Abstract
Starch is the most abundant energy storage molecule in plants and is an essential part of the human diet. This glucose polymer is composed of amorphous and crystalline domains in different forms (A and B types) with specific physicochemical properties that determine its bioavailability for an organism, as well as its value in the food industry. Using two-dimensional (2D) high resolution solid-state nuclear magnetic resonance (SS-NMR) on 13C-labelled starches that were obtained from Chlamydomonas reinhardtii microalgae, we established a complete and unambiguous assignment for starch and its constituents (amylopectin and amylose) in the two crystalline forms and in the amorphous state. We also assigned so far unreported non-reducing end groups and assessed starch chain length, crystallinity and amylose content. Starch was then characterized in situ, i.e., by 13C solid-state NMR of intact microalgal cells. Our in-cell methodology also enabled the identification of the effect of nitrogen starvation on starch metabolism. This work shows how solid-state NMR can enable the identification of starch structure, chemical modifications and biosynthesis in situ in intact microorganisms, eliminating time consuming and potentially altering purification steps.
Highlights
Starch is, with cellulose, the most abundant carbohydrate that is found in nature
The B form was obtained by starch retrogradation, as described in the Material and Methods section, from wild-type C. reinhardtii, yielding a final crystallinity of circa 55% as determined by solid-state nuclear magnetic resonance (SS-NMR) (Table S1)
Amorphous starch was prepared by freeze-drying amylose-rich starch as described by Paris et al [15]
Summary
Composed of a polymer of glucose in semicrystalline granules, it is the major form of energy storage for plants [1]. It is the main energy source in most animal diets and is involved in various food industry processes. Because they are linked with diseases, starches that are resistant to digestive enzymes have been the focus of a growing research emphasis [2]. In addition to its importance in nutrition, starch can be used as an environmentally-friendly low-cost material with no apparent toxicity and can be functionalized for a wide range of applications, such as adhesives, biofilms, biodegradable plastics, pharmacology, etc.
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