Ronald A. Martin
Albert Sklar Chair of Chemistry
Contact Information
Mickle Hall 307
318-869-5221
318-869-5026 (fax)
For more details, see my curriculum vita.
Education
B.A. Chemistry, Oklahoma City University (1963)
Ph.D. Organic Chemistry, University of Oklahoma (1969)
Teaching
Organic Chemistry
Advising
Premedical/Predental
Research with Undergraduates
1. Cardenolides and Pregnanes in the Asclepias Milkweeds
We are examining several of the Asclepias species of milkweeds in order to obtain new steroids belonging to two classes, the cardenolides and pregnanes. Our motivation is twofold - the search for new medicinal compounds and the chemical ecology of milkweeds and insects.
Several plant families produce cardenolides (cardiac glycosides) yet only a few cardenolides, the digitalis drugs, are used medicinally. Today the sole source of the extensively used digitalis drugs is the commercial harvest of flowering foxglove plants. There is considerable interest in finding safer cardiac glycosides than the very toxic digitalis cardiac glycosides such as Lanoxin.
Generally the same plant families yield the chemically related structures, the pregnanes. Although not used medicinally in the United States, some pregnanes have shown antitumor activity in preliminary testing and thus have potential medicinal application.
The majority of the 100+ species of Asclepias milkweeds have not been examined thoroughly for cardenolides although all species examined to date have cardenolides and a unique mixture of such that usually allows chromatographic identification of the species. Only a very few pregnanes have been reported in the milkweeds despite the fact that pregnanes are the theoretical precursors to the cardenolides.
Our research is specifically devoted to the extraction and purification of individual cardenolides and pregnanes from several species of our extensive collection of milkweeds. We rely heavily on chromatographic methods and instrumentation to separate the steroids from other chemical compounds followed by separation of individual cardenolides and pregnanes. Once pure, we determine the chemical structure of the cardenolide or pregnane by spectrometry. From spectra obtained from our department Bruker 300-MHz nuclear magnetic resonance spectrometer we map the position of the carbon and hydrogen atoms in the molecule. If our initial interpretation suggests a previously unreported structure, we obtain mass spectra in order to verify the exact molecular formula that yields precisely the carbon, hydrogen, oxygen atom content.
New cardenolides and pregnanes will be evaluated for initial medicinal activity in collaboration with scientists skilled in these screening methods. However, many of our efforts will yield previously reported cardenolides and pregnanes, yet likely new reports for the species we are studying. Cardiac activity of one new cardenolide has been determined.
2. Chemecology of Milkweeds and Insects
Cardenolides also have an important chemecological role in several plant-insect interactions. The best known example is the uptake and sequestration of the cardenolides by monarch butterflies from their larval food-host plants, the Asclepias milkweeds. Both milkweeds and monarchs utilize the cardenolides in predator protection. We have published several plant-butterfly studies that confirmed a fingerprinting technique for most eastern monarchs. In application, the technique permits the determination of the species of milkweed utilized by the monarch as a caterpillar. The discovery of previously reported and new cardenolides isolated from a species of milkweed adds to our knowledge of the chemecology of cardenolides in milkweeds and monarch butterflies.