HR-μMAS NMR-Based Metabolomics: Localized Metabolic Profiling of a Garlic Clove with μg Tissues

The localization of metabolic profiles within a tissue sample is of particular interest when the sampling size is considerably small, i.e., in the order of a microgram (μg) scale. Small sampling size is inevitable when the sample availability is limited, or when different metabolic profiles are suspected in small disparate sample regions. Capitalizing a recently introduced high-resolution micro-MAS probe (HR-μMAS) for its capability of high-quality NMR data acquisition of μg samples, this study explores the localized metabolic NMR profiling of a single garlic clove and compares the methodology and results with the standard HR-MAS. One advantage of HR-μMAS is the feasibility of analyzing homogeneous μg samples within a large heterogeneous tissue. As a result, the sampling mass (<0.5 mg) allows to selectively profile four homogeneous anatomical garlic regions by HR-μMAS (skin, flesh, inner epidermis, and sprout), in contrast to three regions (skin, flesh, and core ≡ inner epidermis and sprout) by HR-MAS, with a sampling mass of ca. 8 mg. Discriminant analysis was carried out to identify the significant variables in the different regions. It found a significant presence of fructose in both skin and flesh, while sucrose and glucose are predominant in the core. Among the garlic characteristic sulfur compounds, allicin is dominant in the core and allyl mercaptan in both skin and flesh. Quantification analysis was conducted and demonstrated its potential by quantifying metabolites at the μg-level. This study offers an important basis for designing HR-μMAS NMR-based metabolomics studies that can benefit from profiling μg-scale samples.