Dissertations, Theses, and Capstone Projects

Date of Degree

1990

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Fred Naider

Committee Members

Howard Haubenstock

Ruth Stark

Michelle Broido

Subject Categories

Organic Chemistry

Abstract

Nikkomycins and polyoxins are peptidyl nucleoside antibiotics produced by Streptomyces species. They are potent inhibitors of chitin synthetases from pathogenic fungi, and polyoxins have been used in Japan for many years as agricultural fungicides and for insect control. Since chitin synthetase is not found in mammals and both families of antibiotics are not toxic, these inhibitors are potentially attractive for clinical use. Unfortunately these compounds seem to have only a fungistatic action, and they are not used clinically at present.

A procedure for efficiently obtaining nikkomycins X and Z of very high purity from crude antibiotic is described. These nikkomycins are very intractable to separation by conventional chromatography. Our protocol relies on the use of reversed-phase HPLC with a reactive mobile phase (aqueous sodium bisulfite). A polar bisulfite addition product is formed with nikkomycin X, but not nikkomycin Z, within the column bed. This drastically changes the mobility of the former antibiotic and leads to the full resolution of both compounds. The bisulfite addition compound can be easily reverted to nikkomycin X. Recovery yields are in the order of 65-80%.

Five tripeptide analogs (1-5, Table 7) and four dipeptide analogs (9-12, Table 8) of polyoxins and nikkomycins with expected enhanced toxicity toward C. albicans were synthesized, and their biological properties tested against various strains of this yeast. The dipeptides were designed as double-targeted drugs, able to inhibit chitin synthetase and/or release a toxic amino acid upon hydrolysis. One of the tripeptides, L-leucyl-nikkomycin X (3), behaved as a prodrug, releasing a toxic peptide upon intracellular hydrolysis. None of the synthetic antibiotics proved more toxic than nikkomycin.

The conformation of nikkomycin X in aqueous solution was examined by using nuclear Overhauser effects as well as by pH titration studies in which 1H-nmr shifts were monitored. The antibiotic exhibits a very flexible structure, with coexisting syn and anti conformations of the nucleoside moiety. Conformational shifts of the pentose ring, and also along the C4'-C5' bond, occurred during the titration. The studies in aqueous solution were undertaken as a preamble to studies of the antibiotic bound to chitin synthetase and perhaps to peptide permeases of C. albicans. NOe measurements were carried out on nikkomycin X in the presence of fungal membranes containing active enzymes, in an effort to obtain preliminary conformational information for the bound molecule.

Comments

Digital reproduction from the UMI microform.

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