Structural and conformational analyses of 8-hydroxy-2'-deoxyguanosine

Oxidative DNA damage has been shown to involve formation of 8-hydroxy-2'-deoxyguanosine, which may serve as a mispairing lesion during cellular DNA replication. In order to assess the mutagenic potential of this DNA adduct, we examined the possible occurrence of several tautomeric forms and of different base conformations about the deoxyribose. Several spectroscopic and electronic absorption techniques were employed and showed structural changes occurring over a broad pH range. Two pKa's near pH 8 and 12 were observed by pH-solvent partitioning experiments, ultraviolet absorption spectral analyses, and 13C NMR spectroscopic methods. The presence of two pKa's suggested the formation of a dianion, with the second being formed in strong alkali. A comparison of ultraviolet absorption band features of 8-hydroxy-2'-deoxyguanosine with that of different C6,C8-diketo or enol derivatives supported a C8-keto assignment and also provided evidence that this DNA adduct contains a C6-keto group at physiological pH. 13C NMR data showed marked chemical shifts at C6 in solutions of pH 7.4-9.3, indicating the location of the first ionization. Increasing basicity produced further shifts at C5 and C8, indicating the C8 position for the second ionization. Multiple infrared bands were observed in the carbonyl region of the neutral compound, but only a single carbonyl band at 1692 cm-1 remained at pH 9.1, implying a keto group at C8. Determination of the chemical shifts and the nuclear Overhauser enhancements of the N1 and N7 resonances in the proton-decoupled 15N NMR spectrum indicated that both nitrogens were indeed protonated at neutral pH.(ABSTRACT TRUNCATED AT 250 WORDS)     

First principles calculations of the tautomers and pK(a) values of 8-oxoguanine: implications for mutagenicity and repair

8-Oxoguanine is a mutagenic oxidative damage product of guanine that has been the subject of many experimental studies. Despite numerous references to this damaged base, its precise configuration or population of configurations in equilibrium are unknown, as it can be drawn in over 100 potential neutral and ionized tautomeric forms. The structural uncertainty surrounding 8-oxoguanine complicates mechanistic studies of its mutagenicity and capacity to be recognized for repair. Experimental measurements on the tautomeric equilibria and pK(a) values of 8-oxoguanine are complicated by its insolubility in water. Therefore, we used first principles quantum mechanics (density functional theory, B3LYP, in combination with the Poisson-Boltzmann continuum-solvation model) to investigate the relative stabilities and site-specific pK(a) values of various neutral and ionized tautomers of 8-oxoguanine. We show that the major tautomer of neutral 8-oxoguanine in aqueous solution is the 6,8-diketo form 2, and that 8-oxoguanine has increased acidity at N1 relative to guanine. Our calculations on 2'-deoxyguanosine-3',5'-bisphosphate and its 8-oxo analogue support the accepted conclusion that repulsion between the O8 of 8-oxoguanine and O5' of the backbone sugar promote 8-oxoguanine:adenine pairings in the syn:anti conformation. Further, we show that the N7 proton of 8-oxoguanine is difficult to remove either through tautomerization or ionization, consistent with its involvement as an important landmark in distinguishing guanine from 8-oxoguanine. The possibility of additional structural landmarks that distinguish 8-oxoguanine from guanine, and a possible mechanism for glycosylase removal of 8-oxoguanine are discussed.     

Synthesis, characterization, and identification of N7-guanine adducts of isoprene monoepoxides in vitro

Isoprene (IP, 2-methylbuta-1,3-diene) is ubiquitous in the environment through emission by plants, combustion processes, and endogenous formation and exhalation by mammals, including humans. IP is also an industrial chemical, widely used in the manufacture of synthetic rubber and plastics. Like butadiene, IP is metabolized to reactive epoxides, which form adducts with macromolecules, and is a demonstrated carcinogen in mice. To date, DNA adducts of IP monoepoxides have not been reported. We report here on the formation of N7-guanine (N7-Gua) adducts of isoprene-1,2-oxide (IP-1,2-O, 2-ethenyl-2-methyloxirane) and isoprene-3,4-oxide (IP-3,4-O, propen-2-yloxirane). DNA adducts are useful as biomarkers to estimate exposure, as well as to investigate mechanisms of IP carcinogenesis. Incubation of 2'-deoxyguanosine with the monoepoxides followed by deglycosylation gave four N7-Gua adducts that were isolated by HPLC and characterized by high-resolution FAB(+)-MS, ESI(+)-MS, ESI(+)-MS/MS, and (1)H NMR and two-dimensional heteronuclear (1)H, (13)C correlation NMR spectrometry. IP-1,2-O and IP-3,4-O reacted at both terminal and internal oxirane carbons to form the following regioisomeric adducts at Gua N7: N7-(2'-hydroxy-2'-methyl-3'-buten-1'-yl)guanine, N7-(1'-hydroxy-2'-methyl-3'-buten-2'-yl)guanine, N7-(1'-hydroxy-3'-methyl-3'-buten-2'-yl)guanine, and N7-(2'-hydroxy-3'-methyl-3'-buten-1'-yl)guanine. The same adducts were identified by UV spectra, HPLC retention times, and LC/ESI(+)-MS in the neutral thermal hydrolysates of single- and double-stranded calf thymus DNA after incubation with IP monoepoxides. Characterization of the N7-Gua adducts identified in incubations of DNA with IP monoepoxides represents the first step toward establishing biomarkers of IP metabolism and exposure.     

Structural refinement of the 8,9-dihydro-8-(N7-guanyl)-9-hydroxy-aflatoxin B(1) adduct in a 5'-Cp(AFB)G-3' sequence

The structure of the cationic 8,9-dihydro-8-(N7-guanyl)-9-hydroxy-aflatoxin B(1) adduct embedded in a 5'-CpG-3' sequence context and paired with deoxycytosine in the oligodeoxynucleotide d(ACATC(AFB)GATCT) x d(AGATCGATGT) was refined using molecular dynamics calculations restrained by NOE data and dihedral angle restraints obtained from NMR data. The aflatoxin moiety intercalated above the 5' face of the modified guanine. It stacked between C(5) x G(16) and (AFB)G(6) x C(15). The AFB(1) H5, OCH(3), and methylene protons faced into the minor groove, with the methylene protons oriented between the C(15) and G(16) nucleobases. The aflatoxin B(1) H6a, H8, H9, and H9a protons faced the major groove, with H6a and H9a pointing toward the 5' direction from the lesion site. The refined structure was compared to the structure of the aflatoxin B(1) adduct embedded in a 5'-ATGCAT-3' sequence in the oligodeoxynucleotide d(TAT(AFB)GCATA)(2) [Jones, W. R., Johnston, D. S., and Stone, M. P. (1998) Chem. Res. Toxicol.11, 873-881]. The structure of the intercalated aflatoxin B(1) lesion in the ATC(AFB)GAT sequence is similar to its structure in the d(AT(AFB)GCAT) sequence. This is consistent with a mechanism in which the precovalent intercalation of aflatoxin-8,9-exo-epoxide on the 5' face of guanine places the epoxide in close proximity and in the proper orientation to the N7 position of guanine, thus facilitating an S(N)2 reaction. The data provides additional insight into the nature of the disruption of the B-DNA duplex induced by aflatoxin B(1) intercalation. Overall, the results suggest that sequence contributes a minor role in modulating the structure of the cationic guanine N7 AFB(1) lesion in duplex DNA. On the other hand, structural differences are observed when the correctly paired structure is compared to the structure of the cationic AFB(1) adduct mispaired with dA [Giri, I., Johnston, D. S., and Stone, M. P. (2002) Biochemistry 41, 5462-5472].     

A 2-iminohydantoin from the oxidation of guanine

The nucleobase guanine was oxidized with dimethyldioxirane (DMDO) to explore the role of epoxidizing agents in oxidative DNA damage. Treatment of guanine with 10% molar excess DMDO in aqueous solution at 0 degrees C and pH 7.5 followed by workup under mild conditions gave 5-carboxamido-5-formamido-2-iminohydantoin (1) as the sole isolable product in 71% yield. The structure of 1 was established on the basis of mass spectrometry and NMR studies on 1 and its isotopomers generated by the oxidation of [4-(13)C] and [7-(15)N]guanine, which yield [5-(13)C]1 and [7-(15)N]1. The distribution of 13C and 15N labels in the isotopomeric products supports initial epoxidation of the C4-C5 bond of guanine followed by a 1,2-acyl migration of guanine C6. Compound 1 is suggested as a possible primary DNA lesion from putative epoxidizing agents, including hydroperoxides present during biological processes such as lipid peroxidation.     

Computer simulation of the binding of naphthyridinomycin and cyanocycline A to DNA

Cyanocycline A was found to have a pKa of 6.6. Protonation of N14 was established by 1H NMR spectroscopy. In strongly acidic solution the oxazolidine ring opened irreversibly. A model was derived for the binding of naphthyridinomycin and cyanocycline A to the hexanucleotide duplex d(ATGCAT)2, by using the molecular mechanics and dynamics modules of AMBER 3.0. It involved protonation on the oxazolidine-ring nitrogen, reduction of the quinone ring to a hydroquinone, formation of an iminium ion with loss of the C7 substituent, noncovalent binding in the minor groove with the hydroquinone ring in the 3'-direction from guanine, and covalent binding to the 2-amino group of this guanine with C7 adopting the R configuration. This model is consistent with the experimental evidence on the DNA binding of these drugs. An alternative binding mode based on opening of the oxazolidine ring and alkylation at C3a also was feasible according to molecular mechanics calculations. The geometry of naphthyridinomycin does not permit interstrand cross-linking involving both C3a and C7, but formation of a cross-link to protein appears possible. When the covalent naphthyridinomycin-d(ATGCAT)2 models were refined in the presence of water and counterions, the models with the most favorable net binding enthalpies were the same as those produced by simulation in vacuum. Qualitative estimates of the relative entropy changes resulting from adduct formation were based on the number of ordered (hydrogen bonded) water molecules released from d(ATGCAT)2 and from the drug. In all cases but one, d(ATGCAT)2 loses five water molecules. It loses six in the C3a covalent model with 5',S geometry. Naphthyridinomycin hydroquinone loses up to two water molecules, depending on the particular adduct. The 3',R model was again favored for the C7 covalent adduct. Among the C3a covalent models, the one with 5',R geometry lost the second most water molecules, but it had the best binding enthalpy.     

Theoretical study of the protonation and tautomerization of adenosine, formycin, and their 2-NH2 and 2-F derivatives: functional implications in the mechanism of reaction of adenosine deaminase

A quantum chemical study of adenosine, formycin, and their 2-NH2 and 2-F derivatives is performed. The tautomerism of neutral and protonated species as well as the protonation of adenosine, formycin, and their derivatives are theoretically studied using semiempirical MNDO and AM1, as well as ab initio STO-3G methods. Calculations have been performed on a reduced model, in which the ribose moiety has been substituted by a hydroxy-methyl group. Results indicate that adenosine is mainly protonated at the N1 atom, whereas formycin can be protonated on N1 or N3, depending on the tautomeric form (N8-H or N7-H). The quantum chemical study of the N1-protonated molecules shows that a second protonation of adenosine is mainly on the N3 atom, whereas formycin can be protonated on N8 or N3, depending on the tautomeric form. On the other hand, results indicate that the protonation of formycin and its derivatives at the N1 atom leads to a change in their tautomeric preference from N7-H to N8-H. The importance of both tautomerism and protonation reactions in the mechanism of action of adenosine deaminase is studied by means of a quantitative structure activity relationships strategy. Significant correlations were found between several electronic parameters and the logarithm of the maximum rate of deamination (log Vm) of the studied compounds. For formycin and its derivatives, it was necessary to consider their N8-H tautomeric forms. The electronic parameters giving good correlations were as follows: energy of the minimum of the ab initio molecular electrostatic potential on N1, net charge over purine (pyrazolo-pyrimidine) and pyrimidine rings, and the N1 protonation energy. It must be noted that all these parameters are informative in relation to a proton attack. Adenosine and purine ribosides have been studied largely because of their high biological relevance. They are constituents of nucleic acids, intermediates in secondary metabolism, neuromodulators, and neurohormones. Their analogues have been extensively used because of their wide range of pharmacological effects (1). Formycin A (Fig. 1) is one of the most studied analogues of adenosine. It is a natural product extracted from Nocardia interforma (2) with proven antiviral (3-5), antibiotic (2), immunodepressant (6), antitumor (6), and antimetabolic (5) activities.     

Synthesis and antitumor activities of novel pyrimidine derivatives of 2,3-O,O-dibenzyl-6-deoxy-L-ascorbic acid and 4,5-didehydro-5,6- dideoxy-L-ascorbic acid

The new pyrimidine derivatives of 2,3-O, O-dibenzyl-6-deoxy-L-ascorbic acid (8-10) were synthesized by condensation of uracil and its 5-fluoro- and 5-trifluoromethyl-substituted derivatives with 4-(5,6-epoxypropyl)-2, 3-O,O-dibenzyl-L-ascorbic acid (7), while pyrimidine derivatives of 4,5-didehydro-5,6-dideoxy-L-ascorbic acid (14-17) with free C-2' and C-3' hydroxy groups in the lactone ring were obtained by debenzylation of 11-13 with boron trichloride. Z-Configuration of the C4'=C5' double bond and position of the benzyl group in the lactone ring of 14 were deduced from their (1)H and (13)C NMR spectra and connectivities in COSY, ROESY, and HMBC spectra. The exact stereostructure of 13 was confirmed by its X-ray crystal structure analysis. Of all the compounds in the series, compound 16 containing a 5-fluoro-substituted uracil ring showed the most significant antitumor activities against murine leukemia L1210/0 (IC(50) = 1.4 microg/mL), murine mammary carcinoma FM3A/0 (IC(50) = 0.78 microg/mL), and, to a lesser extent, human T-lymphocyte cells Molt4/C8 (IC(50) = 31.8 microg/mL) and CEM/0 cell lines (IC(50) = 20.9 microg/mL).     

Adenosine kinase inhibitors. 4. 6,8-Disubstituted purine nucleoside derivatives. Synthesis, conformation, and enzyme inhibition

6,8-Disubstituted purine nucleosides were synthesized and evaluated as adenosine kinase inhibitors (AKIs). A method was developed to selectively substitute arylamines for halogens at C6 and C8 which utilizes alkali salts of arylamino anions. Regioselectivity was found to be counterion dependent. Potassium and sodium salts add selectively to C6 of 6-chloro-8-iodo-9-(2,3,5-tris-O-tert-butyldimethylsilyl-beta-d-ribofuranosyl)purine (7a) while lithium salts add to C6 and C8 positions. Differential 6,8-bisarylamin-N,N'-diylpurine nucleosides such as 8-anilin-N-yl-6-indolin-N-yl-9-(beta-d-ribofuranosyl)purine (10b) can be prepared by employing stepwise reactions of potassium and then lithium salts of different arylamino anions followed by fluoride ion-induced desilylation. Other C8-substituted compounds were prepared by way of either C8 lithiation chemistry or palladium cross-coupling reactions. Several of these compounds were potent AKIs (e.g. 10b, AK IC(50) = 0.019 microM) and are more potent than the previous best purine-based AKI 5'-deoxy-5'-aminoadenosine (AK IC(50) = 0.170 microM). AK inhibitory potency was greatest for those compounds with (1)H NMR evidence of a predominant anti glycosyl bond conformation, whereas most analogues adopt a syn conformation because of steric repulsions between the C8 substituent and the ribose group. The inhibitors are proposed to bind in the anti conformation with the hydrophobic C6 and C8 substituents contributing to AK affinity in a manner similar to the C4 and C5 aryl substituents of the potent diaryltubercidin nucleoside inhibitor series.     

NMR spectra of the tautomeric mixture of two forms of 6‐azidopurine ribonucleoside labeled with 15N

6‐Azidopurine nucleoside labeled with ¹⁵N at N‐1 position was synthesized. ¹⁵N NMR spectra, ¹⁵N‐¹H and ¹⁵N‐¹³C coupling constants were measured. Two well‐separated sets of signals for two tautomeric forms were detected. Copyright © 2007 John Wiley & Sons, Ltd.     

Quinolactacins A, B and C: novel quinolone compounds from Penicillium sp. EPF-6. II. Physico-chemical properties and structure elucidation

Three novel quinolone compounds, quinolactacins A (1), B (2) and C (3), have been found from the fermentation broth of Penicillium sp. EPF-6, a fungus isolated from the larvae of mulberry pyralid (Margaronia pyloalis Welker). The molecular formulas of 1, 2 and 3 were determined to be C16H18N2O2, C15H16N2O2 and C16H18N2O3, respectively by FAB-MS and NMR spectral analyses. The structures of these compounds have a novel quinolone skeleton with a gamma-lactam ring consisting of C12H8N2O2 as the common chromophore.     

Synthesis and estrogen receptor binding of 6,7-dihydro-8-phenyl-9-[4-[2-(dimethylamino)ethoxy] phenyl]-5H-benzocycloheptene, a nonisomerizable analogue of tamoxifen. X-ray crystallographic studies

Syntheses of the title compound (5), a novel nonisomerizable antiestrogen containing a seven-membered ring, are described. In one method, 6,7-dihydro-9-(4-methoxyphenyl)-5H-benzocycloheptene was brominated at the 8-position and the bromine displaced by phenylzinc chloride with palladium complex catalysis to introduce the 8-phenyl substituent. Alternatively, benzosuberone was alpha phenylated with tricarbonyl(eta 6-fluorobenzene)chromium (0) and the product treated with the appropriate aryllithium reagent to introduce the 9-aryl group last. The relative binding affinities for estrogen receptors in cell cytosol and whole cells and growth inhibitory activity against the MCF-7 human breast tumor cell line in vitro were for 5 comparable to those of tamoxifen (1) and the corresponding six-membered ring analogue (7). X-ray crystallographic analyses of 10 and 15, which are methoxy derivatives of 5 and 7, show that in some respects 5 bears a closer structural relationship to tamoxifen than does nafoxidine (3) or 7. Thus, the aromatic ring, which is fused in the cyclic analogues, was twisted 64, 45, 20, and 19 degrees out of the plane of the double bond for 1, 10, 3, and 15, respectively. Low-temperature NMR studies indicate that 5 is more rigid than tamoxifen; interconversion between enantiomeric conformers is slow on the NMR time scale at -75 degrees C.     

Detection of C8-(1-hydroxyethyl)guanine in liver RNA and DNA from control and ethanol-treated rats

Alcohol consumption is associated with an increased risk of cancer by mechanisms that remain unknown but have been suggested to involve radical metabolites. We have previously shown that the 1-hydroxyethyl radical produced from ethanol oxidation in vitro is able to alkylate nucleic acids to produce C8-(1-hydroxyethyl)guanine [C8-(1-HE)gua] among other products. To assess if this adduct is produced in vivo, we developed a sensitive HPLC-MS/MS method for its detection and analyzed hydrolysates of liver RNA and DNA from control and ethanol-treated rats. Unexpectedly, C8-(1-HE)gua was found to be present in both RNA and DNA from the liver of control Sprague-Dawley rats, and its levels increased slightly, but not significantly, after an acute ethanol dose (5 g/kg). In rat liver, C8-(1-HE)gua endogenous levels were about 10 times higher in RNA (35 +/- 5/10(7) guanine) than DNA (3.7 +/- 1.1/10(7) guanine). These levels were also found in commercial RNA (calf liver and yeast) and DNA (calf thymus), further indicating the endogenous source of the adduct. DNA basal levels of C8-(1-HE)gua were similar to those reported for other 2C guanine adducts such as N7-(2-hydroxyethyl)guanine and N2-ethyl-2'-deoxyguanosine. We speculate that all of these adducts may be generated from DNA attack by products of basal lipid peroxidation. The higher RNA levels of C8-(1-HE)gua are in agreement with the higher accessibility of RNA and nucleotides to reactive intermediates because they are not as protected or as localized as DNA. Chemical modification of RNA has been receiving increasingly attention as an important event in genotoxic mechanisms. Comparison of RNA basal levels of C8-(1-HE)gua, N7-(2-hydroxyethyl)guanine, and N2-ethyl-2'-deoxyguanosine may provide clues about their endogenous sources and biological significance. Yet, the marginal increase of DNA C8-(1-HE)gua upon ethanol administration argues against this adduct playing a major role in the carcinogenic effects of ethanol.     

Assessing the protonation state of drug molecules: the case of aztreonam

Herein we examine the viability of physicochemical approaches based on standard computational chemistry tools to characterize the structure and energetics of flexible drug molecules with various titratable sites. We focus on the case of the monobactam antibiotic aztreonam, whose structure and physicochemical properties have been ascribed to several tautomeric forms, although it is still unclear which protonation states are responsible for its biological activity. First, we experimentally determined the pKa values for aztreonam over the pH range 0.8-7.0 using both 1H NMR and 13C NMR spectroscopy. Second, we carried out quantum chemical calculations on snapshots extracted from classical molecular dynamics simulations. Various levels of approximation were used in the energy calculations: ONIOM(HF/3-21G*:AMBER) for geometry relaxation, B3LYP/6-31+G** for electronic and electrostatic solvation energies, and molecular mechanics for attractive dispersion energy. The value of the free energy of solvation of a proton was treated as a parameter and chosen to give the best match between calculated and experimental pKa values for small molecules. Overall, this computational scheme can give satisfactory results in the pKa calculations for drug molecules.     

Formation and reactions of N(7)-aminoguanosine and derivatives.

Arylamines are mutagens and carcinogens and are thought to initiate tumors by forming adducts with DNA. The major adducts are C(8)-guanyl, and we have previously suggested a role for guanyl-N(7) intermediates in the formation process. N(7)-Aminoguanosine (Guo) was synthesized and characterized, with the position of the NH(2) at N7 established by two-dimensional rotating frame Overhauser enhancement NMR spectroscopy. In DMF, N(7)-NH(2)Guo formed C(8)-NH(2)Guo and the cyclic product C(8):5'-O-cycloGuo. In aqueous media, these products were formed along with 8-oxo-7,8-dihydroGuo, N(7)-NH(2)guanine, and a product characterized as a purine 8, 9-ring-opened derivative (N-aminoformamidopyrimidine). The rate of aqueous decomposition of N(7)-NH(2)Guo increased with pH, with a t(1/2) of 10 h at pH 7 and a t(1/2) of 2 h at pH 9. The rate of migration of NH(2) from N7 to C8 is fast enough to explain the formation of C(8)-NH(2)Guo from the reaction of 2, 4-dinitrophenoxyamine with Guo but not the formation of C(8)-(arylamino)Guo in the reaction of Guo with aryl hydroxylamine esters; however, the fluorenyl moiety may facilitate the proposed rearrangement by stabilizing an incipient negative charge in the transfer. In the reaction of Guo with N-hydroxy-2-aminofluorene and acetylsalicylic acid, a peak with the mass spectrum expected for N(7)-(2-aminofluorenyl)Guo was detected early in the reaction and was distinguished from C(8)-(2-aminofluorenyl)Guo. NMR experiments with [8-(13)C]Guo also provided some additional support for transient formation of N(7)-(2-aminofluorenyl)Guo. We conclude that a guanyl-N(7) intermediate is reasonable in the reaction of activated arylamines with nucleic acids, although an exact rate of transfer of an N(7)-arylamine group to the C8 position has not yet been quantified. The results provide an explanation for the numerous products associated with modification of DNA by activated arylamines. However, the contribution of "direct" reaction at the guanine C8 atom cannot be excluded.     

Novel N7- and N1-substituted cGMP derivatives are potent activators of cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels, key players in olfactory and visual signal transduction, generate electrical responses to odorant- and light-induced changes in cyclic nucleotide concentration. Previous work suggests that substitutions are tolerated solely at the C8 position on the purine ring of cGMP. Our studies with C8, 2'-OH, and 2-NH2-modified cGMP derivatives support this assertion. To gain further insight into determinants important for CNG channel binding and activation, we targeted previously unexplored positions. Modifications at N7 of 8-SH-cGMP (6) are well tolerated by olfactory and retinal rod CNG channels. Toleration of a very large substituent, a 3400 molecular weight PEG, at either N7 or C8 argues for broad accommodation at these positions in the binding site. Modification at N1 of cGMP reduces the apparent affinity for the channel; however, when combined with 8-parachlorophenylthio derivatization, the resulting cGMP analogue is more potent than cGMP itself. These studies establish the N7 and N1 positions of cGMP as targets for modification in the design of novel CNG channel agonists.     

New dibenz[b, f]oxepins from Cercis chinensis Bunge

Three new dibenz[b,f]oxepins: 6-methoxy-7-methyl-8-hydroxydibenz[b, f]oxepin (1), 1,8-dimethoxy-6-hydroxy-7-methyldibenz[b, f]oxepin (2), and 1-hydroxy-6,8-dimethoxy-7-methyldibenz[b,f]oxepin (3), along with two known dibenz[b,f]oxepins pacharin (4) and bauhiniastatin 4 (5), were isolated from Cercis chinensis Bunge (Leguminosae). Their structures were elucidated on the basis of spectroscopic evidence (EI-MS, UV, IR, (1)H, (13)C and 2D NMR). Compounds 1-5 were isolated from the Cercis genus for the first time.     

Cardiotonic agents. 2. Synthesis and structure-activity relationships in a new class of 6-, 7-, and 8-pyridyl-2(1H)-quinolone derivatives

A series of 6-, 7-, and 8-pyridyl-2(1H)-quinolone derivatives with various quinolone substitutents (CH3, Cl, OH, OCH3) was prepared by arylation of pyridine with quinolone via a diazotized aminoquinolone for positive inotropic activity. Several derivatives, especially those with a pyridyl ring in the 6-position, were from 28 to 50 times more potent on left guinea pig atria than ARL-115 and milrinone used as references. Intrinsic activities of the derivatives were almost equivalent to that of ARL-115. These results indicate that pyridyl-2(1H)-quinolone derivatives are a potent new class of positive inotropic agents.     

Modeling the preferred shapes of polyamine transporter ligands and dihydromotuporamine-C mimics: shovel versus hoe

Preferred conformers generated from motuporamine and anthracene-polyamine derivatives provided insight into the shapes associated with polyamine transporter (PAT) recognition and potentially dihydromotuporamine C (4a) bioactivity. Molecular modeling revealed that N(1)-(anthracen-9-ylmethyl)-3,3-triamine (6a), N(1)-(anthracen-9-ylmethyl)-4,4-triamine (6b), N(1)-(anthracen-9-ylmethyl)-N(1)-ethyl-3,3-triamine (7a), N(1)-(anthracen-9-ylmethyl)-N(1)-ethyl-4,4-triamine (7b), and 4a all preferred a hoe motif. This hoe shape was defined by the all-anti polyamine shaft extending above the relatively flat, appended ring system. The hoe geometry was also inferred by the (1)H NMR spectrum of the free amine of 7a (CDCl(3)), which showed a strong shielding effect of the anthracene ring on the chemical shifts associated with the appended polyamine chain. This shielding effect was found to be independent over a broad concentration range of 7a, which also supported an intramolecular phenomenon. The degree of substitution at the N(1)-position seems to be an important determinant of both the molecular shape preferences and biological activity of anthracenylmethyl-polyamine conjugates.     

The importance of the orientation of the C9 substituent to cannabinoid activity

We have found a correlation between cannabinoid psychopharmacological activity and the orientation of the C9 substituent in one class of cannabinoid derivatives. We report here a study of the active cannabinoids delta 9-tetra-hydrocannabinol (delta 9-THC), delta 8-tetrahydrocannabinol (delta 8-THC), and 11 beta-hexahydrocannabinol (11 beta-HHC); the minimally active cannabinoid 11 alpha-hexahydrocannabinol (11 alpha-HHC); and the inactive cannabinoids delta 7-tetrahydrocannabinol (delta 7-THC) and delta 9,11-tetrahydrocannabinol (delta 9,11-THC). Our working hypothesis is that there are two components of cannabinoid structure which confer upon these compounds reactivity characteristics crucial to activity: the directionality of the lone pairs of electrons of the phenyl group hydroxyl oxygen and the orientation of the carbocyclic ring relative to this oxygen. The structures of these six molecules were optimized by using the method of molecular mechanics as encoded in the MMP2(85) program. Other possible minimum-energy conformations of the carbocyclic ring were calculated by driving one torsion angle in this ring by use of the dihedral driver option in MMP2(85). The rotational energy behavior of the phenyl group hydroxyl in each molecule was studied also by using the dihedral driver option in MMP2(85). We found that the carbocyclic ring in 11 alpha-HHC can exist in either a chair or a twist conformation. The carbocyclic ring in delta 9-THC, in delta 8-THC, and in delta 7-THC was found to exist only in a half-chair conformation, while the carbocyclic ring in 11 beta-HHC and in delta 9,11-THC was found to exist only in a chair form. The results of the rotational energy profiles indicated that the minimum-energy positions of the phenyl group hydroxyls are nearly identical in all molecules. These molecules, then, were found to differ only in the conformation of the carbocyclic ring in each. This conformation, in turn, determines the orientation of this ring and its C9 substituent relative to the oxygen of the phenyl group hydroxyl. In order to assess the orientation of the carbocyclic ring with respect to the phenyl group hydroxyl oxygen in each optimized structure, the following nonbonded torsion angles were measured: C10-C10a-C1-O, C8-C7-C1-O, C11-C9-C1-O, and C9-Q-C1-O (where Q is a dummy atom placed midway between C8 and C10).(ABSTRACT TRUNCATED AT 400 WORDS)