Novel ionic liquids-based extraction method that preserves molecular structure from cutin

The biopolyester cutin is ubiquitous in land plants, building the polymeric matrix of the plants outermost defensive barrier - the cuticle. Cutin influences many biological processes in planta however due to its complexity and highly branched nature, the native structure remains partially unresolved. Our aim was to define an original workflow for the purification and systematic characterisation of the molecular structure of cutin. To purify cutin we tested the ionic liquids cholinium hexanoate and 1-butyl-3-methyl-imidazolium acetate. The ensuing polymers are highly esterified, amorphous and have the typical monomeric composition as demonstrated by solid state NMR, complemented by spectroscopic (GC-MS), thermal (DSC) and x-ray scattering (WAXS) analyses. A systematic study by solution-state NMR of cryogenically milled cutins extracted from Micro-Tom tomatoes (the wild type and the gpat6 and cus1 mutants) was undertaken. Their molecular structures, relative distribution of ester aliphatics, free acid end-groups and free hydroxyl groups, differentiating between those derived from primary and secondary esters, were solved. The acquired data demonstrate the existence of free hydroxyl groups in cutin and reveal novel insights on how the mutations impact the esterification arrangement of cutin. Compared to conventional approaches, the usage of ionic liquids for the study of plant polyesters opens new avenues since simple modifications can be applied to recover a biopolymer carrying distinct types/degrees of modifications (e.g. preservation of esters or cuticular polysaccharides), which in combination with the solution NMR methodologies developed here, constitutes now essential tools to fingerprint the multi-functionality and the structure of cutin in planta.