Investigation of chemical structure of nonprotein proteinase inhibitors from dried figs

Two types of the nonprotein proteinase inhibitors, P and B samples were previously isolated from dried figs by gel permeation and high-performance liquid chromatography. In the present study, we investigated the chemical structure of the B sample using both mass spectrometry (MS) and solid-state nuclear magnetic resonance. Fast atom bombardment and matrix-assisted laser desorption ionization-time of flight MS experiments showed losses of 16, 44, 176, and 338 Da. By electrospray ionization tandem MS, the product-ion spectra of m/z 712, 731, and 750 in the parent ions indicated that the fragmentation ion at m/z 175 was the repeated structure of the parent ions. We propose here that the 176- and 338-Da compounds might aggregate each other, probably by polymerizations, ether linkages, and hydrogen bonding, which would result in a mass variety of the B sample as observed in the MS analyses.