The coordination chemistry of the iron(II) complex of the antitumor drug bleomycin has been extensively investigated with a number of spectroscopic and chemical techniques. However, the actual structure of this complex is not established. In this report, we present NMR studies of the paramagnetic Fe(II)BLM and use one- and two-dimensional methods to assign the paramagnetically shifted features to particular protons. The data analysis points toward the primary and secondary amines of the beta-aminoalanine fragment, the pyrimidine and imidazole rings, and the amide nitrogen of the beta-hydroxyhistidine fragment as ligands to the metal center. Correlation of the T1 values with the metal-proton distances derived from the NMR-generated solution structure of HOO-Co(III)BLM [Wu, W., Vanderwall, D. E., Lui, S. M., Tang, X.-J., Turner, C. J., Kozarich, J. W., & Stubbe, J. (1996) J. Am. Chem. Soc. 118, 1268-1280] indicates that the two metallobleomycins share similar structures. The chemical shifts as well as the T1 values of the sugar protons indicate that these fragments are close but not bound to the metal in Fe(II)BLM.

Magnetic dipole couplings between pairs of protons in a series of n-alkanes (hexane through decane) dissolved in a nematic liquid crystal were measured by two-dimensional N M R spectroscopy. The dependence of dipole couplings on the distance between pairs of protons and on the orientation of the molecule with respect to the applied magnetic field make them a potentially rich source of information about molecular conformation and orientation. Interpretation of the dipole couplings, however, is complicated by the fact that the measured couplings are averaged over all molecular conformations and orientations sampled by the molecule. Thus, the couplings do not directly provide internuclear distances and orientations, but instead provide constraints on the time-averaged conformations and orientations of the n-alkane molecules that can be used as a rigorous test of models for intermolecular interactions in liquid crystals. Three models for solute-liquid crystal interactions were examined; the model proposed by Photinos et a1.l was found to be superior to the others in describing this system. The results of the modeling, including order parameters and conformer probabilities for the alkanes, are discussed and compared to n-alkanes in the liquid state.

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The orientation and motion of tetrahydrofuran (THF) in the ternary graphite intercalation compounds Cs-(THF)' .3C24 and K(THF)&24 have been studied by proton NMR. Simulations of the NMR spectra indicate that the THF molecules in C S ( T H F ) I .~C~~ have their mean planes oriented parallel to the layers of the host lattice, while the THF molecules in K(THF)2&4 have their mean planes oriented at an angle between 50" and 75" from the graphite layers. The proton NMR spectra of both compounds show evidence that the THF molecules rotate about the normal to the graphite layers and confirm X-ray diffraction studies showing a degree of orientational disorder in the samples, corresponding to a mosaic spread in the graphite layer orientation. The conformation of the intercalated THF was studied by simulating the experimental NMR spectra using models for the conformational motion of THF. Simulations indicate that the conformation of intercalated THF is different than gas or liquid phase THF, which has been found to have a ring puckering amplitude of 0.38-0.44 A and to undergo nearly free pseudorotation through a series of conformations.

Motionally averaged proton-proton dipolar couplings measured by nuclear magnetic resonance (NMR) spectroscopy can provide information about the conformations and orientations sampled by partially oriented molecules. In this study, the measured dipolar couplings between pairs of protons on n-hexane dissolved in a nematic liquid crystal solvent are used as constraints in a Monte Carlo sampling of the conformations and orientations of n-hexane. Rotation about each carbon-carbon bond in the molecule is modeled by the complete sinusoidal torsional potential of Ryckaert and Bellemans rather than by the three-state rotational isomeric states (RIS) model that has been used in previous studies. Comparison of the results of the simulations using the Ryckaert-Bellemans potential and the RIS model indicates little difference in the values of the adjustable parameters and the quality of the fits to the experimental data. The primary difference between the models appears in the calculated conformer probability distributions for n-hexane, highlighting the importance of the exact shape of the torsional potential used to model carbon-carbon bond rotation in organic molecules.

NMR has proven to be a rich source of inforraation about molecular structure in condensed phases. A particularly good example of this is magnetic dipole couplings between protons on partially-oriented molecules, These couplings are very sensitive to the distance between the coupled protons and to the orientation of their internuclear vector with respect to the applied magnetic field. This spatial dependence gives proton dipole couplings the potential to be excellent probes of molecular conformation and orientation. This potential is usually not fully realized, since couplings between protons on molecules which undergo rapid conformational changes are averaged over all molecular motions which occur. The couplings, however, can provide detailed constraints on the time-averaged conformation of a molecule that. can be used as a rigorous test of models for molecular interactions. NMR experiments to measure proton dipole couplings were performed on a series of n-alkanes (n-hexane through n-decane) dissolved in nematic liquid crystals. Computer modeling of the experimental NMR spectra was done using several different models for intermolecular interactions in these systems. The model of Photinos, 1 i ,! et al. [1] was found to be best in describing the intermolecular interactions in these systems and can provide a statistical picture of the conformation and orientation of the allene molecules in their partially-oriented environment. Order parameters and conformational distributions for the alkanes can be calculated from the modeling. The alkanes are found to have conformational distributions very much like those t found in liquid alkanes. Proton NMR spectra of tetrahydrofuran (THF) intercalated in two graphite intercalation compounds were also measured. Computer simulations of these spectra provide a picture of THF in the constrained environment between the graphene layers where the THF is oriented at a part, icular angle, can translate and rotate freely, but does not appear to pseudorotate.

No abstract

NMR has proven to be a rich source of inforraation about molecular structure in condensed phases. A particularly good example of this is magnetic dipole couplings between protons on partially-oriented molecules, These couplings are very sensitive to the distance between the coupled protons and to the orientation of their internuclear vector with respect to the applied magnetic field. This spatial dependence gives proton dipole couplings the potential to be excellent probes of molecular conformation and orientation. This potential is usually not fully realized, since couplings between protons on molecules which undergo rapid conformational changes are averaged over all molecular motions which occur. The couplings, however, can provide detailed constraints on the time-averaged conformation of a molecule that. can be used as a rigorous test of models for molecular interactions. NMR experiments to measure proton dipole couplings were performed on a series of n-alkanes (n-hexane through n-decane) dissolved in nematic liquid crystals. Computer modeling of the experimental NMR spectra was done using several different models for intermolecular interactions in these systems. The model of Photinos, 1 i ,! et al. [1] was found to be best in describing the intermolecular interactions in these systems and can provide a statistical picture of the conformation and orientation of the allene molecules in their partially-oriented environment. Order parameters and conformational distributions for the alkanes can be calculated from the modeling. The alkanes are found to have conformational distributions very much like those t found in liquid alkanes. Proton NMR spectra of tetrahydrofuran (THF) intercalated in two graphite intercalation compounds were also measured. Computer simulations of these spectra provide a picture of THF in the constrained environment between the graphene layers where the THF is oriented at a part, icular angle, can translate and rotate freely, but does not appear to pseudorotate.