Phytochemical analysis of the southern Australian marine alga, Plocamium mertensii using HPLC-NMR

Introduction: Over the last decade HPLC-NMR has become a robust analytical technique that has been applied to a wide range of studies, particularly plant extracts. There have been only a few applications of the use of HPLC-NMR to profile marine natural product extracts and no studies involving marine algae. The marine alga selected for this study belongs to the genus Plocamium, which is a well known source of polyhalogenated monoterpenes. Objective: To chemically profile the marine alga P. mertensii, using a combination of on-line (HPLC-NMR) and off-line approaches. Methodology: P. mertensii was extracted with 3:1 methanol-dichloromethane and subsequently partitioned into dichloromethane and methanol-soluble fractions. The methanol partition was exclusively profiled by HPLC-NMR (on-flow, stop-flow and time-slice experiments) whilst the dichloromethane partition was investigated by conventional isolation and purification procedures. Results: HPLC-NMR analysis of the methanol partition partially identified the presence of the major compounds 7, 13, 27 and 37, the structures of which were unequivocally elucidated by off-line characterisation of the dichloromethane partition. Two additional minor metabolites (3 and 8) present in the dichloromethane partition were only tentatively identified as these co-occurred in a mixture with compounds 7 and 13. As a result of this study a number of chemical shift reassignments were made for compound 37. Conclusion: This is one of the few reports describing the application of HPLC-NMR to rapidly proftle or dereplicate a marine organism and the first application of HPLC-NMR to successfully profile the chemistry of a marine alga.