1H- and 13C-n.m.r. studies of the antitumour antibiotic luzopeptin. Resonance assignments, conformation and flexibility in solution.

The depsipeptide DNA-intercalating antibiotic luzopeptin was studied in solution by n.m.r. methods. Two-dimensional 1H double-quantum-filtered correlation spectroscopy (DQF-COSY) and nuclear-Overhauser-effect spectroscopy (NOESY) confirm the primary structure and twofold symmetry of luzopeptin and provide details of its three-dimensional conformation in solution. Trans-annular hydrogen bonds between the glycine NH groups and carbonyl oxygen atoms have been identified in the crystalline state [Arnold & Clardy (1981) J. Am. Chem. Soc. 103, 1243-1244], and are important in maintaining an antiparallel beta-sheet conformation. The n.m.r. data indicate that the glycine NH protons are appreciably shielded from the solvent molecules, which suggests that these hydrogen bonds are maintained in solution. The orientation of the quinoline chromophores is defined by two-dimensional NOE cross-peaks that position the N-methyl group of the L-beta-hydroxyvaline residue close in space to both the quinoline H-8 and serine NH proton. This pattern of NOEs is in accord both with the chromophore configuration found in the crystal and one where the quinoline rings are aligned in a parallel manner at right-angles to the depsipeptide ring. The n.m.r. data are consistent with a hydrogen bond between the quinoline hydroxy groups and the quinoline carbonyl oxygen atoms. The pyridazine acetylmethyl groups give NOEs to the C(alpha)H groups of the beta-hydroxy-N-methylvaline residues, showing that the acetyl groups, for at least some of the time, stretch over the depsipeptide ring, occluding one face of the molecule. Both of the latter features are also found in the crystal structure. Resonances in the 13C-n.m.r. spectrum of luzopeptin have been assigned by transferring 1H assignments to their covalently bonded carbon atoms via a heteronuclear shift-correlation experiment (HETCOR). The measurement of spin-lattice relaxation times and 1H-13C NOEs at specific sites in the molecule has led us to conclude that segmental motions within the depsipeptide ring are restricted and that the 13C relaxation data for luzopeptin's protonated carbon atoms are adequately described by isotropic tumbling in solution. Furthermore, relaxation data for the carbon atoms of the quinoline chromophores show that these rings exhibit similar motion to the depsipeptide ring and are not rotating rapidly with respect to it. Taken together all the data imply that luzopeptin is fairly rigid in solution, on the time scale of molecular tumbling, and has, or can readily attain, a staple-like structure suitable for bisintercalation.(ABSTRACT TRUNCATED AT 400 WORDS)