Changes In 13C Chemical Shifts Research Articles

2H,13C-Cholesterol for Dynamics and Structural Studies of Biological Membranes.

We present a cost-effective means of 2H and 13C enrichment of cholesterol. This method exploits the metabolism of 2H,13C-acetate into acetyl-CoA, the first substrate in the mevalonate pathway. We show that growing the cholesterol producing strain RH6827 of Saccharomyces cerevisiae in 2H,13C-acetate-enriched minimal media produces a skip-labeled pattern of deuteration. We characterize this cholesterol labeling pattern by mass spectrometry and solid-state nuclear magnetic resonance spectroscopy. It is confirmed that most 2H nuclei retain their original 2H–13C bonds from acetate throughout the biosynthetic pathway. We then quantify the changes in 13C chemical shifts brought by deuteration and the impact upon 13C–13C spin diffusion. Finally, using adiabatic rotor echo short pulse irradiation cross-polarization (RESPIRATIONCP), we acquire the 2H–13C correlation spectra to site specifically quantify cholesterol dynamics in two model membranes as a function of temperature. These measurements show that cholesterol acyl chains at physiological temperatures in mixtures of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), sphingomyelin, and cholesterol are more dynamic than cholesterol in POPC. However, this overall change in motion is not uniform across the cholesterol molecule. This result establishes that this cholesterol labeling pattern will have great utility in reporting on cholesterol dynamics and orientation in a variety of environments and with different membrane bilayer components, as well as monitoring the mevalonate pathway product interactions within the bilayer. Finally, the flexibility and universality of acetate labeling will allow this technique to be widely applied to a large range of lipids and other natural products.

Application of anisotropic NMR parameters to the confirmation of molecular structure.

The use of anisotropic NMR data, such as residual dipolar couplings (RDCs) and residual chemical shift anisotropies (RCSAs), has emerged as a powerful technique for structural characterization of organic small molecules. RDCs typically report the relative orientations of different 1H-13C bonds; RCSAs report the relative orientations of different carbon chemical shielding tensors and hence are more useful for proton-deficient molecules. This information is complementary to that obtained from conventional NMR data such as J couplings, isotropic chemical shifts, and nuclear Overhauser effects (NOEs)/rotational frame nuclear Overhauser effects (ROEs). Obtaining anisotropic NMR data requires the creation of an anisotropic sample environment through an alignment medium. Here, we focus on the use of compressed or stretched polymeric gels as two different but fundamentally equivalent methods for introducing sample anisotropy. Protocols are provided for the synthesis of the chloroform-compatible poly(methyl methacrylate) and dimethyl sulfoxide (DMSO)-compatible poly(2-hydroxyethyl methacrylate) gels and sample setup with a preparation time of 2-3 d. The bond-specific RDC data and the atom-specific RCSA data are extracted as changes in 1H-13C couplings and 13C chemical shifts, respectively, between two measurements under different alignment conditions, with a total experimental time of 0.5-4 d. NMR data acquisition and important considerations are described in detail. We also provide step-by-step procedures for the density functional theory (DFT) calculations involved and data analysis using the commercial software MSpin. We use three example compounds, namely cryptospirolepine (505 Da), retrorsine (351 Da), and estrone (270 Da), to demonstrate some important aspects of the workflow, such as input data preparation, handling of structural flexibility, and RCSA data correction when necessary.

Detection of Sugars via Chirality Induced in Europium(III) Compounds.

Detection and resolution of simple monosaccharides are difficult tasks because their structure is quite similar. The present study shows that circularly polarized luminescence (CPL) induced in europium complexes provides very specific spectral patterns for fructose, mannose, glucose, and galactose. Differences were also observed between bare Eu(3+) ion and its complexes, when interacting with these sugars. The CPL spectra were measured on a Raman optical activity (ROA) spectrometer, which ensured high fluorescence intensity owing to the strong 532 nm laser excitation. The induced fluorescence was recorded in the same spectrum as the vibrational Raman bands. On the basis of the ligand field theory, most fluorescence spectral peaks could be assigned to f-shell europium transitions. Additional information on the interaction of the lanthanide with the sugar component was provided by measurement of time-dependent fluorescence, as formation of different complexes led to variations in fluorescence decay times. In nuclear magnetic resonance (NMR), the paramagnetic metal ion interacting with the sugars caused specific changes in (13)C chemical shifts. The spectroscopic data and molecular dynamics modeling showed that the interaction between the monosaccharides and Eu ion is rather weak due to the competition of the OH sugar groups with water molecules. However, multiple binding modes are possible, which contributes to the complexity and specificity of the spectra. The induced chirality and fluorescence spectra thus appear to be convenient means for monosaccharide detection and identification.

Detecting O2 binding sites in protein cavities.

Internal cavities are important elements in protein structure, dynamics, stability and function. Here we use NMR spectroscopy to investigate the binding of molecular oxygen (O2) to cavities in a well-studied model for ligand binding, the L99A mutant of T4 lysozyme. On increasing the O2 concentration to 8.9 mM, changes in 1H, 15N, and 13C chemical shifts and signal broadening were observed specifically for backbone amide and side chain methyl groups located around the two hydrophobic cavities of the protein. O2-induced longitudinal relaxation enhancements for amide and methyl protons could be adequately accounted for by paramagnetic dipolar relaxation. These data provide the first experimental demonstration that O2 binds specifically to the hydrophobic, and not the hydrophilic cavities, in a protein. Molecular dynamics simulations visualized the rotational and translational motions of O2 in the cavities, as well as the binding and egress of O2, suggesting that the channel consisting of helices D, E, G, H, and J could be the potential gateway for ligand binding to the protein. Due to strong paramagnetic relaxation effects, O2 gas-pressure NMR measurements can detect hydrophobic cavities when populated to as little as 1%, and thereby provide a general and highly sensitive method for detecting oxygen binding in proteins.

NMR and DFT studies on persistent carbocations derived from benzo[kl ]xanthene, dibenzo[d ,d ′]benzo[1,2-b :4,3-b ′]difuran, and dibenzo[d ,d ′]benzo[1,2-b :4,5-b ′]difuran in superacidic media

Persistent carbocations generated by the protonation of hetero-polycyclic aromatic compounds with oxygen atom(s) were studied by experimental NMR and density function theory calculations. Benzo[kl]xanthene (), dibenzo[d,d′]benzo[1,2-b:4,3-b′]difuran (), and dibenzo[d,d′]benzo[1,2-b:4,5-b′]difuran () were synthesized by the annulation of arenediazonium salts. Compound in FSO3H-SbF5 (4:1)/SO2ClF and in FSO3H-SbF5 (1:1)/SO2ClF ionized to with protonation at C(4) and to with protonation at C(6), and these cations were successfully observed by NMR at low temperatures. The density function theory calculations indicated that and were the most stable protonated carbocations and that should ionize to with protonation at C(6). According to the changes in 13C chemical shifts (Δδ13C), the positive charge was delocalized into the naphthalene unit for , into one benzo[b,d]furan unit for , and into one benzo[b,d]furan unit for . Copyright © 2015 John Wiley & Sons, Ltd.

Studies of the conformational behaviour and preferential interactions with opiate receptors of the cis and trans forms of [Dmet2,pro5]enkephalin and [Dmet2,pro5]enkephalinamide by 1H and 13C NMR, theoretical calculations and 13C relaxation measurements.

[DMet2, Pro5]Enkephalin (I) and [DMet2, Pro5]enkephalinamide (II), two of the more potent opioid peptides, have been studied by 1H and 13C NMR and by theoretical calculations. Their biological activity is tentatively related to their conformational behaviour. A cis⇌ trans equilibrium around the Phe-Pro bond is brought about by the terminal prolyl residue. The cis/trans ratios lie around 50/50 for peptide I (55%cis in dimethylsulfoxide, 35 %cis in water) and around 20/80 for peptide II, the more potent compound, in both the solvents. Almost all the signals, both in 1H and 13C spectra, give separate resonances for the cis and trans rotamers, in dimethylsulfoxide as well as in water. A temperature variation allows one to determine the rotational barrier (ΔG+TC, ∼ 81.5 kJ/mol) for the cis⇌trans interconversion. The analysis of the vicinal coupling constants and theoretical calculations suggests that Tyr-dMet-Gly-Phe-Pro exists as a class of highly puckered conformations with short end-to-end distances at r= 0.4 nm and r= 0.6 nm for the trans and cisconfigurations respectively. The populations of the side-chain rotamers are predominantly trans/gauche (tg) for the tyrosine residue. This feature, which is quite general in enkephalins, is related to the crucial role of the tyramine moiety for opioid activity. The orientation of the Phe-4 side-chain is independent of the spatial disposition of the following prolyl residue. pH titrations provide evidence for the existence of head-to-tail interactions. Hence, in (2H6)dimethylsulfoxide, and to a lesser extent in 2H2O, the titration of the Pro-5 residue is clearly felt on the Tyr-1 moiety, whereas the deprotonation of this latter residue leads to changes in the chemical shifts of the pyrrolidine ring protons. The β protons of the Tyr-1 and Phe-4 residues are strongly magnetically nonequivalent; the opposite is true for Met-enkephalin where they show the same chemical shifts below pH 6. The titration effects suggest that no strong conformational change occurs during the amino or carboxyl group titration and indicate an important steric hindrance on the NMR time scale for the two aromatic side chains. The 13C NMR spectra of peptide I in (2H6)dimethylsulfoxide and in a mixture of (2H6)dimethylsulfoxide/2H2O (507sol;50) exhibit for almost all the resonances well-separated signals, corresponding to the cis and trans conformers. The large changes in 13C chemical shifts occurring Qn the backbone and side-chain carbons during the titration steps confirm the through-space effects already deduced from the proton titration curves. Hence, several C-α (Met-2, Phe-4) and C-β (Met-2, Pro-5) atoms are affected by both the titrations of the two terminal parts of the peptide. Such effects were not observed in Met-enkephalin or Leu-enkephalin. The 13C relaxation times for the cis and trans rotamers were compared, in (2H6)dimethylsulfoxide, under strictly the same conditions. All the NT1 values are small and lie in the same range both for the carbons of the peptide backbone and for the carbons of the lateral chains. The restriction in the segmental motion of the sidechains is shown by the NT1 values of the 13C of the pyrrolidine ring and by the C-β and C-γ of the methionine residue which lie near those of the backbone carbons. No significant internal rotation of the aromatic ring of the tyrosyl residue seems to occur. A restricted degree of freedom for the C-α-C-β carbons is shown for the phenylalanyl residue which contrasts with Metenkephalin, Leu-enkephalin and a larger peptide such as α-endorphin. All these results, both in dimethylsulfoxide and in water seem to exclude a random conformation for the compounds I and II. The titration effects are related to strong compression effects due to the presence of a d met-2 residue wedged between two relatively inflexible aromatic residues. Moreover. the results indicate that, despite the absence of a classical β-turn, these compounds exhibit folded structures induced by their high content of hydrophobic side chains. The conformational behaviour of peptides I and II can be related to their high specificity for the μ-receptor site. Hence, it can be noticed that, in the trans form of the peptides the first part, Tyr-DMet-Gly, may adopt the angular requirements (ψ1=−169°, φ2= plus;83°, ψ2=+53°, φ3=+150°) of the active conformation that we recently proposed for the interaction at the μ-receptor site. Moreover it is possible that the trans form, more populated in the highly potent compound II, might better represent the active form in the interaction at the receptor site.

Dynamics Studies of a Malachite Green−RNA Complex Revealing the Origin of the Red-Shift and Energetic Contributions of Stacking Interactions

RNA plays a central role in many biological processes and is therefore an important target for drug development. In recent years an increasing wealth of structural and functional information about RNA−ligand complexes has been obtained using in vitro selected RNAs (aptamers). However, all those studies focused on structure and changes of the nucleic acid and mostly considered the ligand as a rigid target. Experimental characterization of the dynamics of a malachite green (MAG)−RNA aptamer complex revealed surprisingly asymmetric changes in the 13C chemical shift of the ligand methyl groups, which indicate that the dye undergoes changes in its conformation and charge distribution upon binding, in addition to the red-shift in its maximum absorption frequency. Earlier computational work explored the electrostatic influence of the highly charged RNA backbone and surrounding counterions for these asymmetrical changes in 13C chemical shift. The work presented here examines the dynamical nature of the MAG molecule inside the RNA ligand-binding site using molecular dynamics with explicit solvent to explore the intermolecular and dynamic influences on the red-shift and to calculate the binding affinity. An induced fit behavior, similar to that caused by the electrostatic field alone, is observed. The binding strength of a series of malachite green derivatives correlates with the structural flatness of the ligand. These findings are important for the rational design of RNA ligands and for understanding the properties of RNA−ligand complexes.

NMR analysis of cellulose dissolved in aqueous NaOH solutions

Low molecular weight cellulose (degree of polymerization = ca 15) was dissolved in 4-- 30% NaOD/D2O, and relationships between 1H- and 13C-chemical shifts of the cellulose and NaOD concentrations were studied in terms of the dissociation of three hydroxyl groups of cellulose in aqueous NaOH solutions. All C---H proton resonances were shifted upfield linearly with an increase in the NaOD concentration, indicating that all C---H protons of cellulose undergo the electron-shielding effect by NaOH. On the other hand, the shifting patterns of carbon resonances varied among the six carbons: C1 and C4 carbons undergo the electron-shielding effect, whereas C2, C3, C5, and C6 carbons experience the electron-deshielding effect by NaOH. Changes in 13C-chemical shifts of cellulose carbons in 4--30% NaOD/D2O indicate that C3---OH has the highest resistance to dissociation in aqueous NaOH of the three hydroxyl groups of an anhydroglucose residue. It is plausible, at least from the aspects of 13C-chemical shifts, that cellulose molecules dissolving in 20--30% NaOH behave differently from those swollen in 20--30% NaOH as alkali- cellulose

The Radical Ions of Dipleiadiene (Dicyclohepta[de,ij]naphthalene) and Its 12b,12c‐Homo Derivative. An ESR and ENDOR study

AbstractThe radical anions and the radical cations of dipleiadiene (dicyclohepta[de,ij]naphthalene; 1) and its 12b, 12c‐homo derivative 2 were characterized by ESR and ENDOR spectroscopy. Their singly occupied orbitals are related to the degenerate nonbonding MOs of a 16‐membered π‐perimeter. The π‐spin distribution over the perimeter is similar in the radical cations 1.+ and 2.+, and an analogous statement holds for the radical anions 1.− and 2.−. However, deviations of the π‐system from planarity lead to a decrease in the absolute values of the negative coupling constants of the perimeter protons in 2.+ and 2.− relative to those in 1.+ and 1.−. The hyperfine data for the perimeter protons in the radical ions correlate with the changes in 13C chemical shifts on passing from the neutral compounds to the corresponding diions. It is concluded from the coupling constants of the CH2 protons in the radical ions of 2 that the cation 2.+ exists in the methano‐bridged form (A) of the neutral 2 (and, presumably, also of the dication 22+), whereas the anion 2.− adopts the bisnorcaradiene form (B) of the dianion 22−.

Conformational Analysis of Some Radiation-Induced Decomposition Products of Thymidine Using13C NMR Analysis

The 13C NMR spectra of several diastereomers of radiation-induced decomposition products of thymidine and thymine (its homologue without a sugar moiety at N-1) were interpreted in terms of the conformational features of the pyrimidine ring. The study was based on the analysis of the changes in 13C chemical shifts of the pyrimidine ring due to through-space interactions between the different substituents. First, γ effects between the substituents at C-6 (hydroxyl, hydroperoxyl or 2,2,4,4-tetramethylpiperidone-N-oxyl) and the methyl group at C-5 were considered. Then these steric effects were combined with hydrogen bonding interactions between the hydroxyl group at C-5 and the carbonyl at C-4. This made it possible to account for the observed changes in the 13C chemical shifts of the pyrimidine ring. The 5,6-saturated pyrimidine moiety is proposed to adopt a half-chair conformation with the substituent at carbon C-6 in a pseudo-axial position.

Investigation of hydrogen bond structure in benzoic acid solutions

Significant changes in 13C chemical shifts of benzoic acid (BA) were observed for the carboxyl and quarternary carbon atoms on going from non-polar benzene to solvents with different hydrogen bond donor or acceptor abilities. Shielding effects of 6.145 and 5.413 ppm at the carboxyl carbon in dimethyl sulfoxide (DMSO) and acetone, respectively, are due to the redistribution of electron density upon the formation of new intermolecular hydrogen bonds with solvent molecules. 1H NMR upfield shifts of the carboxyl proton in DMSO and chloroform solutions in comparison with benzene solution, and shifts in the CO stretching frequency, ν CO, observed in infrared spectra of BA complexes are consistent with these results. Structural parameters and interaction energies of the hydrogen-bonded complexes determined by quantum-chemical calculations substantiate the experimental observations. Using the semiempirical PM3 hamiltonian, hydrogen bonding was treated as the donor-acceptor mechanism within the natural bond orbital (NBO) formalism. Two correlations, which reflect hydrogen bonding in different solvents, are proposed: one between the 13C chemical shifts and the change in p-character of the carboxyl carbon atom and the other between the carboxyl CO stretching frequency and the calculated interaction energy of the hydrogen-bonded complexes.

Interaction of calcium channel antagonists with calcium: structural studies on nicardipine and its Ca2+ complex.

Conformational features of nicardipine in acetonitrile, in the absence and presence of Ca2+, were investigated by one-dimensional NMR and difference absorption spectroscopy techniques. The data show that in acetonitrile solution the antiperiplanar form of nicardipine is dominant. The addition of Ca2+ to the drug solution caused marked changes in the difference absorbance spectra in the 200-400 nm region and in many of its 1H and 13C NMR resonances. The changes were most significant up to a ratio of 0.5 Ca2+:drug. Analysis of the binding data showed the predominant species to be a 2:1 drug:Ca2+ "sandwich" complex with an estimated dissociation constant of 100 microM at 25 degrees C. One-dimensional nuclear Overhauser effect (NOE) experiments revealed through-space connectivities in the drug before and after Ca2+ binding. These changes in conjunction with the changes in 1H and 13C chemical shifts suggest a structure in which the 4-aryl ring substitute of the pyridine moiety moves closer to the C3-side chain in the presence of Ca2+. This attraction is achieved via the chelation of the Ca2+ ion by the oxygen atoms in the m-NO2 of the aryl group and the COOCH2 group in the side chain of the dihydropyridine ring, and gives rise to a stable synperiplanar conformation. A preference for this conformation was also observed in the Ca2+ complex of nifedipine in acetonitrile as inferred from the rather limited NOE data obtained. Our study provides a detailed solution structure for nicardipine and also leads to a suggestion of a role for Ca2+ in the action of this and possibly other dihydropyridines.

Interaction of calcium channel antagonists with calcium: structural studies on verapamil and its Ca2+ complex.

The conformation of the calcium channel antagonist verapamil has been determined in acetonitrile, in the absence and presence of Ca2+, using two-dimensional 1H-NMR and molecular modeling techniques. Interproton connectivities in the drug molecule were identified from the observed NOESY cross peaks and interproton distances were estimated from the magnitudes of the volume integrals of the cross peaks. The molecular modeling program utilized the Monte Carlo simulation to generate a random ensemble of conformers complying with the NOESY-derived distance constraints. The energies of these conformers were subsequently computed. The minimum-energy structure of the free drug obtained in this manner exhibited some significant differences from the structure of verapamil determined by X-ray crystallography. In particular, the torsional angles in the middle region of the molecule containing the aliphatic "backbone" were such that the two aromatic rings at either end of the drug molecules were moved farther apart from each other in solution than in the crystal structure. The nearly perpendicular orientation of the aromatic rings seen in the crystal was, however, maintained in the solution structure as well. The addition of Ca2+ to a solution of verapamil in acetonitrile caused marked changes in the difference absorbance of the drug in the 200-300-nm region and in many of its 1H-NMR resonances. The changes were most significant up to a mole ratio of about 0.5 Ca2+:drug. Analysis of the binding data at 25 degrees C showed the presence of both 2:1 and 1:1 drug:Ca2+ complexes in equilibrium, the former "sandwich" complex being dominant at the lower cation concentrations with an estimated dissociation constant of about 300 microM. All of the NOESY cross peaks of the free drug remained on addition of 0.5 mol ratio of Ca2+ to verapamil in deuterated acetonitrile and only two new connectivities were observed. Using the interproton distances calculated from these NOESY data, molecular modeling of the 2:1 drug:Ca2+ complex was carried out to yield the minimum-energy conformer. In this conformer, Ca2+ was coordinated to two methoxy oxygens from each of the two drug molecules. The implications of the verapamil-Ca2+ interaction are discussed in terms of available experimental data on the binding of verapamil to the dihydropyridine-sensitive channel and in terms of a hypothesis on the formation of a drug-Ca(2+)-receptor complex in the lipid bilayer environment.

29Si and 13C nmr spectra of trimethylsiloxy and hydroxy adamantanes reinvestigated at low concentration in deuteriochloroform solutions

29Si and 13C NMR chemical shifts are reported for mono- and bis(trimethylsiloxy)adamantanes as well as for mono- and dihydroxyadamantanes dissolved in deuteriochloroform. The chloroform induced changes in 29Si and 13C chemical shifts can be qualitatively accounted for by hydrogen-bonding considering basicity and solvent accessibility of oxygen atoms in the solute molecules. The steric effects observed on the 29Si shifts are not due to an interaction of the terminal hydrogen atoms of the trimethylsilyl groups but are due to proximity of two oxygen atoms on one cyclohexane ring of the adamantane skeleton.

Nuclear magnetic resonance spectra of heterocycles. Analysis of carbon-13 spectra of thioxanthenones using calculated chemical shifts, substituent constants, and PRDDO molecular-orbital calculations

Changes in 13C chemical shifts of a series of 2-substituted thioxanthenones indicate that the 2-substituent causes electronic changes in the unsubstituted ring. Analysis of the 13C n.m.r. data using the dual-substituent parameter (d.s.p.) method and PRDDO molecular-orbital calculations shows that the 2-substituent alters, in a regular fashion, the electron density at C-6 and C-7, and at carbons adjacent to sulphur. The analysis also suggests that transmission of substituent effects from the 2-substituent to the unsubstituted ring takes place via a π-polarization mechanism operating through the heterocyclic sulphur.

13C- and 31P-NMR studies of the conformation of carcinogen-modified nucleic acid dimers

The effect of the carcinogen acetylaminofluorene (AAF) on nucleic acid structure was examined using 13C- and 31P-NMR spectroscopies. Conformational effects were compared in two AAF-modified dinucleoside monophosphates (ApG and GpA) and two AAF-modified deoxydinucleotides (dpApG and dpGpA). Changes in adenine 13C chemical shifts on formation of the AAF-adduct and as a function of temperature provided evidence of base stacking. Differences in fluorene 13C chemical shifts between the AAF-modified dimer and AAF-modified monomer provided evidence of fluorene stacking. The effect of forming the adduct on the phosphate backbone was examined using 31P-NMR. A correlation was demonstrated between the degree of adenine-fluorene stacking on one hand and the change in conformation of the backbone conformation on the other.

Lithium dialkylamides. 13C parameters and slow proton transfer

The changes in 13C chemical shifts for the structural change R 2NH → R 2NLi have been measured for a series of dialkylamines. These lithiation shifts are largest at the alpha carbon (3.7 – 9.7 ppm) and decrease in the order α > β > γ > δ. The rates of lithium-hydrogen interchange between R 2NH and R′ 2NLi have been determined. The activation energies are large (9 – 17 kcal/mole) and increase as the size of R or R′ increases. The slow exchange permits the direct measurement of acidity differences between pairs of amines using 13C nmr.

Hydrogen-1 and carbon-13 magnetic resonance studies of nonactin-calcium complex.

For an understanding of the cation selectivity and general binding characteristics of macrotetralide antibiotic nonactin (NA) with ions of different sizes and charges, the nature of binding of divalent caption, Ca2+, to NA and conformation of the NA-Ca2+ complex have been studied by use of 270-MHz proton nuclear magnetic resonance (1H NMR) and carbon-13 nuclear magnetic resonance (13C NMR). The calcium ion induced significantly large changes in chemical shifts for H7, H2, H3, and H5 protons of NA and relatively small changes for H18 and H21 protons. Changes in 13C chemical shift were quite large for carbonyl carbon, C1; it is noteworthy that in the NA-K+ complex, H2 and H21 protons practically do not shown any change during complexation and carbonyl carbon shows a much smaller chemical shift change. From an analysis of calcium ion titration data, it is concluded tht NA forms an equimolar complex with Ca2+. The study of complexation of NA and dry acetone-d6 and in acetone-d6 containing 0.1 mol fraction of D2O indicated that only unhydrated Ca2+ is bound to the NA, similar to complexes of NA with other cations. The binding constant for NA-Ca2+ in dry acetone is of the order of 10(4) (in mole fraction units); this gets marginally reduced to 7.5 X 10(3) in the case of wet acetone. The study further indicated that the overall conformation for NA-Ca2+ is similar to that of NA-K+ reported earlier, with significant local conformational changes occurring in order to accommodate the doubly charged, smaller sized calcium ion.

Interactions of 4-nitroquinoline 1-oxide with deoxyribodinucleotides

The interactions of 4-nitroquinoline 1-oxide (NQO), a potent mutagen and carcinogen, with several self- and non-self-complementary deoxydinucleotides were probed by using absorption spectra of the charge transfer bands and 1H and 13C NMR spectra. Absorption spectra were analyzed by using Benesi-Hildebrand-type equations to yield stoichiometries and equilibrium constants of complex formation. Non-self complementary dimers form weak l:1 complexes [dpTpG:NQO, K(25 degrees C) = 22 M-1] while self-complementary dimers form strong 2:1 complexes [dpCpG)2:NQO, K(25 degrees C) = 2.2 X 10(4) M-2]. A mixture of dpTpG and dpCpA with NQO gives a 2:1 complexes [dpCpG)2:NQO, K(25 degrees C) = 2.2 X 10(4) M-2]. A mixture of dpTpG and dpCpA, K(25 degrees C) = 8.6 X 10(3) M-2]. Analyses of the changes in 13C and 1H NMR chemical shifts with complex formation gave approximate orientations for the intercalation of NQO with self-complementary dimer minihelixes. In the (dpCpG)2:NQO and (dpGpC)2:NQO complexes, the NO2 group of NQO probably lies in the major grove and the NO2, NO containing NQO ring is stacked near the purine imidazole ring. In the (dpTpA)2:NQO and (dpApT)2NQO complexes, the NO2 seems to project into the minor grove and the NQO benzenoid ring is over the purine imidazole ring.

A 13C nmr study of metal ion binding to pyridoxine

13C nmr confirms that coordination of metal ions by pyridoxine is through the C-3 and C-4′ oxygens in aqueous solution. Nitrogen appears to become more effective as a donor site in water–dimethylsulfoxide mixtures, while with increasing proportions of DMSO some cations are coordinated by DMSO rather than by pyridoxine. Changes in 13C spin–lattice relaxation times on metal ion coordination are more informative about metal binding sites than changes in 13C chemical shift. Some analogous results are reported for pyridoxamine.