The Conformational Analysis of Substance P Analogs Using High-Field NMR Techniques

Abstract

High-field nuclear magnetic resonance measurements were carried out on substance P fragments SP_4–11 [pGlu⁵]-SP_5–11 and [pGlu⁶]SP_6–11 both at 400 and at 500 MHz. A spectral simulation was carried out on two of these peptides and the coupling constants were interpreted in terms of the conformations. The J_NH-CHa coupling constants are all ～8 Hz, with the exception of glycine, indicating no preferred conformation for the backbone. For the amino acids other than p-Glu, a comparison of the coupling constant data suggests the same relative rotamer populations for the side chains. Proton longitudinal relaxation time data were measured for all three peptides and support the above conclusions.     

Pressure dependence of side chain <Superscript>13</Superscript>C chemical shifts in model peptides Ac-Gly-Gly-Xxx-Ala-NH<Subscript>2</Subscript>

For evaluating the pressure responses of folded as well as intrinsically unfolded proteins detectable by NMR spectroscopy the availability of data from well-defined model systems is indispensable. In this work we report the pressure dependence of ¹³C chemical shifts of the side chain atoms in the protected tetrapeptides Ac-Gly-Gly-Xxx-Ala-NH₂ (Xxx, one of the 20 canonical amino acids). Contrary to expectation the chemical shifts of a number of nuclei have a nonlinear dependence on pressure in the range from 0.1 to 200 MPa. The size of the polynomial pressure coefficients B ₁ and B ₂ is dependent on the type of atom and amino acid studied. For H^N, N and C^α the first order pressure coefficient B ₁ is also correlated to the chemical shift at atmospheric pressure. The first and second order pressure coefficients of a given type of carbon atom show significant linear correlations suggesting that the NMR observable pressure effects in the different amino acids have at least partly the same physical cause. In line with this observation the magnitude of the second order coefficients of nuclei being direct neighbors in the chemical structure also are weakly correlated. The downfield shifts of the methyl resonances suggest that gauche conformers of the side chains are not preferred with pressure. The valine and leucine methyl groups in the model peptides were assigned using stereospecifically ¹³C enriched amino acids with the pro-R carbons downfield shifted relative to the pro-S carbons.     

Stereospecific assignments of 1H-nmr methyl lines and conformation of valyl residues in the lac repressor headpiece

Two-dimensional ¹H-nmr methods are described to obtain information on the sidechain conformation of valyl residues of the lac repressor headpiece and to assign the resonances of their methyl groups stereospecifically. The spin–spin coupling constants (J_αβ) between C^αand C^β protons are obtained from two-dimensional correlated spectroscopy experiments. Large values for J_αβ(10–12 Hz) corresponding to trans orientations for these protons (g⁺ conformation) are found for all valyl residues in α-helical segments. For these valyl residues, the distance between one methyl group (γ₁)and the valyl amide proton is much shorter than for the other methyl group, so that stereospecific resonance assignments follow from relative intensities of the corresponding cross peaks in a two-dimensional nuclear Overhauser enhancement spectrum. Thus, streospecific assignments could be made for the methyl groups of Val 9, 20, 23, and 38 (of a total of eight valyl residues).     

Quantitative f torsion angle analysis in Desulfovibrio vulgaris flavodoxin based on six f related 3 J couplings

Quantitative φ-dihedral angle determinations of non-glycine and non-proline residues in Desulfovibrio vulgaris flavodoxin are carried out on the exclusive basis of ³ J coupling constants. In total 124 ³ JHNH _α , 123 ³ JHNC _′i , 118 ³ JHNC _β , 117 ³ JC′ _i–1Hα , 109 ³ JC′ _i–1C′i , and 103 ³ JC′ _i–1Hβ values form the experimental basis for translating J coupling data into geometry information using various combinations of Karplus parameters given in the literature. In addition, each backbone torsional angle φ is adjusted assuming different models of local geometry, either a rigid torsion, a Gaussian distribution centered at a distinct angle, or a two-site jump model. Numerical optimization is followed by a statistical significance evaluation to assess the results. It is found that experimental coupling constants of most of the residues involved in secondary structure elements agree best with those predicted from rigid local conformations. For dihedral angles in loop regions, mobility effects are not negligible, and a single torsion (Glu 42) is likely to adopt two distinct adjustments. However, α-helix conformations with –60° < φ < –45° give rise to an alternate solution with φ≈+170° with similar statistical significance when using the four traditionally determined proton-involved ³ J couplings. This ambiguity is efficiently avoided only when taking advantage of the complete data set comprising six available experimental ³ J coupling constants and of the degeneracy intrinsic to the Karplus relation. The optimized φ conformations are compared with reference values from the crystal structure of flavodoxin.     

Theoretical description of the magnetic properties of μ3-hydroxo bridged trinuclear copper(II) complexes

A theoretical study of the magnetic properties, using density functional theory, of a family of trinuclear μ₃-OH copper(II) complexes reported in the literature is presented. The reported X-ray crystal structures of [Cu₃(μ₃-OH)(aat)₃(H₂O)₃](NO₃)₂·H₂O (HUKDUM), where aat: 3-acetylamine-1,2,4-triazole; [Cu₃(μ₃-OH)(aaat)₃(H₂SO₄)(HSO₄)(H₂O)] (HUKDOG), where aaat: 3-acetylamine-5-amine-1,2,4-triazole; [Cu₃(μ₃-OH)(PhPyCNO)₃(tchlphac)₂] (HOHQUR), where PhPyCNO: phenyl 2-pyridyl-ketoxime and tchlphac: acid 2,4,5-trichlorophenoxyacetic; [Cu₃(μ₃-OH)(PhPyCNO)₃(NO₃)₂(CH₃OH)] (ILEGEM); [Cu₃(μ₃-OH)(pz)₃(Hpz)₃(ClO₄)₂] (QOPJIP), where Hpz?=?pyrazole; [Cu₃(μ₃-OH)(pz)₃(Hpz)(Me₃CCOO)₂]?2Me₃CCOOH (DEFSEN) and [Cu₃(μ₃-OH)(8-amino-4-methyl-5-azaoct-3-en-2-one)₃][CuI₃] (RITXUO), were used in the calculations. The magnetic exchange constants were calculated using the broken-symmetry approach. The calculated J values are for HUKDUM J₁?=??68.6 cm^?1, J₂?=??69.9 cm^?1, J₃?=??70.4 cm^?1; for HUKDOG, J₁?=??73.5 cm^?1, J₂?=??58.9 cm^?1, J₃?=??62.1 cm^?1; for HOHQUR J₁?=??128.3 cm^?1, J₂?=??134.1 cm^?1, J₃?=??120.4 cm^?1; for ILEGEM J₁?=??151.6 cm^?1, J₂?=??173.9 cm^?1, J₃?=??186.9 cm^?1; for QOPJIP J₁?=??118.3 cm^?1, J₂?=??106.0 cm^?1, J₃?=??120.6 cm^?1; for DEFSEN J₁?=??74.9 cm^?1, J₂?=??64.0 cm^?1, J₃?=??57.7 cm^?1 and for RITXUO J₁?=??10.9 cm^?1, J₂?=?+14.3 cm^?1, J₃?=??35.4 cm^?1. The Kahn-Briat model was used to correlate the calculated magnetic properties with the overlap of the magnetic orbitals. Spin density surfaces show that the delocalization mechanism is predominant in all the studied compounds.

Étude par r.m.n. de quatre disaccharides peracétylés sélectivement enrichis en 13C

Four peracetylated disaccharides ¹³C-labelled at the C-1′ position and having α-d-(1′→3), β-d-(1′→3), α-d-(1′→4), and β-d-(1′→4) linkages were prepared starting from the commercially available d-[1-¹³C]glucose. They were studied on the basis of their ³J_¹³CH coupling constants in relation with the conformation in solution of oligosaccharides as models for the corresponding polymer. A method of analysis of the n.m.r. spectra is described and the coupling constants J_{¹³C-1′H} given, particularly the ²J coupling (in the same cycle and with sign determination) and the ³J coupling (through the glycosidic bond). In that case, the values obtained give experimental information on the ψ angle values. They are compared with the known X-ray data for similar compounds.     

Conformations of methyl 3,4-dideoxy-DL-glyc-3-enopyranosides

The conformational equilibrium (°H₁ ?¹H°) has been established for 26 diastereoisomeric methyl 3,4-dideoxy-DL-glyc-3-enopyranosides on the basis of the coupling constant J_1,2. The position of equilibrium depends on the interplay of steric and polar factors. The concept of the allylic effect is useful in explaining some conformational phenomena.     

Impact of two-bond <Superscript>15</Superscript>N–<Superscript>15</Superscript>N scalar couplings on <Superscript>15</Superscript>N transverse relaxation measurements for arginine side chains of proteins

NMR relaxation of arginine (Arg) ¹⁵N_ε nuclei is useful for studying side-chain dynamics of proteins. In this work, we studied the impact of two geminal ¹⁵N–¹⁵N scalar couplings on measurements of transverse relaxation rates (R ₂ ) for Arg side-chain ¹⁵N_ε nuclei. For 12 Arg side chains of the DNA-binding domain of the Antp protein, we measured the geminal ¹⁵N–¹⁵N couplings ( ² J _NN ) of the ¹⁵N_ε nuclei and found that the magnitudes of the ² J _NN coupling constants were virtually uniform with an average of 1.2 Hz. Our simulations, assuming ideal 180° rotations for all ¹⁵N nuclei, suggested that the two ² J _NN couplings of this magnitude could in principle cause significant modulation in signal intensities during the Carr–Purcell-Meiboom–Gill (CPMG) scheme for Arg ¹⁵N_ε R ₂ measurements. However, our experimental data show that the expected modulation via two ² J _NN couplings vanishes during the ¹⁵N CPMG scheme. This quenching of J modulation can be explained by the mechanism described in Dittmer and Bodenhausen (Chemphyschem 7:831–836, 2006). This effect allows for accurate measurements of R ₂ relaxation rates for Arg side-chain ¹⁵N_ε nuclei despite the presence of two ² J _NN couplings. Although the so-called recoupling conditions may cause overestimate of R ₂ rates for very mobile Arg side chains, such conditions can readily be avoided through appropriate experimental settings.     

Performance of a neural-network-based determination of amino acid class and secondary structure from 1H-15N NMR data

A neural network which can determine both amino acid class andsecondary structure using NMR data from ¹⁵N-labeled proteinsis described. We have included nitrogen chemical shifts,³J_HN_H coupling constants, -protonchemical shifts, and side-chain proton chemical shifts as input to athree-layer feed-forward network. The network was trained with 456 spinsystems from several proteins containing various types of secondarystructure, and tested on human ubiquitin, which has no sequence homologywith any of the proteins in the training set. A very limited set of data,representative of those from a TOCSY-HSQC and HNHA experiment, was used.Nevertheless, in 60% of the spin systems the correct amino acid classwas among the top two choices given by the network, while in 96% ofthe spin systems the secondary structure was correctly identified. Theperformance of this network clearly shows the potential of the neuralnetwork algorithm in the automation of NMR spectral analysis.     

Biodegradation of Methyl Orange by alginate-immobilized Aeromonas sp. in a packed bed reactor: external mass transfer modeling

Azo dyes are recalcitrant and xenobiotic nature makes these compounds a challenging task for continuous biodegradation up to satisfactorily levels in large-scale. In the present report, the biodegradation efficiency of alginate immobilized indigenous Aeromonas sp. MNK1 on Methyl Orange (MO) in a packed bed reactor was explored. The experimental results were used to determine the external mass transfer model. Complete MO degradation and COD removal were observed at 0.20 cm bead size and 120 ml/h flow rate at 300 mg/l of initial dye concentration. The degradation of MO decreased with increasing bead sizes and flow rates, which may be attributed to the decrease in surface of the beads and higher flux of MO, respectively. The experimental rate constants (k _ps) for various beads sizes and flow rates were calculated and compared with theoretically obtained rate constants using external film diffusion models. From the experimental data, the external mass transfer effect was correlated with a model J _D = K Re ^?(1 ? n). The model was tested with K value (5.7) and the Colburn factor correlation model for 0.20, 0.40 and 0.60 bead sizes were J _D = 5.7 Re ^?0.15, J _D = 5.7 Re ^?0.36 and J _D = 5.7 Re ^?0.48, respectively. Based on the results, the Colburn factor correlation models were found to predict the experimental data accurately. The proposed model was constructive to design and direct industrial applications in packed bed reactors within acceptable limits.     

Proton and phosphorus-31 magnetic relaxation studies on the interaction of polyriboadenylic acid with Mn2+

Proton and phosphorus-31 nuclear spin–lattice relaxation times T₁ and spin–spin relaxation times T₂ have been measured on the single-stranded polyriboadenylic acid [poly(A)]–Mn²⁺ system in a neutral D₂O solution in the temperature range 10°–90°C at 100 and 40.5 MHz, respectively, with the Fourier transform nmr method. Minimum values of T₁ have been found for all these nuclei, which have enabled the exact estimation of apparent distances from Mn²⁺ to H₂, H₈, H_1′, and the phosphorus nucleus to be 4.7, 4.1, 5.2, and 3.0 Å, respectively. The electron spin of Mn²⁺ penetrates into the phosphorus nucleus, giving ³¹P hyperfine coupling of more than 10⁶ Hz. Evidence of penetration of the electron spin into H₈ and H₂ is also obtained, suggesting direct coordination of nitrogen atoms of the adenine ring to the Mn²⁺ Ion. Combined with the result from proton relaxation enhancement of water, it is concluded that every Mn²⁺ ion added is bound directly to two phosphate groups with a Mn²⁺–phosphorus distance of 3.3 Å, while a part of the Mn²⁺ ions are simultaneouly bound to the adenine ring. It is estimated that 39 ± 13% and 13 ± 5% of Mn²⁺ are coordinated by N₇ and N₃ (or N₁), respectively. The motional freedom of poly(A) in the environment of the Mn²⁺ binding site has been found to be quenched to the extent that the rotational motion becomes several times slower than that of the corresponding Mn²⁺–free poly(A). The activation energies for the molecular motion are, however, practically unchanged from those for Mn²⁺–free poly(A), and are found to be 8.3, 8.5, 6.1, and 8.7 kcal/mol for H₈, H₂, H_1′, and phosphorus, respectively. T₂ of phosphorus is determined by the dissociation rate (k_?1) of Mn²⁺ from the phosphate group for the whole temperature range studied with activation enthalpy of 6.5 kcal/mol. The dissociation rates of Mn²⁺ from the adenine ring are also estimated from proton T₂ values below 50°C.     

Proton magnetic resonance studies of 9-(β-d-xylofuranosyl)adenine 3′,5′-cyclic monophosphate and 9-(β-d-arabinofuranosyl)adenine 2′,5′-cyclic monophosphate

A detailed ¹H 220-MHz n.m.r. study of 9-(β-d-xylofuranosyl)adenine 3′,5′-cyclic monophosphate (3′,5′-xylo-cAMP, 1) and 9-(?-d-arabinofuranosyl)adenine 2′,5′-cyclic monophosphate (2′,5′-ara-cAMP, 2) in D₂O solution is described. The sugar-ring conformations in 1 and 2 are shown to be ³E and ²E, respectively, and the phosphate rings are in a chair form. An unusual ⁴J_P,H coupling of 2.4 Hz is observed between H-4′ and phosphorus in 1 and a vicinal J_P,H of 30.8 Hz between H-5′ and phosphorus in 2. This latter coupling verifies a similar value found previously in the ara-cytidine analog of 2. A comparison of the conformational properties of cyclic nucleotides having fused phosphate and sugar rings has been made, together with an assessment of the use of the Karplus constants in such ring-systems.     

Synthesis and multinuclear magnetic resonance spectroscopy of the novel ionic Pt(II) mixed-ligands complexes cis- and trans-[Pt(amine)2(pyrimidine)2](NO3)2

Novel ionic mixed-ligands complexes of the types cis- and trans-[Pt(amine)₂(pm)₂](NO₃)₂ (where pm = pyrimidine) were synthesized and studied in the solid state by IR spectroscopy and in aqueous solution by multinuclear (¹⁹⁵Pt, ¹H and ¹³C) magnetic resonance spectroscopy. The results of the solution NMR characterization have shown that the isolated compounds are pure. In ¹⁹⁵Pt NMR, the cis RNH₂ complexes were observed at slightly lower fields (ave. −2441 ppm) than the equivalent trans analogues (ave. −2448 ppm). For Me₂NH, the difference between the two isomers is larger (29 ppm). The complexes are observed at lower fields (difference of 100 ppm) than the corresponding [Pt(amine)₄]²⁺ complexes, which might indicate the presence of π-backdonation in the Pt-pm bond. In ¹H NMR, the coupling constants ³J(¹⁹⁵Pt-¹H_amine) are larger in the cis compounds (38-48 Hz) than in the trans analogues (30-36 Hz). The ³J(¹⁹⁵Pt-¹H_pm) values are also larger for the cis isomers. In ¹³C NMR spectroscopy, the coupling constants ³J(¹⁹⁵Pt-¹³C_amine) are 36 Hz (ave.) for the cis complexes and 26 Hz (ave.) for the trans isomers, while the ²J(¹⁹⁵Pt-¹³C_amine) are 18 Hz (cis) and 14 Hz (trans), respectively. The ³J(¹⁹⁵Pt-¹³C_5(pm)) values are 36 Hz (cis) and 28 Hz (trans). A few ²J(¹⁹⁵Pt-¹³C_pm) couplings were observed (7-10 Hz).     

1H, 13C and 15N chemical shift assignments of Na-FAR-1, a helix-rich fatty acid and retinol binding protein of the parasitic nematode Necator americanus

The fatty acid and retinol-binding (FAR) proteins are a family of unusual helix-rich lipid binding proteins found exclusively in nematodes, and are secreted by a range of parasites of humans, animals and plants. Na-FAR-1 is from the parasitic nematode Necator americanus, an intestinal blood-feeding parasite of humans. Sequence-specific ¹H, ¹³C and ¹⁵N resonance assignments have been obtained for the recombinant 170 amino acid protein, using three-dimensional triple-resonance heteronuclear magnetic resonance experiments. Backbone assignments have been obtained for 99.3 % of the non-proline HN/N pairs (146 out of 147). The amide resonance of T45 was not observed, probably due to rapid exchange with solvent water. A total of 96.9 % of backbone resonances were identified, while 97.7 % assignment of amino acid sidechain protons is complete. All Hα(166), Hβ(250) and Hγ(160) and 98.4 % of the Hδ (126 out of 128) atoms were assigned. In addition, 99.4 % Cα (154 out of 155) and 99.3 % Cβ (143 out of 144) resonances have been assigned. No resonances were observed for the NH_n groups of R93 N_εH_ε, arginine, N_η1H₂, N_η2H₂, histidine N_δ1H_δ1, N_ε1H_ε1 and lysine N_ζ3H₃. Na-FAR-1 has a similar overall arrangement of α-helices to Ce-FAR-7 of the free-living Caeorhabditis elegans, but with an extra C-terminal helix.     

Observation of H-bond mediated 3hJH2H3 coupling constants across Watson-Crick AU base pairs in RNA

^3hJ_H2H3trans-hydrogen bond scalar coupling constants have been observed for the first time in Watson-Crick AU base pairs in uniformly ¹⁵N-labeled RNA oligonucleotides using a new ^2hJ_NN-HNN-E. COSY experiment. The experiment utilizes adenosine H2 (AH2) for original polarization and detection, while employing ^2hJ_NNcouplings for coherence transfer across the hydrogen bonds (H-bonds). The H3 protons of uracil bases are unperturbed throughout the experiment so that these protons appear as passive spins in E. COSY patterns. ^3hJ_H2H3coupling constants can therefore be accurately measured in the acquisition dimension from the displacement of the E. COSY multiplet components, which are separated by the relatively large ¹J_H3N3coupling constants in the indirect dimension of the two-dimensional experiment. The ^3hJ_H2H3scalar coupling constants determined for AU base pairs in the two RNA hairpins examined here have been found to be positive and range in magnitude up to 1.8 Hz. Using a molecular fragment representation of an AU base pair, density functional theory/finite field perturbation theory (DFT/FPT) methods have been applied to attempt to predict the relative contributions of H-bond length and angular geometry to the magnitude of ^3hJ_H2H3coupling constants. Although the DFT/FPT calculations did not reproduce the full range of magnitude observed experimentally for the ^3hJ_H2H3coupling constants, the calculations do predict the correct sign and general trends in variation in size of these coupling constants. The calculations suggest that the magnitude of the coupling constants depends largely on H-bond length, but can also vary with differences in base pair geometry. The dependency of the ^3hJ_H2H3coupling constant on H-bond strength and geometry makes it a new probe for defining base pairs in NMR studies of nucleic acids.     

Quantitative two-dimensional nmr study of dermenkephalin,a highly potent and selective δ-opioid peptide

Dermenkephalin, H-Tyr-(D ) Met-Phe-His-Leu-Met-Asp-NH₂, a highly potent and selective δ-opioid peptide isolated from frog skin, was studied in DMSO-d₆ solution by two-dimensional nmr spectroscopy, including the determination of NH temperature coefficients, the evaluation of ³J coupling constants from phase-sensitive correlated spectroscopy (COSY) and the volumes of nuclear Overhauser effect (NOE) correlations. The two-dimensional NOE spectroscopy (NOESY) spectrum of dermenkephalin revealed sequential, medium-, and long-range effects. To put this information on a quantitative basis, special attention was devoted to J cross-peak suppression, quantification of the NOE volumes and analysis of the overlaps, normalization of the NOEs against diagonal peaks and H_ββ′ geminal interactions. Although most of the dihedral angles deduced from the ³J_Nα coupling constants together with several N_iα_i and α_iN_{i + 1} NOEs pointed to a partially extended peptide backbone, several N_i N_{i + 1} NOEs and β_i N_{i + 1} interactions argued in favor of a folded structure. Moreover, several long-range correlations of strong intensities were found that supported a close spatial proximity between the side chains of D -Met² and Met⁶, Tyr¹ and His⁴, Tyr¹ and Asp⁷, and His⁴ and the C-terminal amide group. In Phe, the g^? rotamer in the side chain is deduced from the ³J_αβ coupling constants and αβ and Nβ NOE correlations. Whereas the amide proton dependency was not indicative of stable hydrogen bonds, the nonuniform values of the temperature coefficient may reflect an equilibrium mixture of folded and extended conformers. The overall data should provide realistic starting models for energy minimization and modelization studies. © 1993 John Wiley & Sons, Inc.     

Four mononuclear platinum(II) complexes: synthesis,DNA/BSA binding,DNA cleavage and cytotoxicity

Four new platinum(II) complexes: Pt^II L1·H₂O (C1, H₂ L1 = C₂₀H₁₆N₂O₂), Pt^II L2Cl₂ (C2, L2 = C₂₂H₁₆N₂O₂), Pt^II L3Cl₂·H₂O (C3, L3 = C₂₀H₁₆N₂), Pt^II L4Cl₂·0.4H₂O (C4, L4 = C₁₈H₁₄N₄) have been synthesized and characterized by using various physico-chemical techniques. The binding interaction of the four platinum(II) complexes C1–C4 with calf thymus (CT)-DNA has been investigated by UV–Vis and fluorescence emission spectrometry. The apparent binding constant (K _app) values follow the order: C3 > C1 > C2 > C4. In addition, fluorescence spectrometry of bovine serum albumin (BSA) with the four platinum(II) complexes C1–C4 showed that the quenching mechanism might be a static quenching procedure. For C1–C4, the number of binding sites was about one for BSA and the binding constants follow the order: C3 (7.08 × 10⁵M^?1) > C1 (2.82 × 10⁵M^?1) > C2 (0.85 × 10⁵M^?1) > C4 (0.15 × 10⁵M^?1). With the single condition change such as absence of an external agent, the DNA cleavage abilities of C3 exhibit remarkable changes. In addition, the cytotoxicity of C3 in vitro on tumor cells lines (MCF-7, HepG2 and HT29) were examined by MTT and showed better antitumor effects on the tested cells.     

Synthesis and multinuclear magnetic resonance spectroscopy of the novel ionic Pt(II) mixed-ligands complexes cis- and trans-[Pt(pyrazine)2(Ypy)2](NO3)2 where Ypy = pyridine derivative

Novel ionic mixed-ligands complexes of the types cis- and trans-[Pt(pz)₂(Ypy)₂](NO₃)₂ (where Ypy is a pyridine derivative and pz = pyrazine) were synthesized and studied mainly in the solid state by IR spectroscopy and in aqueous solution by multinuclear (¹⁹⁵Pt, ¹H and ¹³C) magnetic resonance spectroscopy. The trans isomers with ligands containing a methyl group in ortho position on the pyridine ring could not be synthesized. The results of the solution NMR characterization have shown that the isolated compounds are pure. In ¹⁹⁵Pt NMR, the cis complexes containing a methyl group in ortho positions were observed at lower field (average −2337 ppm) than the other cis compounds (average −2427 ppm), which is explained by the solvent effect. The trans isomers were observed at very slightly lower fields (average −2422 ppm) than the equivalent cis complexes (average −2427 ppm). In ¹H NMR, the coupling constants ³J(¹⁹⁵Pt-¹H_Ypy) and ³J(¹⁹⁵Pt-¹H_pz) are larger in the cis compounds (∼40 Hz) than in the trans complexes (∼31 Hz). A few ⁴J(¹⁹⁵Pt-¹H_pz) were observed (∼16 Hz). In ¹³C NMR spectroscopy, the coupling constants ³J(¹⁹⁵Pt-¹³C_pz) and ³J(¹⁹⁵Pt-¹³C_Ypy) are also larger in the cis configuration (∼30 and ∼38 Hz, respectively) than in the trans isomers (∼20 Hz). One ⁴J(¹⁹⁵Pt-¹³C_pz) could be calculated (17 Hz). The presence of the syn and anti rotamers were observed in all the cis complexes containing a pyridine derivative with a -CH₃ group in ortho position. They were observed in ¹⁹⁵Pt, ¹H and ¹³C NMR spectroscopy. The proportion of the two rotamers is about 55% and 45%.     

Conformational dependence of the vicinal proton coupling constant for the Cα?Cβ bond in peptides

We use the nmr data concerning the C^αH? C^βH fragment in eight peptides with rigid side chains to parametrize a Karplus correlation between the vicinal proton J_αβ coupling constant and the dihedral angle θ. When considering molecules containing the fragment C^αH^α? C^βH^βH^β′, the three-dimensional structure of the model peptides does not need to be known with accurate precision, since each set of J_αβ and J_αβ′ coupling constants is then related to the coefficients of the Karplus equation. A good correlation is observed with the Karplus equation, which is in substantial agreement with the J_αβ coupling constants reported for rigid as well as rotating C^α? C^β bonds in peptides.