Technical note: epimerization of ergopeptine alkaloids in organic and aqueous solvents.

Abstract

Purified ergopeptine alkaloids are often used in studies related to tall fescue toxicosis without regard to epimerization that occurs when ergopeptines are solvated. The objectives of this study were to measure the rates of alpha-ergocryptine epimerization to alpha-ergocryptinine at room temperature and at -40 degrees C, and to measure the rate of ergovaline epimerization to ergovalinine at 37 degrees C. Alpha-ergocryptine tartrate was stable (< 0.5% epimerization) in protic or aprotic solvents when stored at -40 degrees C for 20 to 52 d. At room temperature, alpha-ergocryptine epimerization in chloroform did not occur; epimerization was modest in acetone and acetonitrile (< 5%) but was substantial in methanol (78% by 38 d) and in a 70:30 water methanol mix (47% by 42 d). Ergovaline epimerization to ergovalinine occurred at 37 degrees C in 0.1 M phosphate buffers (pH 3, 7.5, and 9) in 9% aqueous solutions of fetal bovine serum (FBS), and in water, methanol, and acetonitrile. The degree of epimerization at 37 degrees C was solvent-dependent. Epimerization rates with respect to time were roughly linear in phosphate buffer (pH 3 only), water, methanol, and acetonitrile; epimerization rates resembled first-order kinetics in phosphate buffers (pH 7.5 and 9) and in the presence of FBS (pH 3, 7.5 and in Dulbecco's culture media). Epimerization equilibria (48 to 63% ergovaline) were reached within approximately 1 to 19 h. Results from this study indicate that researchers conducting studies with purified ergopeptines should carefully control the storage conditions of solvated ergopeptines and measure isomeric composition under the actual experimental conditions used in experiments.