The main nonpolar chlorophyll c from Emiliania huxleyi (prymnesiophyceae) is a chlorophyll c_2-monogalactosyldiacylglyceride ester: a mass spectrometry study

摘要

The main nonpolar chlorophyll c-like pigment was extracted from Emiliania huxleyi (Lohm.) Hay et Mohler (strain CCMP 370) cultures and isolated by preparative column chromatography and HPLC. The pigment, whose visible spectrum closely resembled that of chlorophyll c_2, was studied by low-resolution fast atom bombardment mass spectrometry, showing a very high mass molecular ion (m/z 1313). The fragment ions, either in the direct spectrum or obtained by tandem mass spectrometry with collision-induced dissociation of the molecular ion, were compatible with the consecutive losses of two fatty acids (14:0 and 18:4), glycerol, and a hexose, leaving a chlorophyll c_2 backbone, suggesting the molecule consists of a chlorophyll c_2 residue linked, via an ester bond, to the sugar moiety of a monohexosyldiacylglycerol. The identities of the two fatty acid residues (14:0 and 18:4n-3) were subsequently corroborated by gas chromatography of the corresponding methyl esters. Chemical hydrolysis-derivatization-gas chromatography-mass spectrometry demonstrated the occurrence of glycerol and that galactose is the constituent sugar. The porphyrin obtained on acid hydrolysis showed chromatographic and visible spectral properties identical to pheoporphyrin c_2. This evidence led us to propose a tentative structure whose molecular formula, C_(76)H_(96)O_(14)N_4Mg, was supported by the values of exact mass measurements by high-resolution fast atom bombardment mass spectrometry. This novel structure represents the highest molecular weight natural chlorophyll described to date.