Structure dereplication challenges

The study of natural products in plant extracts is an interesting challenge to LC-MS. Generally, the relevant compounds must be detected as minor components in complex mixtures. A combination of LC separation, especially to resolve isomeric stractures, and MS detection is needed. Furthermore, structural information is needed for the identification and dereplication of the unknown plant constituents. Because of the complexity of the sample pretreatment procedures involved in the isolation, MS in most cases is the only applicable spectrometric technique too much of a purified component would be needed for IR and NMR analysis. On-line analysis in relatively erode samples is obligatory for the detection of minor constitnents. When electrospray ionization (ESI) or atmospheric-pressnre chemical ionization (APCl) are applied for analyte ionization, structural information mnst be obtained by application of colhsion-indneed dissociation (CID), either via in-sonree CID or preferably via MS-MS or MS". LC-MS and LC-MS-MS have proved to be extremely snccessful in this area. 

LC/NMR in various combinations with LC/UV-DAD, LC/MS, LC/MSMS, LC/IR, and/or LC/CD has been used in many applications related to the online identification of natural products. In this field, the challenge for hyphenated techniques is important since often the characterization of completely unknown molecules is required in very complex biological matrices. In this case, LC hyphenated techniques are used for the chemical evaluation of biologically active fractions or extracts and for dereplication purposes. As full structure assignment is often needed, all online spectroscopic data are taken into consideration. Most applications are performed in the stop-flow mode and extensive 2D NMR correlation experiments are measured. For unknown online determination the need for data is often mandatory. This type of information can be deduced from HSQC and HMBC indirect measurements and very recently it has been demonstrated that even direct measurements were possible in a crude plant extract. For this application the LC peak of interest was preconcentrated by trapping on SPE and the measurement was performed on a cryogenic flow... 

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