Negative Nuclear Overhauser Effect Research Articles

NMR Studies on Effects of Tetraalkylammonium Bromides on Micellization of Sodium Dodecylsulfate

The effects of tetraalkylammonium bromides (TAABs) on the micellization of sodium dodecylsulfate (SDS) are studied using pyrene solubilization and several nuclear magnetic resonance (NMR) techniques. Two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY) experiments confirm that tetraalkylammonium (TAA(+)) ions associate with SDS to form mixed micelles. TAA(+) ions attach to the surface of the mixed micelles and become inserted into the hydrophobic core of the mixed micelles. Because TAA(+) ions appear in the hydrophobic interior of the TAA-SDS mixed micelles, the micropolarity inside the mixed micelles sensed by pyrene might not reflect the true hydrophobicity of the micellar core. Using proton chemical shift analysis, the degree of hydration on the surface of the mixed micelles is determined from the chemical shift change of SDS α-CH2 protons. The self-diffusion coefficients of SDS and TAA(+) ions in the TAAB/SDS/D2O solutions are measured by using pulse-field gradient NMR, and the fraction of TAA(+) ions associated with the SDS to form the mixed micelles is calculated from the self-diffusion data. Moreover, secondary micelle formation for SDS and TAA(+) ions is observed on the basis of (1)H chemical shift analysis and the self-diffusion data. The 2D NOESY experiments also reveal unusual tumbling behavior of SDS alkyl protons. For Pr4NBr/SDS and Bu4NBr/SDS solutions, positive and negative nuclear Overhauser effects are simultaneously observed among the SDS alkyl protons.

Weak Interactions between Folate and Osmolytes in Solution

Previous osmotic stress studies on the role of solvent in two structurally unrelated dihydrofolate reductases (DHFRs) found weaker binding of dihydrofolate (DHF) to either enzyme in the presence of osmolytes. To explain these unusual results, weak interactions between DHF and osmolytes were proposed, with a competition between osmolyte and DHFR for DHF. High osmolyte concentrations will inhibit binding of the cognate pair. To evaluate this hypothesis, we devised a small molecule approach. Dimerization of folate, monitored by nuclear magnetic resonance, was weakened 2-3-fold upon addition of betaine or dimethyl sulfoxide (DMSO), supporting preferential interaction of either osmolyte with the monomer (as it possesses a larger surface area). Nuclear Overhauser effect (NOE) spectroscopy experiments found a positive NOE for the interaction of the C3'/C5' benzoyl ring protons with the C9 proton in buffer; however, a negative NOE was observed upon addition of betaine or DMSO. This change indicated a decreased tumbling rate, consistent with osmolyte interaction. Osmotic stress experiments also showed that betaine, DMSO, and sucrose preferentially interact with folate. Further, studies with the folate fragments, p-aminobenzoic acid and pterin 6-carboxylate, revealed interactions for both model compounds with betaine and sucrose. In contrast, DMSO was strongly excluded from the pterin ring but preferentially interacted with the p-aminobenzoyl moiety. These interactions are likely to be important in vivo because of the crowded conditions of the cell where weak contacts can more readily compete with specific binding interactions.

Microemulsions of water in supercritical carbon dioxide: an in-situ NMR investigation of micelle formation and structure

High-pressure NMR spectroscopy was used for the first time to investigate microemulsions of water in supercritical carbon dioxide. The emulsions were formed using a family of anionic perfluoropolyether ammonium carboxylate surfactants. This system holds promise as a reaction medium for conducting homogeneous catalytic reactions within the aqueous micellular cores while, at the same time, exploiting the facile mass transfer properties of the supercritical fluid. Ammonium hexafluorophosphate was used as a water-soluble ionic guest to investigate micelle formation and structure. Under micelle-forming conditions, the PF 6 − guest, surfactant, and water were uniformly dispersed throughout the CO 2 phase, as demonstrated by in situ NMR imaging. In addition, the micelles were observed to form even in the absence of mechanical stirring. This spontaneous formation of micelles demonstrates that the NMR spectral properties were obtained under conditions that result in the production of thermodynamically stable microemulsions. The nuclear overhauser effect (NOE) was used to probe the micellular structure through dipole–dipole interactions between the PF 6 − anion and the fluorinated backbone of the surfactant. A strong negative homonuclear NoE was observed between the PF 6 − guest and the fluorine moiety that is located directly adjacent to the surfactant's carboxylate head group. This highly specific negative NOE indicates an ordered arrangement, where the PF 6 − anion and carboxylate ion are located in close proximity to one another. This close association of two negatively charged ionic groups in an aqueous environment is unusual and suggests that the PF 6 − guest is concentrated within the electric double layer that forms at the micellular interface.

Structure and thermodynamics of metal binding in the P5 helix of a group I intron ribozyme

The solution structure of an RNA hairpin modelling the P5 helix of a group I intron, complexed with Co(NH3)63+, has been determined by nuclear magnetic resonance. Co(NH3)63+, which possesses a geometry very close to Mg(H2O)62+, was used to identify and characterize a Mg2+ binding site in the RNA. Strong and positive intermolecular nuclear Overhauser effect (NOE) cross-peaks define a specific complex in which the Co(NH3)63+ molecule is in the major groove of tandem G·U base-pairs. The structure of the RNA is characterized by a very low twist angle between the two G·U base-pairs, providing a flat and narrowed major groove. The Co(NH3)63+, although highly localized, is free to rotate to hydrogen bond in several ways to the O4 atoms of the uracil bases and to N7 and O6 of the guanine bases. Negative and small NOE cross-peaks to other protons in the sequence reveal a non-specific or delocalized interaction, characterized by a high mobility of the cobalt ion. Mn2+ titrations of P5 show specific broadening of protons of the G·U base-pairs that form the metal ion binding site, in agreement with the NOE data from Co(NH3)63+. Binding constants for the interaction of Co(NH3)63+ and of Mg2+ to P5 were determined by monitoring imino proton chemical shifts during titration of the RNA with the metal ions. Dissociation constants are on the order of 0.1 mM for Co(NH3)63+ and 1 mM for Mg2+. Binding studies were done on mutants with sequences corresponding to the three orientations of tandem G·U base-pairs. The affinities of Co(NH3)63+ and Mg2+ for the tandem G·U base-pairs depend strongly on their sequences; the differences can be understood in terms of the different structures of the corresponding metal ion-RNA complexes. Substitution of G·C or A·U for G·U pairs also affected the binding, as expected. These structural and thermodynamic results provide systematic new information about major groove metal ion binding in RNA.

Interactions of polyamines with the DNA octamers d(m5CG)4 and d(GGAATTCC): A1H-NMR investigation

The interaction of two DNA octamers, d(m5CG)4 and d(GGAATTCC), with the polyamines spermine4+ and spermidine3+, has been studied by means of 1H-nmr nuclear Overhauser effect (NOE) difference measurements. The experiments were performed at 10°C and for a polyamine charge to DNA charge (i.e., phosphate) ratio of 0.4, where the solution of d(m5CG)4 contains about 50% Z-form of the DNA. The results show that the polyamine intramolecular NOEs for the protons on the propyl chains are similarly negative with the two oligonucleotides, while those on the butyl chain show slightly more negative NOE with d(m5CG)4 than with d(GGAATTCC). The fully N-methylated analogues of spermine (Me10Spn4+) and spermidine (Me8Spd3+) as well as the diamines 1,3-diaminopropane (DAP2+) and 1,4-diaminobutane (putrescine2+) have been studied for the ability to transform d(m5CG)4 from the B- to the Z-form. 1H-nmr spectra showed the order spermine4+ > spermidine3+ > Me10Spn4+ > Me8Spd3+ > 1,3-diaminopropane2+ > putrescine2+, with spermine showing the largest relative amount of Z-DNA. 1H-nmr pulsed-gradient self-diffusion measurements of the triamines showed a large difference in the interaction of Spd and Me8Spd with the two different duplexes. With the same duplex (either of the two), however, no difference between Spd and Me8Spd can be seen. Within a two-state model this is interpreted as a larger fraction of bound polyamines with d(m5CG)4 than with d(GGAATTCC). © 1999 John Wiley & Sons, Inc. Biopoly 49: 41–53, 1999

Characterization of a buried neutral histidine in Bacillus circulans xylanase: internal dynamics and interaction with a bound water molecule.

NMR spectroscopy was used to characterize the dynamic behavior of His149 in Bacillus circulans xylanase (BCX) and its interaction with an internal water molecule. Rate constants for the specific acid- and base-catalyzed exchange following bimolecular kinetics (EX2) of the nitrogen-bonded H epsilon 2 of this buried, neutral histidine were determined. At pDmin 7.0 and 30 degrees C, the lifetime for this proton is 9.9 h, corresponding to a protection factor of approximately 10(7) relative to that predicted for an exposed histidine. The apparent activation energies measured for specific acid and base catalysis (7.0 and 17.4 kcal/mol) indicate that exchange occurs via local structural fluctuations. Consistent with its buried environment, the N epsilon 2-H bond vector of His149 shows restricted mobility, as evidenced by an order parameter S2 = 0.83 determined from 15N relaxation measurements. The crystal structure of BCX reveals that a conserved, buried water hydrogen-bonds to the H epsilon 2 of His149. Strong support for this interaction in solution is provided by the observation of a negative nuclear Overhauser effect (NOE) and positive rotating-frame Overhauser effect (ROE) between His149 H epsilon 2 and a water molecule with the same chemical shift as the bulk solvent. However, the chemical shift of H epsilon 2 (12.2 ppm) and a D/H fractionation factor close to unity (0.89 +/- 0.02) indicate that this is not a so-called low-barrier hydrogen bond. Lower and upper bounds on the lifetime of the internal water are estimated to be 10(-8) and 10(-3) s. Therefore the chemical exchange of solvent protons with those of His149 H epsilon 2 and the diffusion or physical exchange of the internal water to which the histidine is hydrogen-bonded differ in rate by over 7 orders of magnitude.

Proton nuclear magnetic resonance studies of monoacylated anthocyanins from the wild carrot: Part 1. Inter- and intra-molecular interactions in solution

Inter- and intra-molecular interactions of five anthocyanins (one non-acylated and four acylated with either sinapic acid or 4-methylated sinapic acid) obtained from cell culture of the wild carrot (Daucus carotassp. carota) have been studied by one- and two-dimensional proton nuclear magnetic resonance (NMR) spectroscopy. Temperature and concentration dependencies of proton chemical shifts show that self-association of non-acylated anthocyanins depends upon the number and position of the sugar units. It was found for the first time that there were non-acylated anthocyanins that were not protected by self-association. Both intra- and inter-molecular interactions were found in the acylated anthocyanins under study. These compounds form strong intra-molecular π-complexes between the sinapoyl group and the anthocyanidin nucleus, the double bond of sinapic acid being involved as well as its aromatic ring. Upon increasing the concentration of the anthocyanins or lowering the temperature of the NMR sample, the π-complexes form multinuclear complexes as shown by the resultant negative nuclear Overhauser effect (NOE) values. Spin diffusion was observed in the acylated anthocyanins for temperatures below −40°C. It was also concluded that, because anthocyanins fall in the range of medium-size molecules with NOEs of small magnitude, and because they readily form complexes, two-dimensional NOE experiments are more reliable for structural elucidation of the anthocyanins than are one-dimensional steady-state NOE experiments. © 1998 John Wiley & Sons, Ltd.

NMR and molecular dynamics study of muroctasin—Implications for the bioactive conformation

Muroctasin (N2-[(N-acetylmuramoyl)-L-alanyl-D-isoglutaminyl]-N6-stearoyl-L-lysine), a potent muramyl dipeptide (MDP) analogue with increased lipophilicity, has been studied in DMSO-d6 solution with nmr methods and molecular dynamics (MD) simulations. The proton spectra show the presence of the α- and β-anomers of the pyranosyl ring in the ratio of 3:1. The molecule is in the negative nuclear Overhauser effect (NOE) regime at 25°C and 300 MHz, but shows some positive cross peaks in the stearoyl-lysine moiety in the NOE spectroscopy (NOESY) spectra; at 600 MHz only the stearoyl moiety remains in the positive regime. The temperature dependence of the alanyl amide proton chemical shift (−2.7 ppb/K) points to its involvement in intramolecular H bonding. An extensive conformational search in vacuo indicates that the preferred conformations contain the β-turn like structure characterized by the intramolecular H-bond N-Acetyl-CO-HN-Ala; it coexists with the type II β-turn characterized by the H-bond Mur-CO-HENT-iGln only in some high energy structures obtained by the conformational search (relative energy > 20 kcal/mol). Such an S-shaped conformation was never found in the 0.5 ns simulation involving explicit DMSO solvent molecules where only the Ala amide proton is significantly protected from the solvent; some (short-lived) γ-turns in the alanyl-isoglutaminyl backbone are also observed. No conformational preferences of the stearoyl-lysine moiety could be deduced from the NOE data; the increased flexibility of this part of the molecule relative to the muramoyl moiety is in agreement with the shorter local correlation times evident from NOESY spectra. The set of conformations that are most similar to the set of candidates for the bioactive conformation of MDP obtained by the CCLUES procedure [P. Pristovsěk et al. (1995), J. Chem. Inf. Comput. Sci. Vol. 35, pp. 633–639] contains no low-energy structures, but some local conformational features found in solution are preserved. © 1997 John Wiley & Sons, Inc. Biopoly 42: 659–671, 1997

Simultaneous observation of positive and negative NOE in biomolecules: a bistable jump model for segmental motion with modulation of internuclear distances

Recently a negative nuclear Overhauser effect (NOE) among the backbone protons as well as a positive NOE between the backbone and ring protons of tryptophan in a peptide was reported. While the negative NOE is indicative of slow overall reorientation of the molecule, simultaneous Occurrence of a positive NOE is indicative of internal motions in the molecule. To explain these observations we develop here a model in which the overall motion is anisotropic reorientation and the internal motion modulates the internuclear distance. Analytical expressions for the spectral density functions for dipole-dipole relaxation are obtained when the internal motion is a bistable jump. Steady-state and transient NOE calculations are performed for two relaxation-coupled spins. The results confirm that when the overall motion is in the slow motion limit and the side chain internal motion is in the fast motion limit, the NOE within the backbone protons is negative and that between the backbone and the side chain protons is positive. This model further shows that the NOE between the backbone and the side chain is proportional to the weighted average of the inverse sixth power of the distance between the interacting spins only when the overall motion is isotropic, dominates the relaxation process and is much faster than the internal motion. With use of this model, multispin steady-state NOES are computed to explain the experimental observation of positive and negative NOE’s and their temperature dependence in a tryptophan-containing polypeptide.

1H and 31P nuclear magnetic resonance studies of spermine binding to the Z-DNA form of d(m 5CGm 5CGm 5CG) 2: Evidence for decreased spermine mobility

The binding of spermine to the d(m 5CGm 5CGm 5CG) duplex has been studied by proton and phosphorus nuclear magnetic resonance techniques in order to investigate the mobility and nature of spermine bound to the resulting Z-DNA complex. A characterization of the B to Z transition as a function of increasing spermine concentration demonstrated doubling of the non-exchangeable proton and the phosphorus peaks at a ratio of about 1:1 (spermine/duplex) and a re-simplification of the spectrum at 2:1 (spermine/duplex) where about 90% or the DNA was fully converted into the Z-form. However, some of the Z-DNA proton chemical shifts differed between the 1:1 and 2:1 titration points. Since these differences involved primarily the exchangeable terminal imino and amino protons, they could result from end effects. Discrepancies were generally not observed with the non-terminal proton shifts nor with the phosphorus shifts. These proton and phosphorus chemical shift changes are fully consistent with a, B to Z transition. Complexed spermine peaks appear about 0.1 parts per million upfield from the uncomplexed form. The spermine and both the B and Z-DNA hexamer signals are noticeably broadened at the 1:1 ratio but the remaining signals re-sharpen at the 2:1 ratio. Both one-dimensional and two-dimensional studies revealed negative nuclear Overhauser effect (NOE) contacts between each spermine proton. Therefore, spermine has a longer correlation time than that observed for unbounded spermine. These results are contrasted with the positive NOE contacts observed for the B-DNA-spermine complexes reported by Wemmer et al. using the dodecamer d(CGCGAATTCGCG) 2 and reported here using the hexamer d(ATGCAT) 2. While the mobility of spermine in the Z-DNA complex is significantly less than that of the B-DNA complex, no clear evidence of intermolecular spermine-DNA proton NOE contacts is observed.

Anti-plasmin inhibitor. VI. Structure of phlorofucofuroeckol A, a novel phlorotannin with both dibenzo-1,4-dioxin and dibenzofuran elements, from Ecklonia kurome Okamura.

Phlorofucofuroeckol A (1), a novel phlorotannin with both dibenzo-1,4-dioxin and dibenzofuran elements, has been isolated from the brown alga Ecklonia kurome OKAMURA as a potent anti-plasmin inhibitor. Its structure has been elucidated to be 1,11-di(3,5-dihydroxyphenoxy)- benzofuro[3,2-a]dibenzo[b,e][1,4]dioxin-2,4,8,10,14-pentaol on the basis of the spectral data, in particular, by means of negative nuclear Overhauser effect (NOE) and long-range carbon-proton couplings (2JCH and 3JCH). Phlorofucofuroeckol A inhibited the action of alpha 2-macroglobulin (IC50 = 1.0 micrograms/ml) and alpha 2-plasmin inhibitor (IC50 = 0.3 micrograms/ml), the main plasmin inhibitors in plasma.

400 MHz two-dimensional nuclear Overhauser spectroscopy on anesthetic interaction with lipid bilayer

Interaction between a volatile anesthetic, methoxyflurane, and dipalmitoylphosphatidylcholine (DPPC) vesicle membrane was analyzed by nuclear Overhauser effect (NOE) difference spectroscopy and two-dimensional nuclear Overhauser spectroscopy (NOESY). The NOE difference spectra were obtained by selectively irradiating methoxy protons (hydrophobic end) of the anesthetic: a negative nuclear Overhauser effect of -2.94% was observed with the choline methyl protons of DPPC. The NOESY spectra revealed a cross-peak between the anesthetic methoxy protons and the choline methyl protons. A dipole-dipole interaction exists between the hydrophobic end of the anesthetic and the hydrophilic head group of DPPC. No other cross-peaks were observed. The anesthetic orients itself at the membrane/water interface by interacting with the hydrophilic surface of the DPPC membrane, leaving the hydrophilic end of the anesthetic molecule in the aqueous phase. The preferred residence site of dipolar volatile anesthetics is the membrane/water interface.

Asernestioside C, a new minor saponin from the roots of Astragalus ernestii COMB.; First example of negative nuclear Overhauser effect in the saponins.

A minor saponin, asernestioside C (1), was isolated from the roots of Astragalus ernestii COMB. (Leguminosae) and its structure was determined from the two-dimensional nuclear magnetic resonance spectra and nuclear Overhauser effect (NOE) difference spectra. In this case, negative NOE was observed between the 3-H and the anomeric proton of 3-O-sugar moiety and between the 24-H and the 26-H3. Examples of negative NOE were also found in some other saponins.

Structure of an anti-plasmin inhibitor, eckol, isolated from the brown alga Ecklonia kurome Okamura and inhibitory activities of its derivatives on plasma plasmin inhibitors.

Eckol (1), a novel phlorotannin with a dibenzo-1,4-dioxin skeleton, has been isolated from the brown alga Ecklonia kurome Okamura as a potent and specific anti-plasmin inhibitor. Its structure has been elucidated based on the spectral data, in particular, by means of negative nuclear Overhauser effect (NOE), and finally established as 1-(3,5-dihydroxyphenoxy)-2,4,7,9-tetrahydroxydibenzo-1,4-dio xin by X-ray analysis. Some partially methoxylated derivatives of eckol were prepared by methylation with diazomethane and also by selective dimethylation of eckol permethylate (1b) to establish the structural requirements for inhibitory activities on alpha 2-macroglobulin and alpha 2-plasmin inhibitor, the main plasmin inhibitors in plasma.

Spectrophotometric and NMR analysis of the interaction of fluorinated mono- and bifunctional intercalators with DNA, poly(dA-dT), and poly(dG-dC).

AbstractWe have synthesized and investigated the DNA binding properties of three fluorinated acridine derivatives—a monomer (I), a short dimer (II) and a long dimer (III). Only III has a sufficiently long chain bridging the two acridine nuclei to permit binding by bisintercalation. Analysis of the equilibrium and kinetic binding properties of these compounds to poly(dA‐dT) demonstrates that they behave very similarly to their unfluorinated parent compounds. Helix extension, as determined by viscosity measurements, shows that both compounds I and II bind by monointercalation while III binds by bisintercalation. These results are confirmed by 19F‐nmr analysis, which indicates, in particular, that the two chromophores of III share the same molecular environment as that of I in the presence of either calf thymus DNA or poly(dA‐dT). Negative nuclear Overhauser effects in the presence of DNA indicate tight binding such that the motion of the ligands is governed by the polynucleotide dynamics. Optical titrations establish that in 4M NaCl, both I and III bind to calf thymus DNA, but no binding was observed with poly(dG‐dC). This result is in contrast to those for dimers of ethidium, which show substantial binding to polynucleotides under high salt conditions. Nuclear magnetic resonance experiments, however, carried out at considerably higher concentrations, show that compound I does indeed bind to poly(dG‐dC) under these high salt conditions, albeit weakly, and leads to a conversion of the polynucleotide from a left‐handed to a right‐handed conformation.

Intermolecular nuclear Overhauser effect and atomic pair potential approaches to wheat germ agglutinin-sugar binding.

The detailed binding mechanism of wheat germ agglutinin (WGA) with N-acetylglucosamine (GlcNAc) was investigated using intermolecular 1H-1H nuclear Overhauser effect (NOE) and atomic pair potential (APP) calculations. Negative NOE was observed on the 1H spectrum of 1-O-methyl derivative of GlcNAc in a solution containing WGA, when the aromatic region of the WGA spectrum was irradiated. Analyses of the time dependence of NOE revealed that H2 and the N-acetyl methyl protons of the sugar are in close proximity to the aromatic protons of WGA in the bound state. This was confirmed and further elucidated by the APP calculations. According to the calculation, the major binding force comes from a hydrogen-bonding between C3-OH of sugar and an acidic residue present in each of the two binding sites of WGA: Glu115 in site 1 and Asp29 in site 2. The binding is further assisted by the N-acetyl group which interacts with a few more polar amino acid residues in the binding sites. The optimized binding mode suggested by the APP calculations supports the NMR results in that H2 and a part of the N-acetyl methyl protons are within 4.5 A distance from protons of both Tyr64 and Tyr73 in site 1 and of Tyr159 in site 2.

Simultaneous observation of positive and negative nuclear overhauser effects in oligopeptides due to segmental motion

Nuclear Overhauser effect (NOE) studies of the symmetrical cystine peptides Boc-Cys-(Val) n-Trp-OMe (n=1–3) in dimethylsulfoxide, Boc-Cys-(Val) n-Trp-OMe have resulted in the simultaneous observation of both positive and negative NOEs. Positive NOEs are observed on the Trp C2H and C4H protons of the indole ring upon irradiation of Trp C α H and C βH 2 resonances in the peptides where n=1 and 2. Negative NOEs are observed between backbone NH and C α H protons. The magnitudes of the observed NOEs are sensitive to changes in molecular size and solvent viscosity. The results demonstrate that NOEs may be a useful probe of sidechain segmental motion in oligopeptides.

Locations and dynamical perturbations for lipids of cationic forms of procaine, tetracaine, and dibucaine in small unilamellar phosphatidylcholine vesicles as studied by nuclear Overhauser effects in 1H nuclear magnetic resonance spectroscopy

Locations and dynamical perturbations for lipids of local anesthetics ( procaine · HCl , tetracaine · HCl , and dibucaine · HCl ) in sonicated egg yolk phosphatidylcholine (PC) vesicles have been studied by 1H− 1H nuclear Overhauser effect (NOE) measurements. It was found that tetracaine and dibucaine bind much strongly to the neutral lipids than does procaine and that their mobilities are lowered to such an extent that spin diffusion is transmitted (i.e., ω 2τ c 2 ⪢ 1 ). The intermolecular NOEs between drugs and PC were more effective in the case of dibucaine than with tetracaine, indicating that dibucaine binds to the lipids more strongly than tetracaine; this order agrees well with that of anesthetic potency. However, it was only tetracaine that gave any appreciable dynamical perturbation to the PC vesicles when they were monitored by the extent of transfer of the negative NOE from α-methylene protons to choline methyls, olefinic methines, acyl methylenes and terminal methyl protons. This finding was interpreted as being due to the differences in the locations of these drugs in small unilamellar vesicles: (1) procaine interacts with lipids very weakly at the outer surface of the vesicles; (2) tetracaine binds to the lipids both at the outer and inner halves of the bilayer, inserting its rod-like molecule in a forest of acyl chains of PC; (3) dibucaine binds tightly to the polar head-group of PC, which resides only at the outer half of the bilayer vesicles. It was concluded that the relative order of anesthetic potency within these drugs can be correlated not with the ability to affect membrane fluidity but with the ability to bind to lipids at the polar head-group of the bilayer vesicles.

Bisintercalation of ditercalinium into a d[CpGpCpG]2 minihelix: Structure and dynamics aspects—a 400‐MHz 1H‐nmr study

AbstractThe structure of the complex formed in aqueous solution at pH 5.5 between ditercalinium, a potent antitumoral 7H‐pyrido[4,3‐c]carbazole rigid dimer, and the self‐complementary tetranucleotide d[CpGpCpG], was investigated by 400‐MHz 1H‐nmr. For a 1:2.5 drug‐to‐helix ratio, the dimer was only found in bound form, whereas free and complexed tetranucleotide were in slow exchange. This allowed unambiguous assignment of the protons in the complex through exchange polarization transfer measurements. The tetranucleotide existed as a right‐handed double helix in the complex. The strong upfield shifts measured on most aromatic protons on both drug and nucleobases as well as on DNA imino protons were consistent with bisintercalation of the dimer. According to the negative nuclear Overhauser effects generated to protons on the convex edge of the bound drug rings by saturation of sugar protons, it was concluded that ditercalinium was intercalated with its rigid bis‐ethyl bispiperidine spacer fitting the major groove of the helix. Difference in antitumor activity of various pyridocarbazole dimers is discussed in relation to the binding kinetics and the complex geometry determined in this study.

1H-NMR studies of a monointercalating drug into a d[CpGpCpG]2 minihelix.

AbstractThe structure of the complex formed between the 7H‐pyridocarbazole monomer [{(2‐piperidyl)‐2,1‐ethane‐yl} {10‐methoxy‐7H‐pyrido[4,3‐c]carbazolium} dimethane sulfonate] and the autocom‐plementary tetranucleotide d(CpGpCpG)2 in aqueous solution is analyzed by 270‐MHz and 400‐MHz 1H‐nmr. The strong upfield shifts observed on most aromatic resonances of both the drug and the nucleotide are interpreted as the result of intercalation of the 7H‐pyridocarbazole monomer in the base‐paired minihelix of d(CpGpCpG). The observation of intermolecular negative nuclear Overhauser effects induced in some drug resonances by irradiation of sugar protons confirms this conclusion. A privileged orientation of the drug in the intercalation site with the quaternizing ethyl piperidine chain protruding in the major groove is proposed.