Structural Mass Spectrometry: Rapid Methods for Separation and Analysis of Peptide Natural Products (original) (raw)

2012, Journal of Natural Products

A significant challenge in natural product discovery is the initial discrimination of discrete secondary metabolites alongside functionally similar primary metabolic cellular components within complex biological samples. A property that has yet to be fully exploited for natural product identification and characterization is the gas phase collision cross section, or, more generally, the mobility-mass correlation. Peptide natural products possess many of the properties that distinguish natural products as they are frequently characterized by a high degree of intramolecular bonding, and possess extended and compact conformations among other structural modifications. This report describes a rapid structural mass spectrometry technique based on ion mobility-mass spectrometry for the comparison of peptide natural products to their primary metabolic congeners using mobility-mass correlation. This property is empirically determined using ion mobility-mass spectrometry, applied to the analysis of linear versus modified peptides, and used to discriminate peptide natural products in a crude microbial extract. Complementary computational approaches are utilized to understand the structural basis for the separation of primary metabolism derived linear peptides from secondary metabolite cyclic and modified cyclic species. These findings provide a platform for enhancing the identification of secondary metabolic peptides with distinct mobility-mass ratios within complex biological samples. The diverse activities of peptide natural products are partially a function of their unique structural attributes, which are defined by incorporation of nonproteinogenic amino acids and by extensive 'post-translational' modifications including macrocyclization, heterocyclization, and oxidation/elimination reactions. These modifications are responsible for the diversity of reported cyclic and heterocyclic scaffolds. Additionally, conformational restraints introduced through these modifications have been implicated in the biological activity of these secondary metabolites, as the entropic loss is less significant upon active site binding compared to non-constrained analogues. 1 Some microbial peptide secondary metabolites with medical relevance include cyclosporin (immunosuppressive), bialaphos (herbicide), vancomycin and penicillin (antibiotic), to name a few, and their activities have inspired the continuing search for new peptide natural products from microbial sources. This search has been reinvigorated in recent years by the discovery of a large reservoir of cryptic peptide gene clusters, both ribosomally encoded and nonribosomally encoded, in microbial