Tracking the deprotonation site of dehydroandrographolide with electrospray ionization mass spectrometry by deuteration

Dehydroandrographolide (DA) is a diterpene compound of biological interest that contains one α,β-unsaturated lactone group and two hydroxy groups. In the ESI (electrospray ionization) negative ion mode mass spectral analysis of 15-dideuterodehydroandrographolide (15-D2 -DA), the deuterium nucleus at the γ position of the α,β-unsaturated lactone was more easily dedeuterated than deprotonation of the protons from the hydroxy groups. Exploring the rationality of deuteration as a tool for deprotonation position tracking is significant for gas-phase acidity.The mass spectra of DA and 15-D2 -DA in positive and negative ion mode were acquired by liquid chromatography/ion trap time-of-flight mass spectrometry (LC/IT-TOF) systems. The deprotonation and dedeuteration energies at specific sites were calculated by the B3LYP and M06-2X density functional theory (DFT) methods with the program Gaussian 16.The [M + H]+ ion of 15-D2 -DA was 2 amu larger than that of DA due to the substitution of two hydrogens with two deuteriums; however, the anion base peak of 15-D2 -DA was only 1 amu larger than that of the [M - H]- ion of DA. Dedeuteration at the C15 site was proposed according to the mass spectral data. The deprotonation (dedeuteration) energies calculated by the B3LYP/6-311++G(3df,3pd)//B3LYP/6-31 + G(d) and M06-2X-D3/ma-TZVP methods showed that the C-H and C-D bonds at the C15 site have lower deprotonation (dedeuteration) energies than the energies of the hydroxy groups of DA, making their deprotonation (dedeuteration) more thermodynamically favourable.Deuteration of DA provided direct evidence of the deprotonation site of DA in the ESI source of the mass spectrometer, and the DFT method well predicted the gas-phase deprotonation site of DA.