κ2-N,N-dpkfah)Cl] · acetone

Reaction between [Re(CO)5Cl] and di-2-pyridyl ketone 2-furoic acid hydrazone (dpkfah) (1) in refluxing toluene gave fac-[Re(CO)3(κ2-N,N-dpkfah)Cl] (2). Spectroscopic and electrochemical measurements disclosed sensitivity of 2 to its surroundings. 1H-NMR measurements showed that the amide proton exchanged with solvent protons, and its chemical shift is solvent and temperature dependent, while the chemical shifts of aromatic protons are solvent and temperature independent. Electronic absorption spectra of 2 divulged two intra-ligand charge transfer transitions (ILCT) in protophilic solvents and a single ILCT transition in non-protophilic solvents. Optical measurements on protophilic solutions of 2 established an equilibrium between 2 and its conjugate base, fac-[Re(CO)3(κ2-N,N-dpkfah-H)Cl]− (3). Thermo-optical measurements confirmed that the interconversion between 2 and 3 and gave ΔG ø values of −26.48 and 22.99 kJ mol−1, respectively, for the protonation of DMF and DMSO by 2. Optosensing measurements showed that [MCl2] (M = Zn, Cd, or Hg) in concentrations as low as 1.00 × 10−7 mol L−1 can be detected and determined using protophilic solutions of 2. Electrochemical measurements showed 2 to be more stable in CH3CN than DMF. Single-crystal X-ray structural analysis on fac-[Re(CO)3(κ2-N,N-dpkfah)Cl] · acetone (4) obtained from an acetone solution of 2 confirmed the solvent–complex interaction and revealed two symmetry-independent molecules in the asymmetric unit. The extended structure of 4 disclosed parallel stacks connected via a network of classic and non-classic hydrogen bonds.

Detoxification of amitriptyline by oligochitosan derivatives.

Oligo-chitosans were chemically modified with dinitrophenyl groups for selective and rapid adsorption of amitriptyline by forming pi-pi complexes. 1H-NMR was utilized not only for characterization of modified chitosans but also for monitoring the aromatic-aromatic interaction. The variation in the chemical shift of aromatic protons was followed to monitor the aromatic-aromatic interaction. Upfield shift of aromatic protons of dinitrophenyl groups supports aromatic-aromatic interactions with amitriptyline. Drug uptake test by HPLC reveals that dinitrophenyl chitosan particles (1-2 microm) at 0.4 wt% (w/v) in a saline solution (pH 6.9) adsorb 90% amitriptyline within 30 min.

Synthetic, Infrared, 1H And 13C NMR Spectral Studies on Potassium Salts of N-Chloroarylsulphonamides

Several N-chloroarylsulphonamides of the configuration, 4-X-C6H4SO2(K)NCl・xH2O (where X = H, CH3, C2H5, F, Cl or Br) and i-X, j-YC6H3SO2(K)NCl・xH2O (where i-X, j-Y = 2,3-(CH3)2; 2,4- (CH3)2; 2,5-(CH3)2; 2-CH3,4-Cl; 2-CH3,5-Cl; 3-CH3,4-Cl; 2,4-Cl2 or 3,4-Cl2) are prepared, characterised, and their infrared spectra in the solid state and NMR spectra in solution are measured and correlated. Comparison of the infrared spectra of the potassium salts of N-chloro-arylsulphonamides with the corresponding arylsulphonamides shows that the strong absorptions in the range 947 - 933 cm−1 are due to N-Cl stretching vibrations. The effect of ring substitution on the N-Cl frequencies is non-uniform. The frequencies in the ranges 1404 - 1370 cm−1 and 1149 - 1125 cm−1 are respectively assigned to S=O asymmetric and symmetric vibrations. The effect of substitution in the phenyl ring in terms of electron withdrawing and electron donating groups is non-systematic. Empirical correlations relating the chemical shifts to the structures are considered. The chemical shifts of aromatic protons and carbons in all the N-chloroarylsulphonamides have been calculated by adding substituent contributions to the shift of benzene, as per the principle of substituent addition. Considering the approximation made, the agreement between the calculated and experimental chemical shifts is reasonably good.

1H and 13C NMR Spectral Studies on N-(Aryl)-Substituted Acetamides, C6H5NHCOCH3-iXi and 2/4-XC6H4NHCOCH3-iXi (where X = Cl or CH3 and i = 0, 1, 2 or 3)

35 N-(Phenyl)-, N-(2/4-chlorophenyl)- and N-(2/4-methylphenyl)-substituted acetamides are prepared, characterised and their NMR spectra studied in solution state. The variation of the chemical shifts of the aromatic protons in these compounds follow more or less the same trend with changes in the side chain. The chemical shifts remain almost the same on introduction of Cl substituent to the benzene ring, while that of methyl group lowers the chemical shifts of the aromatic protons. But only 13C-1 and 13C-4 chemical shifts in these compounds are sensitive to variations of the side chain. The incremental shifts in the chemical shifts of the aromatic protons and carbons due to -COCH3−iXi or NHCOCH3−iXi groups in all the N-(phenyl)-substituted acetamides, C6H5NHCOCH3−iXi (where X = Cl or CH3 and i = 0, 1, 2 or 3) are calculated. These incremental chemical shifts are used to calculate the chemical shifts of the aromatic protons and carbons in all the N-(2/4-chlorophenyl)- and N-(2/4-methylphenyl)-substituted acetamides, in two ways. In the first way, the chemical shifts of aromatic protons or carbons are computed by adding the incremental shifts due to -COCH3−iXi groups and the substituents at the 2nd or 4th position in the benzene ring to the chemical shifts of the corresponding aromatic protons or carbons of the parent aniline. In the second way, the chemical shifts are calculated by adding the incremental shifts due to -NHCOCH3−iXi groups and the substituents at the 2nd or 4th position in the benzene ring to the chemical shift of a benzene proton or carbon, respectively. Comparison of the two sets of calculated chemical shifts of the aromatic protons or carbons of all the compounds revealed that the two procedures of calculation lead to almost the same values in most cases and agree well with the experimental chemical shifts.

Infrared, 1H and 13C NMR Spectra of N,N-Dichloroarylsulphonamides, 4-X-C6H4SO2NCl2 (X = H, CH3, C2H5, F, Cl or Br) and i-X, j-YC6H3SO2NCl2 (i-X, j-Y = 2,3-(CH3)2, 2,4-(CH3)2, 2,5-(CH3)2, 2-CH3, 4-Cl, 2-CH3, 5-Cl, 3-CH3, 4-Cl, 2,4-Cl2 or 3,4-Cl2)

Several mono- and di-substituted N,N-dichloroarylsulphonamides of the configuration, 4-XC6H4SO2NCl2 (where X = H, CH3, C2H5, F, Cl or Br) and i-X, j-YC6H3SO2NCl2 (where i- X, j-Y = 2,3-(CH3)2, 2,4-(CH3)2, 2,5-(CH3)2, 2-CH3,4-Cl, 2-CH3,5-Cl, 3-CH3,4-Cl, 2,4-Cl2 or 3,4-Cl2), respectively, were prepared, characterised and their infrared spectra in the solid state and NMR spectra in solution state were measured and correlated. Comparison of the infrared spectra of the N,N-dichloroarylsulphonamides with the corresponding arylsulphonamides and Nchloroarylsulphonamides revealed that the infrared absorption bands in the ranges, 790 - 735 cm−1 and 595 - 546 cm−1 are due to N-Cl asymmetric and symmetric stretching vibrations, respectively, and that the effect of ring substitution on the N-Cl frequencies is not consistent. The frequencies in the ranges 1384 - 1333 cm−1 and 1181 - 1143 cm−1 are, respectively, assigned to S=O asymmetric and symmetric modes of vibration. The effect of substitution in the phenyl ring in terms of electron withdrawing and electron donating groups is non-systematic. Since the chemical shift depends on the electron density around the nucleus, empirical correlations relating the chemical shifts to the structures have been considered. The chemical shifts of aromatic protons and carbons in all the N,Ndichloroarylsulphonamides have been calculated by adding substituent contributions to the shift of benzene. Considering the approximation made, the agreement between the calculated and experimental chemical shifts is good.

Infrared and NMR (1H & 13C) Spectra of Sodium Salts of N-Bromo-Mono and Di-Substituted-Benzenesulphonamides

Fifteen sodium salts of mono and di-substituted N-bromobenzene-sulphonamides of the configuration, 4-X-C6H4SO2NaNBr (where X = H; CH3; C2H5; F; Cl; Br; or NO2) and i-X,j- YC6H3SO2NaNBr (where i-X, j-Y = 2,3-(CH3)2; 2,4-(CH3)2; 2,5-(CH3)2; 2-CH3,4-Cl; 2-CH3,5-Cl; 3-CH3,4-Cl; 2,4-Cl2 or 3,4-Cl2) are prepared and characterised by measuring their infrared spectra in the solid state and NMR spectra in solution. The N-Br vibrational frequencies, νN−Br of N-bromoarylsulphonamides vary in the range, 945 - 925 cm−1, while the N-Cl vibrational frequencies, νN−Cl, are observed in the range 950 - 927 cm−1 for the corresponding N-chloroarylsulphonamides. Asymmetric and symmetric SO2 stretching vibrations appear in the ranges, 1391 - 1352 cm−1 and 1148 - 1131 cm−1 for the monosubstituted N-bromoarylsulphonamides, while for the disubstituted N-bromocompounds they absorb in the ranges 1391 - 1331 cm−1 and 1149 - 1121 cm−1, respectively. The chemical shifts of aromatic protons and carbon-13 in all the N-bromoarylsulphonamides have been calculated by adding substituent contributions to the shift of benzene and compared with the observed values. The agreement between the calculated and experimental chemical shifts for different protons or carbon-13 is quite good.

Spectroscopic Studies with N-Chloroarylsulphonamides: IR and 1H, 13C and 23Na Nmr Spectra of Sodium Salts of N-Chloro-Mono- and Di-Substituted-Benzenesulphonamides, 4-X-C6H4SO2NANCl (X = H; CH3; C2H5; F; Cl; Br; I or NO2) and i-X, j-YC6H3SO2NANCl (i-X, j-Y = 2,3-(CH3)2; 2,4-(CH3)2; 2,5-(CH3)2; 2-CH3, 4-Cl; 2-CH3, 5-Cl; 3-CH3, 4-Cl; 2,4-Cl2 or 3,4-Cl2)

In an effort to introduce N-chloroarylsulphonamides of different oxydising strengths, sixteen sodium salts of N-chloro-mono- and di-substituted benzenesulphonamides of the configuration, 4- X-C6H4SO2NaNCl (where X = H; CH3; C2H5; F; Cl; Br; I or NO2) and i-X, j-YC6H3SO2NaNCl (where i-X, j-Y = 2,3-(CH3)2; 2,4-(CH3)2; 2,5-(CH3)2; 2-CH3,4-Cl; 2-CH3,5-Cl; 3-CH3,4-Cl; 2,4- Cl2 or 3,4-Cl2) are prepared, characterized through their infrared spectra in the solid state and NMR spectra in solution. The υN-Cl frequencies vary in the range 950 - 927 cm−1. Effects of substitution in the benzene ring in terms of electron donating and electron withdrawing groups have been considered, and conclusions drawn. The chemical shifts of aromatic protons and carbon-13 in all the N-chloroarylsulphonamides have been calculated by adding substituent contributions to the shift of benzene. Considering the approximation employed the agreement between the calculated and experimental chemical shift values for different protons or carbon-13 is quite good. Effects of phenyl ring substitution on chemical shift values of both 1H and 13C are also graphically represented in terms of line diagrams.

Infrared and NMR Spectra of Arylsulphonamides, 4-X-C6H4SO2NH2 and i-X, j-YC6H3SO2NH2 (X=H; CH3; C2H5; F; Cl; Br; I or NO2 and i-X, j-Y=2,3-(CH3)2; 2,4-(CH3)2; 2,5- (CH3)2; 2-CH3, 4-Cl; 2-CH3, 5-Cl; 3-CH3, 4-Cl; 2,4-Cl2 or 3,4-Cl2)

Several arylsulphonamides of the configuration, 4-X-C6H4SO2NH2 (where X= H; CH3; C2H5; F; Cl; Br; I or NO2) and i-X, j-YC6H3SO2NH2 (where i-X, j-Y=2,3-(CH3)2; 2,4-(CH3)2; 2,5-(CH3)2; 2-CH3,4-Cl; 2-CH3,5-Cl; 3-CH3,4-Cl; 2,4-Cl2 or 3,4-Cl2) were prepared, and their infrared spectra were measured in the solid state. The NMR spectra were recorded in solution. N-H asymmetric and symmetric stretching vibrations absorb in the ranges, 3390 - 3323 cm-1 and 3279 - 3229 cm-1, respectively. Asymmetric and symmetric SO2 stretching vibrations appear as strong absorption lines in the ranges, 1344 - 1317 cm-1 and 1187 - 1147 cm-1, respectively. Sulphonamides exhibit S-N stretching vibrational absorptions in the range, 924 - 906 cm-1. The effect of substitution in the phenyl ring in terms of electron withdrawing and electron donating groups could not be generalised, as the effect is non-systematic. The chemical shift is highly dependent on the electron density around the nucleus or associated with the atom to which it is bonded. Hence empirical correlations relating the chemical shifts to the structures have been discussed. The chemical shifts of aromatic protons and carbons in all the arylsulphonamides have been calculated by adding substituent contributions to the shift of benzene, the principle of substituent addition. Considering the approximation made, the agreement between the calculated and experimental chemical shift values is reasonably good. Generally, electron-withdrawing groups shows high chemical shifts compared to electron-donating groups.

A Study on the Spectra Characterization of Ruthenium(Ii) Complexes

A series of Ru(II) complexes have been synthesized, and their electronic spectra and NMR spectroscopy properties were characterized. the chemical shifts of aromatic protons of [Ru(bpy)3] (PF6)2, [Ru(phen)3](ClO4)2 and [Ru(bqdi)3](PF6) move downfield, but the resonance peaks of cis-Ru(bpy)2Cl2 shift upfield. Within the visible spectra of the ruthenium(II) complexes appear a relatively high oscillator strength which is referred to as the π(Ru)→* (ligands) transition.

Benzhydroximic acids — NMR study of trimethylsilyl derivatives

NMR spectra ( 1H 13C, 15N, and 29Si) of trimethylsilylated para and meta substituted benzhydroxamic acids were studied in chloroform solutions. The silylation products have the structure of Z-O,O′-bis(trimethylsilyl) derivatives of benzhydroximic acid, independently of the ring substituent. According to aromatic proton chemical shifts, the geometry of the hydroximic group and its torsion angle with the ring plane are not affected by the para substituent. The chemical shifts of the nuclei in the hydroximic part of the molecule show surprisingly strong dependence on the remote ring substituent. The two 29Si chemical shifts exhibit essentially the same sensitivity to substitution despite the fact that the Si(O 1) is one bond closer to the substituent than the Si(O 4) silicon. It is suggested that while electron donor substituents increase the shielding of the silicon atoms they also increase the basicity of the oxygen in the SiO moiety, thus leading to the stronger hydrogen bonding with the solvent. Association with chloroform partially compensates the direct substituent effect on the shielding in the case of Si(O 1) silicon. The influence of other factors not covered by substituent constants is demonstrated by excellent correlations with the chemical shifts in analogous tert-butyldimethylsilyl derivatives.

Molecular recognition of phenethylamine, tyramine and dopamine with new anionic cyclophanes in aqueous media

Water-soluble cyclophanes functionalized by amide groups and pendant carboxymethyl groups have been synthesized, in a single step, by a condensation reaction between ethylenediaminetetraacetic (EDTA) dianhydride and bis(4-aminophenyl) ether or bis(4-aminophenyl)methane: cyclophanes obtained are 2,9,25,32-tetraoxo-4,7,27,30-tetrakis(carboxymethyl)-1,4,7,10,24,27,30,33-octaaza-17,40-dioxa[10.1.10.1]paracyclophane (1) and 2,9,25,32-tetraoxo-4,7,27,30-tetrakis(carboxymethyl)-1,4,7,10,24,27,30,33-octaaza[10.1.10.1]paracyclophane (2). Their complexation with 2-phenylethylamine (phenethylamine), 2-(4-hydroxyphenyl)ethylamine (tyramine) and 2-(3,4-dihydroxyphenyl)ethylamine (dopamine), which have biologically important activities, has been studied by 1H NMR spectroscopy in aqueous media. The formation constants of 1∶1 host–guest complexes, K = [HG]/[H][G], have been determined as: log K = 0.8 for 1–phenethylamine; 1.2 for 1–tyramine; 1.2 for 1–dopamine; 1.6 for 2–phenethylamine; 2.0 for 2–tyramine. Dopamine and 2 form a complex with low water-solubility. The chemical shifts of aromatic protons of the host and guest molecules suggest the formation of inclusion complexes in solutions. The formation of the host–guest complexes is assisted by a hydrogen bond and/or an electrostatic interaction between the pendant –CH2CO2– group of the host and the –CH2CH2NH3+ arm of the guest molecule. The two types of molecular recognition sites of the new cyclophanes result in the selective complex formation with the aromatic amines.

Terminal base pairs of oligodeoxynucleotides: imino proton exchange and fraying.

We have estimated the dissociation constant of the terminal base pairs of the B-DNA duplexes formed by 5'-d(CGCGATCGCG) and 5'-d(TAGCGCTA) by two methods, one based on the change in imino proton chemical shift with temperature and the other on the apparent pK shift of the imino proton, as monitored by the change in chemical shift of aromatic protons. These methods do not rely on imino proton exchange, whose rate was also measured. (1) The effect of ammonia on the imino proton exchange rate of the terminal pair of the 5'-d(CGCGATCGCG) duplex is 67 times less than on the isolated nucleoside. This provides an upper limit on the exchange rate from the closed pair. In fact, the effect is just as predicted from the dissociation constant, assuming that there is no exchange at all from the closed pair and that, as has been argued previously, external catalysts act on the open state as they do on the isolated nucleoside. The inhibition of catalyzed proton exchange in the closed pair, despite exposure of one face of the pair to solvent, is a new feature of the exchange process. It will allow determination of the dissociation constant of terminal pairs from the exchange rate. (2) Intrinsic catalysis of proton exchange is less efficient for the terminal pair than for an internal one. A possible explanation is that proton transfer across the water bridge responsible for intrinsic catalysis is slower, as expected if the open-state separation of the bases is larger in a terminal pair. This observation may lead to a direct method for the study of fraying. (3) At 0 degrees C, the dissociation constant of the second pair of the 5'-d(CGCGATCGCG) duplex is close to the square of the constant for the terminal pair, as predicted from a simple model of fraying. The enthalpy and entropy of opening of the terminal pairs may be compared with those of nearest neighbor interactions derived from calorimetry [Breslauer, K. J., et al. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 3746-3750].

Mono-, bi- and polynuclear complexes of diphenylmethane with Cr, Co and Ru. Synthesis and investigation by 1H, 13C and 17O NMR

Complexes of diphenylmethane (Ph2CH2): Ph2CH2Cr(CO)3 (1), Ph2CH2[Cr(CO)3]2 (2), Ph2CH2Co4 (CO)9 (3), Ph2CH2[Co4(CO)9]2 (4), Ph2CH2Cr(CO)3Co4(CO)9 (5) and Ph2CH2Ru6C(CO)14 (6) have been prepared and characterized by 1H and 13C-and17O-NMR spectroscopy. Strong shielding effects are caused by the metal valence electrons on the 1H- and 13C-NMR chemical shifts of aromatic protons and carbons in π-coordinated ring(s) diphenylmethane. Generally, the order of these shielding effects on the nuclei of the aromatic rings in 1H-NMR was Co4(CO)9 <Ru6C(CO)14 <Cr(CO)3 and in 13C-NMR Co4(CO)9<Cr(CO)3<Ru6C(CO)14. In addition, aromatic solvent exhibits an enhanced shielding effect on the 1H-NMR chemical shifts of the π-coordinated ring induced probably by aromatic solvent induced shifts (ASIS). The 1H-NMR chemical shifts of the exocyclic methylene protons are shielded or deshielded depending on the solvent, the metal and the degree of π-coordination. These findings can be explained by the varying conformational states adopted by the flexible ligand. The13C-NMR chemical shifts of the methylene carbon are generally shielded supporting the above explanation. This conformational flexibility can be of extreme importance in controlling the catalytic activity of these organometallic compounds. In chromium and heterobimetallic chromium cobalt derivatives1, 2 and5, 17O-NMR spectroscopy proved to have excellent sensitivity comparable with that of 13C-NMR. In cobalt clusters3 and4 no 17O-NMR lines were observed, which is probably because of strongly broadened 17O-NMR signals of carbonyls undergoing dynamic exchange. In the ruthenium cluster6 only one broad 17O-NMR line at 30°C was observed. An inverse relation between the 13C- and 17O-NMR chemical shifts of the carbonyl groups can be explained by the effect of π-backbonding.

Stabilization of an anthrapyrazole antitumour agent, DuP 937, on complexation with heptakis(2,6-di-O-methyl)-β-cyclodextrin in aqueous solution

DuP 937, an anthrapyrazole antitumour agent that is chemically unstable in aqueous solution, was shown, by absorption spectroscopy, to form an inclusion complex with heptakis(2,6-di-O-methyl)-beta-cyclodextrin (DM beta CD) in aqueous solution. Proton nuclear magnetic resonance spectroscopy was used to determine the stoichiometry and association constant of the complex. The 1:1 stoichiometry of the complex was established by the continuous variation method by following changes in the chemical shifts of aromatic protons of DuP 937. The complex association constants determined by different techniques used in this study were in the same order of magnitude. The kinetics of degradation of DuP 937 in aqueous solution were investigated as a function of DM beta CD concentration at pH 5.5 and 60 degrees C. The results indicated about a seven-fold increase in the stability of DuP 937 in the presence of DM beta CD in aqueous solution.

The synthesis and characterization of amine‐terminated poly(aryl ether ketone)s as a function of side group and molecular weight

AbstractIt is shown that amine‐terminated poly(aryl ether ketone)s based on the reaction of 4,4'‐difluorobenzophenone, and a substituted hydroquinone [either methylhydroquinone (MePK), t‐butylhydroquinone (tBPK), or phenylhydroquinone (PhPK)] of controlled molecular weight and high amine‐termination efficiency can be synthesized by a two‐step reaction technique. Attempts to synthesize analogous materials by a one‐step method were shown to be unsuccessful. The side groups are shown to have a large influence on the aromatic proton chemical shifts and this effect is characterized. The side groups and molecular weight are also shown to influence the thermal transitions of the respective polymers. The tBPK polymer possessed the highest glass transition temperature, while the MePK polymer was found to be the only semi‐crystalline polymer; a unit cell is proposed. The side groups and molecular weight effects are also characterized as a function of thermal stability and mechanical properties. © 1994 John Wiley &amp; Sons, Inc.

Correlation of substituent – inducted chemical shifts of aromatic protons: Substituted benzonitriles and methyl benzoates

Proton NMR spectra in deuteriochloroform are reported for 53 meta- and para-substituted benzonitriles and 61 methyl benzoates. The substituent-induced chemical shifts (SCS) were correlated with dual substituent parameters (DSP), with 13C SCS of the adjoining carbon, and with the other 1H SCS using the principal component analysis (PCA). They are controlled by different factors in each position to the variable substituent. In the position 4 the long-range polar effects, as expressed by DSP, are decisive, the conjugative component being more important. Remarkable is also very close correlation of 13C and 1H SCS in this position which has no analogy in the other position. In the position 3 there is an additional specific effect of heavier halogens. In the position 2 the short-range effect can be quantified by special constants, only for a subgroup of simple donor substituents it is proportional to DSP. In all positions SCS are practically independent of the second substituent, so that the total chemical shifts can be calculated by an additive scheme. The coupling constants J(2,3), J(5,6), and J(2,6) depend on substitution in the same manner, probably are controlled mainly by substituent electronegativity.

Mechanism of adenylate kinase. Structural and functional demonstration of arginine-138 as a key catalytic residue that cannot be replaced by lysine

Replacement of the arginine-138 of adenylate kinase (AK) by lysine or methionine resulted in a decrease in kcat by a factor of 10(4), increases in Km by a factor of 10-20, and relatively little changes in dissociation constants. Proton nuclear magnetic resonance (NMR) studies were then undertaken to obtain structural information for quantitative interpretation of the kinetic data. Since the lysine mutant (R138K) represents a conservative mutation with surprisingly large effects on kinetics, structural studies were focused on the wild type (WT) and R138K. The results and conclusions are summarized as follows: (i) The aromatic spin systems of WT and R138K were assigned from total correlated spectroscopy (TOCSY). Comparison of the chemical shifts of aromatic protons, one-dimensional spectra, TOCSY, and nuclear Overhauser enhanced spectroscopy (NOESY) indicated that the conformation of R138K was almost unperturbed relative to that of WT. Thus Arg-138 is not important for the tertiary structure. (ii) Proton NMR titrations with AMP and MgATP suggested that substrate binding affinities and substrate-induced conformational changes are nearly identical between WT and R138K. Thus arginine-138 should not be involved in stabilizing the first substrate in the binary complex. (iii) Notable differences were observed between the proton NMR spectra of the WT and R138K complexes with the reaction mixture, which agrees with the perturbation in the Km values of R138K. The differences were analyzed in detail by using a "static reaction mixture'--p1, p5-bis(5'-adenosyl)pentaphosphate (MgAP5A). The aromatic spin systems of WT + MgAP5A and R138K + MgAP5A were partially assigned from various two-dimensional spectra.(ABSTRACT TRUNCATED AT 250 WORDS)