Ab initio studies of the reaction of hydrogen transfer from DNA to the calicheamicinone diradical.

BACKGROUND: The biological activity of enediyne chemotherapeutic (anti-cancer) agents is attributed to their ability to cleave duplex DNA. Part of the reaction of cleavage is the abstraction of hydrogens from the deoxyribose moiety of DNA by the biradical formed via a Bergman rearrangement. METHODS: The mechanism of the reaction of abstraction of two hydrogen atoms from two deoxyribophosphate molecules by the calicheamicinone biradical is studied with ab initio calculations at Hartree-Fock and post-Hartree-Fock level. The Titan program is used to perform the calculations. RESULTS: It is found that the reactions are exothermic and thus thermodynamically reasonable. CONCLUSIONS: The mechanism of DNA cleavage by the enediyne-containing drugs is likely to proceed by the abstraction of the hydrogens from deoxyribose by the biradical formed by the drug. Further studies should determine in which way the modification of the drug's structure would make this reaction even more exothermic and, thus, more likely to occur.

NMR methods for determination of enantiomeric excess.

With the dramatic recent growth in importance of asymmetric syntheses, new applications or approaches for analyses of enantiomeric excess (% e.e.) of samples continue to become more essential. Nuclear magnetic resonance (NMR) spectroscopy provides a wide range of powerful methods which are complementary to chromatographic or electrophoretic-based approaches. This present review focuses on representative and selective recent examples (through mid-1999) of English language reports on NMR methods for % e.e. determination.

Ab initio calculations on (-)-calicheamicinone and some of its reactions involved in its activation and interaction with DNA.

Ab initio calculations were performed on (-)-calicheamicinone, and on the product (Z or E) of the Michael addition via a reaction with methanethiol. It is found that the sulfur moiety position versus the rest of the molecule is quite flexible. The Michael adduct featuring the carbamate group E to the sulfur moiety is more stable than the Z isomer. The Bergman reaction of the diradical formation is strongly exothermic.

Semi-empirical, ab initio and molecular modeling studies on the DNA binding of a calicheamicinone-polyamide conjugate.

AM1 semi-empirical and ab initio calculations were performed on certain synthetic polyamide conjugates of the aglycone of the minor groove binding antibiotic calicheamicin. Geometry optimized conformations and heats of formation were obtained. The binding of the optimized conformations of the drug to both alternating and non-alternating (AT)n and to (G)n x (C)n sequences were studied and the energies of binding were compared to each other. The results can be utilized in the design of novel enediyne-based drugs.

1H NMR studies of drugs with achiral and chiral lanthanide shift reagents: applications to the anticonvulsant pheneturide.

The anticonvulsant pheneturide, PNT, has been studied by 300 MHz 1H NMR in CDCl3 at ambient temperatures with the achiral lanthanide shift reagent (LSR) Eu(FOD)3, and with the chiral LSR, Eu(HFC)3. Both LSRs produced spectral simplification of the aryl proton signal region, and substantial lanthanide-induced shifts (LIS). With added Eu(HFC)3, enantiomeric shift differences (DeltaDeltadelta) were induced for most nuclei of PNT, indicating substantial potential for direct determination of enantiomeric excess. Valley heights between corresponding signals in the PNT enantiomers as low as 3.6% were achieved for the meta resonance. Least squares line-fitting was applied to the variation of chemical shift vs. [LSR]/[PNT] molar ratios for both LSRs. Tentative assignments were made for the NH absorptions based on two-dimensional NMR (COSY45), as well as their relative magnitudes of LIS, DeltaDeltadelta, and lanthanide-induced line broadening. The PNT conformation reported in the crystal is believed to be retained in solution with added LSR. The relative senses of magnetic nonequivalence were found to be the same among the three sets of aryl protons, and among the three kinds of protons in the ethyl moiety, with high levels of added chiral LSR, using 2D NMR.

1H NMR spectral simplification with achiral and chiral lanthanide shift reagents--IV. Thiopental and barbiturate analogues.

The 60 MHz (1)H NMR spectra of racemic thiopental, 1, have been studied with the achiral shift reagent, tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato) europium(III), 2, and the chiral tris[3-(trifluoromethylhydroxymethylene)-d-camphorato] europium(III), 3, and tris[3-(heptafluoropropylhydroxymethylene)-d-camphorato] europium(III), 4. Enantiomeric shift differences, DeltaDeltadelta, were clearly observed for all three methyl signals of 1 with 3 or 4, with larger values obtained using the former reagent. Thus, a 0.216 molal solution of 1 in CDCl(3) at 28 degrees C with a 3:1 molar ratio of 0.359 displayed DeltaDeltadelta values of about 17 Hz for the proximal methyl of the methylbutyl group (at the chiral centre), 13 Hz for the CH(3) of the ethyl group, and 6 Hz for the distal CH(3) of the methylbutyl group. Results are compared for those obtained with 2 and 3 using secobarbital, talbutal, butabarbital and pentobarbital.

Optical purity determination, conformer populations and 1H NMR spectral simplification with lanthanide shift reagents--part IX. A method for improved analytical precision for "DOEt", 2,5-dimethoxy-4-ethylamphetamine.

The 60 MHz (1)H NMR spectra of the potent hallucinogen 2,5-dimethoxy-4-ethylamphetamine ("DOEt"), 1, have been studied in CDCl(3) at 28 degrees with the achiral shift reagent, tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)europium(III), 2, and the chiral reagents tris[3-(trifluoromethylhydroxymethylene)-d-camphorato] europium(III), 3, and tris[3-(heptafluoropropylhydroxymethylene)-d-camphorato] europium(III), 4. Distinct enantiomeric shift differences, DeltaDeltadelta, were observed for the amphetamine CH(3) and the adjacent CH resonances using either 3 or 4 that permit direct optical purity determinations. A novel use of an external computing integrator as an accessory to a basic NMR is described; interfacing these instruments permits improved analytical precision for the reported optical purity determinations using nonracemic mixtures of known compositions. Relative abundances of the different conformers with respect to C(alpha)C(beta) bond rotation in the arylethylamine moiety is discussed based on coupling constants. Results are compared with the related hallucinogen, 3,4-methylenedioxyamphetamine.

Optical purity determination and (1)H-NMR spectral simplification with lanthanide shift reagents - VIII. An indacrinone precursor, 6,7-dichloro-5-methoxy-2-methyl-2-phenyl-1-indanone.

The 60 MHz (1)H NMR spectra of racemic 6,7-dichloro-5-methoxy-2-methyl-2-phenyl-1 indanone (1) have been studied with the achiral shift reagent, tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato) europium (III) (2) and the chiral reagents, tris[3-(trifluoromethylhydroxymethylene)-d-camphorato] europium (III) (3) and tris[3-(heptafluoropropylhydroxymethylene)-d-camphorato] europium (III) (4). While some enantiomeric shift differences, DeltaDeltadelta, were seen for some protons of 1 with 3, dramatically larger values were obtained with 4. The latter would be the reagent of choice for direct optical purity determinations of 1 using the 2-methyl resonance. Optimum conditions would use a 4:1 molar ratio of 0.1-0.25, providing near-baseline resolution for the peaks and freedom from overlap with interfering peaks. Less than 3% of the minor enantiomer should be detectable. The results are consistent with lanthanide complexation at the carbonyl. The value of DeltaDeltadelta for the C(2) methyl of 1.1 ppm with a 4:1 molar ratio of 1.50 appears to be among the highest reported values for simple ketones with 3 or 4.

(1)H NMR spectral simplification with achiral and chiral lanthanide shift reagents. Tranylcypromine, trans-2-phenylcyclopropanamine.

The 60 MHz (1)H NMR spectra of racemic tranylcypromine, 1, have been studied with the achiral shift reagent, tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato) europium(III), 2, and the chiral tris[3-(trifluoromethylhydroxymethylene)-d-camphorato] europium(III), 3. Appreciable values of the enantiomeric shift differences, DeltaDeltadelta, were observed for each cyclopropyl proton except for the proton beta and trans to the amino group (furthest from the expected europium complexation site). The proton alpha to the amino group showed DeltaDeltadelta as high as about 38 Hz for a 3:1 ratio of 0.71; the DeltaDeltadelta decreased at higher 3:1 ratios. Optical purity evaluations should be most practical using this alpha proton with 3:1 ratios near 1.07 to minimize interference due to peak overlap.

Quantitative NMR assay for aspirin, phenacetin, and caffeine mixtures with 1,3,5-trioxane as internal standard.

The method for (1)H NMR determination of aspirin, phenacetin and caffeine (APC) mixtures has been improved by the use of 1,3,5-trioxane as an internal standard. The trioxane absorption occurs in a peak-free region of the spectrum and produces no interferences with any of the analytes. Compared to the original method with caffeine as an external standard, the present method appears to offer better accuracy and precision. Average errors relative to the correct results were: aspirin, 1.0%; phenacetin, 0.8%; and caffeine 1.8%, for known standard mixtures. Coupling constants, (1)J((13)CH), were determined for the methyl groups of aspirin and caffeine and for the trioxane methylene group to clarify potential (13)C satellite interferences.

Optical purity determination and (1)H NMR spectral simplification with lanthanide shift reagents - V. Mephenytoin, 5-ethyl-3-methyl-5-phenyl-2,4-imidazolidinedione.

The 60-MHz 1H NMR spectra of racemic mephenytoin, I, have been studied with the achiral shift reagent, tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octane-dionato) europium (III), II, and the chiral reagent, tris[3-(trifluoromethylhydroxy-methylene)-d-camphorato] europium (III), III. Moderate values of the enantiomeric shift differences, Delta, were clearly observed for the NCH3, NH, aryl and CCH3 resonances in CDCl3 solution at 28 degrees C with added III. The NCH3 and NH absorptions could be useful for direct assays of optical purity. Thus, for a 0.34 molal solution of I in CDCl3 with a molar ratio III:I of 0.138, the value of Delta for the NCH3 resonance was 3.1 Hz (0.052 ppm) with 32% valley resolution.