First Principles Insight into the α-Glucan Structures of Starch: Their Synthesis, Conformation, and Hydration

Carbohydrates constitute the most abundant group of organic compounds found in nature. Oxygenic photosynthesis, the process energizing carbon dioxide fixation in the biosphere, is estimated to 1011 tons of dry weight biomass per year, most of it being carbohydrate.(1) For human consumption, the abundance of starch and the possibility to carry out large-scale purification, derivatization and processing provide unique and straightforward options to design starch crops harboring new valuable functionalities offering diversified uses in the food and nonfood sectors.2,3 These include raw materials for the design of advanced and healthy foods to combat obesity and other lifestyle-related diseases(4) or to replace gelatin.(5) Today, starch constitutes a major raw material in the bioethanol production6,7 and in the future starch is expected to play an important role in providing resources for the increasing demand for CO2-neutral energy. The global annual starch production by man approximates 3000 million tons and the industrial production of pure, refined starch now exceeds 60 million tons.(8) The simple and compact structure of starch and its human analogue glycogen has proven to be very successful for providing energy to living organisms and as energy storage reservoirs in biological systems. The metabolism and architecture of these two polymers are highly dependent on the presence of water. Understanding of the detailed structure and molecular models of complex α-glucans in an aqueous environment would be useful tools in the attempt to provide science-based recommendations in our efforts to build a bio-based society where starches play a major role as bulk polymers. Advances within these areas are dependent on the availability of complex α-glucans of defined chemical structures that mimic the key features of starch and other complex α-glucans and thus offer the opportunity to gain detailed knowledge of the molecular structure of hydrated starch and α-glucan systems. This review provides an overview of this rapidly expanding and challenging field of research with main focus on starch structure and hydration. Starch9−11 and glycogen12−14 are synthesized by sets of specific enzyme activities that directly determine their molecular structures and physical properties. The extent of crystallinity, aggregation and hydration is of fundamental importance for starch and its human analogue glycogen. Starch is deposited in the plant as a stable form in highly organized, semicrystalline granules15,16 (Figure ​(Figure1)1) having specific crystalline polymorphs (Figure ​(Figure2)2) as determined by powder X-ray crystallography.(17) Glycogen is not crystalline, but the importance of correctly structured glycogen granules12−14 can be exemplified by the occurrence of specific Mendelian inherited glycogen-dependent disorders,(18) such as the epileptic Lafora disease(19) or the Cori disease.(20) These two diseases are characterized by deposition of aberrant “starch-like” glycogen structures resulting in the inability to properly store and mobilize deposited glycogen. Open in a separate window Figure 1 Principle of the “top-down” strategy of starch analysis. (A) A cross section of a wheat starch granule (Confocal microscopic image by Mikkel A. Glaring). (B) A schematic drawing of the layered structure of amylopectin. Alternating amorphous and crystalline lamellae are repeated with 9 nm spacing. (C) A cross section of the parallel helices pack to form either hexagonal (A-type or B-type) or pseudo hexagonal packing. Alternating amorphous and crystalline lamellae are repeated with 9 nm spacing. (D) State-of-the art modeling concern tiny double helical structure and nanocrystallite packing. (E) Structure of chemically synthesized branched pentasaccharide mimicking the branch point in starch and glycogen.