A complete analysis has been made for each of the high-resolution proton magnetic resonance spectra of naphthalene, anthracene, pyrene, perylene, triphenylene, and coronene at infinite dilution in CS2 or CCl4. A partial analysis has been made for phenanthrene. The analyses yield absolute values for the chemical shifts and spin coupling constants. A correlation has been made between the π-bond order of the particular (C–C) bond and the spin coupling constant. Theoretical values for the ``ring current shifts'' were obtained using London type molecular orbital calculations. The agreement between experimental and theoretical values is satisfactory in that the relative positions of the proton shifts are correctly predicted in all cases.

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Determination of the mechanism of intramolecular nuclear Overhauser effects (NOE) in peptides and depsipeptides is essential to the use of this technique in conformational analysis of these and related biomolecules. Towards this end, 1H NMR double-resonance studies were conducted on valinomycin in (CD3)2SO at 90 MGZ (FT mode) and 250 MGZ (correlation mode). The NOE's are positive at the lower frequency and negative at the higher frequency. Consideration of the theoretical dependence of the NOE on the proton-proton internuclear correlation time and on the resonance frequency indicates that these results are explained by a predominantly dipolar relaxation mechanism. It is demonstrated that exchange modulation of scalar coupling does not contribute significantly to the NOE. A formalism for the NOE's of loosely coupled spin systems is presented which takes into account the effects of high magnetic-field strengths and long correlation times. An approximate analysis of the NOE data assuming a single correlation time for the entire molecule and ignoring cross-relaxation effects was used to evaluate various models that have been proposed for the conformation of valinomycin. The III-1 model of Patel and Tonelli (Patel, D.J., and Tonelli, A.E. (1973), Biochemistry 12, 486) fits the NOE and peptide NHCalphaH coupling constant data and is probably a preferred orientation in dimethyl sulfoxide. These experiments illustrate how intramolecular NOE data provide a valuable auxiliary method to other techniques for delineating the preferred solution conformation of peptides, depsipeptides, and other biomolecules.

A density matrix description of intermolecular nuclear Overhauser experiments in high resolution NMR systems is developed. The formalism is specific for a binary solution of solute A in solvent B. The solute and solvent have homonuclear spin systems with magnetogyric ratios γI and γS, respectively. The steady state intermolecular nuclear Overhauser effect (INOE) experiment, where the solvent spins are saturated and the frequency sweep NMR spectrum of the solute is examined, offers a technique complementary to intramolecular double resonance for the analysis of relaxation in high resolution NMR systems. For the INOE experiment, the density matrix equations may be expressed completely in the spin space of a single solute molecule. The resulting INOE equations are straightforward to use, and explicit solutions are given for the Overhauser enhancement of each line in an AX spin system with internal dipolar relaxation and different intermolecular dipolar interactions at the sites of the A and X spins. Theory is compared with experiment for a dilute sample where the solute is the AX2 spin system, 1,1,2-trichloroethane, and the solvent is tetramethylsilane. Saturation of the tetramethylsilane signal led to 37% enhancement in each of the transitions of the A proton and to 16% enhancement in each of the transitions of the X protons. An analysis of these enhancements allowed the accurate determination of the fraction of internal dipolar interaction in the solute. It is suggested that the INOE technique can be useful for molecular conformation analysis.

Solvent proton nuclear magnetic dispersion studies at 25, 100, and 300 MHz have been performed on protocatechuate 3,4-dioxygenase (PCD) and its complexes with 3-chloro-4-hydroxybenzoate and 3-fluoro-4-hydroxybenzoate. Longitudinal and transverse relaxation rates were measured for these compounds and for the apoenzyme. The paramagnetic enhancement of solvent T1 is interpreted in terms of dominant dipole-dipole relaxation of fast-exchanging solvent protons with a negligible contribution from outer sphere relaxation and an electronic spin relaxation time of 0.5 ns for the high-spin ferric ion. A discrepancy between the observed T2 at 300 MHz and that calculated by assuming the usual dipolar relaxation provides evidence for an additional Curie-spin dipolar or hyperfine interaction between the proton and iron. Quantitation of the additional relaxivity provides an estimated chemical exchange lifetime of 0.1-0.14 microseconds, which suggests proton exchange by a hydroxide ligand. Proton-to-iron distances are 2.7-3.1 A in PCD and lengthen to 3.6-4.1 A in the halohydroxybenzoate complexes.