Conformer interconversion in the LC analysis of triostin A and its under-N-methylated synthetic analogue.

Multiple peak formation and interconversion in the liquid chromatographic (LC) analysis of triostin A and its under-N-methylated synthetic analogue, [N-MeCys3,N-MeCys7]-TANDEM (MCTANDEM), are investigated as a function of column temperature. Slow interconversion between chromatographic peaks, ascribed to the presence of the n- and p-solution conformers of the peptides, is exhibited in the normal-phase elution profiles of triostin A and in the reversed-phase elution profiles of MCTANDEM. A chromatographic model is developed to estimate the kinetics of conformer interconversion. Reversed-phase LC analysis of the n- and p-conformers of MCTANDEM yields a value of 0.01/s for the apparent interconversion rate constant (kn-p) at 25 degrees C, with a corresponding activation energy of 16 kcal/mol. Normal-phase LC analysis of the n- and p-conformer interconversion of triostin A dissolved in chloroform results in a value of 0.04/s for kn-p at 25 degrees C, with a corresponding activation energy of 18 kcal/mol. For triostin A, normal-phase LC findings as a function of column temperature are compared with 1H nuclear magnetic resonance (NMR) line-width measurements between 80 degrees C and 140 degrees C for the n- and p-conformers, which yield an activation energy of 19 kcal/mol and an extrapolated value of 0.02/s at 25 degrees C for kn-p in deuteriochloroform.