Calicheamicin gamma 1I: an antitumor antibiotic that cleaves double-stranded DNA site specifically.

Calicheamicin gamma 1I is a recently discovered diyne-ene--containing antitumor antibiotic with considerable potency against murine tumors. In vitro, this drug interacts with double-helical DNA in the minor groove and causes site-specific double-stranded cleavage. It is proposed that the observed cleavage specificity is a result of a unique fit of the drug and DNA followed by the generation of a nondiffusible 1,4-dehydrobenzene--diradical species that initiates oxidative strand scission by hydrogen abstraction on the deoxyribose ring. The ability of calicheamicin gamma 1I to cause double-strand cuts at very low concentrations may account for its potent antitumor activity.

Applications of fast atom bombardment mass spectrometry and fast atom bombardment mass spectrometry-mass spectrometry to the maduramicins and other polyether antibiotics.

Fast atom bombardment mass spectrometry (FAB MS) and fast atom bombardment mass spectrometry-mass spectrometry (FAB MS/MS) were used to study the monovalent glycoside polyether antibiotics maduramicin alpha, beta and delta and the maduramicin alpha salts, their derivatives and degradation products. Also, representative compounds from three major classes of polyether antibiotics were studied: the monovalent polyethers, nigericin and monensin A, the divalent polyether lasalocid A and the monovalent glycoside polyethers septamycin, BL580 delta, etheromycin and carriomycin. The respective FAB fragment and decomposition ions were correlated with the known structures. The FAB spectra of all the polyethers contained metal-adduct molecular ions. Protonated molecular ions were absent. All the polyethers having a beta-hemiketal carboxylic acid group produced an abundant ion, often the base peak of the spectra, 62 daltons less than the corresponding metal-adduct molecular ion. The gas phase mechanism proposed for the formation of this fragment ion is an unusual unimolecular reaction which is initiated by an intramolecular proton transfer from the carboxylic acid to the hydroxy group of the beta-hemiketal, and, then followed by the concerted losses of water and carbon dioxide to produce the corresponding polyether olefin.

Chemistry of maduramicin. II. Decarboxylation, abnormal ketalization and dehydration.

The behavior of the free acid and ammonium salt of maduramicin towards heat and alcohols is examined. In refluxing lower alcohols the free acid material is decarboxylated. In addition a bisketal decarboxylated compound as well as an A-ring monoketal decarboxylated derivative are formed. Heating the ammonium salt of the ionophores in suspension in water, or dissolved in inert solvents such as heptane or xylene can cause decarboxylation as well as concomitant dehydration of the F-ring. Reaction of dansyl chloride with the free acid of maduramicin can cause dehydration of the B-ring under very mild conditions.

Chemistry of maduramicin. I. Salt formation and normal ketalization.

The formation of monovalent and divalent salt complexes of maduramicin is described and the 13C NMR chemical shift assignments of these materials are tabulated and discussed. In the spectrum of the thallium salt, 20 of the 47 signals are split due to the thallium-carbon coupling. Similarly the preparation of both the free acid and the sodium salt complexes of the normal methyl and ethyl ketal derivatives of maduramicin are outlined. Their 13C NMR spectra are fully assigned together with discussion of displacements observed due to this derivatization.

Nuclear magnetic resonance studies on the antibiotic avoparcin, Conformational properties in relation to antibacterial activity.

Avoparcin is a commercially important glycopeptide antibiotic which is active against Gram-positive bacteria. Recently, beta-avoparcin, the major component of the avoparcin mixture, and several other analogues have been structurally characterized and tested for their antibacterial activity. Varying degrees of activity were observed for only minor structural differences. For example, beta-avoparcin and epi-beta-avoparcin, which differ only in the stereochemistry at position 1' of the NH2-terminal phenylsarcosine subunit, exhibit a 10- to 100-fold difference in antibacterial activity. Following the supposition that the conformational properties of these molecules may explain the differences in their antibacterial activity, we have analyzed the conformations of beta-avoparcin, epi-beta-avoparcin, and other avoparcin analogues in aqueous solutions using NMR methods. On the basis of an analysis of the 1H chemical shifts, 1H-1H nuclear Overhauser enhancement data, and pH titration experiments, it was concluded that the conformations of all of the analogues are similar at the COOH terminus. However, for beta-avoparcin and epi-beta-avoparcin, conformational differences were observed in the NH2-terminal region of the molecules. In this paper, we present a detailed description of the over-all conformations of these two glycopeptides as deduced from an in-depth analysis of their corresponding 1H NMR spectra and discuss the possible relationships this may have with their markedly different antibacterial activities.

Nuclear magnetic resonance studies on the interaction of avoparcin with model receptors of bacterial cell walls.

On the basis of nuclear Overhauser enhancement and 1H chemical shift data obtained in aqueous solution, a model is proposed for the interaction of beta-avoparcin and epi-beta-avoparcin with acetyl-D-alanyl-D-alanine (Ac-D-Ala-D-Ala) and diacetyl-L-lysyl-D-alanyl-D-alanine (Ac2-L-Lys-D-Ala-D-Ala). For the beta-avoparcin: Ac2-L-Lys-D-Ala-Ala complex, the COOH-terminal end of the tripeptide is located near the NH2 terminus of the antibiotic with the tripeptide extending across the peptide backbone of beta-avoparcin toward its COOH-terminal end. In our proposed structure, the three amino acid residues of the peptide span the entire length of the antibiotic, and the aliphatic side chain of the lysine residue extends over the D-ring of beta-avoparcin. The structure of the epi-beta-avoparcin:Ac2-L-Lys-D-Ala-D-Ala complex was found to be similar to the beta-avoparcin complex at the binding site for the lysine residue at the COOH-terminal end of the antibiotic, but differed in the interactions at the NH2 terminus. These results are consistent with the similarities in the COOH-terminal conformations and the differences in conformations at the NH2 terminus found for beta-avoparcin and epi-beta-avoparcin which were described in the preceding paper [Fesik, S. W., I. M. Armitage, G. A. Ellestad, and W. J. McGahren, Mol. Pharmacol. 25:275-280 (1984)]. The association constants (measured by UV methods) for both beta-avoparcin:peptide complexes were greater than those measured for epi-beta-avoparcin and correlated with their differences in antibacterial activity. Epi-beta-avoparcin exhibited no measurable binding to the dipeptide: however, a significant affinity was measured for the tripeptide, indicating that the interactions with the NH2 terminus of the antibiotics provide binding energy for the antibiotic peptide complex but that the COOH-terminal end of the antibiotics also plays an important role in the binding interaction. These results are interesting in light of the similarities in the structural and conformational features at the COOH terminus for all of the glycopeptide antibiotics.

Components and degradation compounds of the avoparcin complex.

The isolation and characterization of many of the components of the avoparcin complex are described. A number of mild degradations products from this complex are similarly treated. Conditions for the analytical and preparative HPLC resolution of these materials are outlined.

Epimerization and stereochemistry of avoparcin.

The epimerization of avoparcin entities is discussed in some detail. The absolute stereochemistry of avoparcin is now known since the N-methyl terminal amino acid of the aglycone has been isolated and shown to exhibit negative optical rotation and hence has the R-configuration. The same amino acid has been isolated from an epimerized solution of avoparcin and found to have positive rotation and hence the S-configuration. A comparison is made of the CD curves of beta-avoparcin and epi-beta-avoparcin. Some discussion on the effect of protonation of the terminal N-methyl group on the antibacterial activity of avoparcin is included.