Diterpenoids from Gutierrezia sarothrae and G. microcephala: chemical diversity, chemophenetics and implications to toxicity in grazing livestock

Authors: Gardner, Dale; Cook, Daniel; Larsen, Scott; Stonecipher, Clinton - Clint; JOHNSON, ROBERT - Brigham Young University

Submitted to: Phytochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2020
Publication Date: 10/1/2020
Citation: Gardner, D.R., Cook, D., Larsen, S.W., Stonecipher, C.A., Johnson, R. 2020. Diterpenoids from Gutierrezia sarothrae and G. microcephala: chemical diversity, chemophenetics and implications to toxicity in grazing livestock. Phytochemistry. 178. https://doi.org/10.1016/j.phytochem.2020.112465.
DOI: https://doi.org/10.1016/j.phytochem.2020.112465

Interpretive Summary: The snakeweeds, which include the two primary plants known as broom snakeweed (Gutierrezia sarothrae) and threadleaf snakeweed (G. microcephala), are perennial plants found in many parts of western US rangelands and are of interest because of possible toxicity in grazing livestock. The toxic components however are not known but several potential toxic compounds have been suggested including a class of compounds known as diterpene acids. Reported here is the chemical analysis of one hundred sixty-two samples of snakeweed which were collected from 55 locations in Colorado, Oklahoma, New Mexico, Texas, Wyoming and Utah. Plants from each collection were classified taxonomically from pressed samples. Samples were chemically analyzed to determine the diterpene acid content of each collection and then grouped into what is termed as chemotypes or a chemical fingerprint based on this chemical diterpene acid content. The chemical fingerprints were found to be diverse showing at least 8 different chemotypes. From each of the chemotypes the major diterpenoid acids were isolated and characterized as to their complete chemical structure. In all, twenty-one diterpenoids were identified. Only four of the 21 compounds isolated had been previously reported as occurring in snakeweeds species G. sarothrae or G. microcephala, while another 8 compounds were previously reported from other related species. Nine of the isolated diterpene acids were found to not have been previously reported and their complete structure elucidation is reported. The finding of several different chemotypes within the snakeweeds helps to explain the possible inconsistencies in toxic effects on grazing livestock reported from the field.

Technical Abstract: Broom snakeweed (Gutierrezia sarothrae) and threadleaf snakeweed (G. microcephala) are suffrutescent plants found in many parts of western US rangelands and are possibly toxic to grazing livestock. The toxic components are not known, but it has been suggested that the diterpenoid acids may be both toxic and abortifacient. One hundred sixty-two samples of snakeweed were collected from 55 locations in Colorado, Oklahoma, New Mexico, Texas, Wyoming and Utah and were taxonomically classified. Samples were analyzed by GC-MS in a chemophenetic analysis and grouped into individual chemotypes based on diterpene acid content. The GC-MS profiles were found to be diverse showing at least 8 different chemotypes. From each of the chemotypes the major diterpenoid acids were isolated and characterized by IR, MS and NMR spectroscopy. Twenty-one diterpenoids were identified and found to be a mix of furano, lactone, di-acid and esters of labdane, ent-labdane and chlerodane acids and alcohols. Only four of the 21 compounds isolated had been previously reported as occurring in G. sarathorea or G. microcephala, while another 8 compounds were previously reported from other Gutierrezia or related species. Nine of the isolated diterpenoids were found to not have been previously reported and their structure elucidation is reported.