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BackgroundLevan and levan-type fructo-oligosaccharides (LFOs) have various potential applications in pharmaceutical and food industries due to their beneficial properties such as their low intrinsic viscosity and high water solubility. Previous studies showed that they exhibited prebiotic effects, anti-inflammatory and anti-tumor activities against Sarcoma-180 tumor cells of human. Despite their various potential applications, the structural and molecular properties of LFOs of various chain lengths are not well understood.ResultsWe employed the replica-exchange molecular dynamics simulations method (REMD) in AMBER14 to elucidate structural and molecular properties of LFOs with chain lengths of 5 (LFO5), 10 (LFO10) and 15 (LFO15) residues in two models of generalized Born implicit solvent (GBHCT and GBOBC1). For LFO10 and LFO15, four distinct conformations (helix-like, partial helix, zig-zag and random structures) were characterized by their upper-middle and lower-middle torsions. For LFO5, two distinct conformations (partial helix and random structures) were characterized by their middle torsion and molecular angle of residues 1, 3 and 5. To determine hydrogen bonds important for the formation of helix-like structures of LFO10 and LFO15, occurrence frequencies of hydrogen bonds were analyzed, and the O6(i)--H3O(i+1) hydrogen bond was found with the highest frequency, suggesting its importance in helix formation. Among three dihedral angles between two fructosyl units [ϕ (O5’-C2’-O6-C6), ψ (C2’-O6-C6-C5) and ω (O6-C6-C5-C4)], dihedral angle distributions showed that ω was the most flexible dihedral angle and probably responsible for conformational differences of LFOs.ConclusionsOur study provides important insights into the structural and molecular properties of LFOs, which tend to form helical structures as the chain length increases from 5 to 15 residues. This information could be beneficial for the selection of LFOs with appropriate lengths and properties for pharmaceutical and biological applications.Electronic supplementary materialThe online version of this article (doi:10.1186/s12859-016-1182-7) contains supplementary material, which is available to authorized users.

Levan has various potential applications in the pharmaceutical and food industries, such as cholesterol-lowering agents and prebiotics, due to its beneficial properties, which depend on its length and branching degree. A previous study also found that the branching degree of levan affected anti-tumor activities against SNU-1 and HepG2 tumor cell lines. Despite its promising potential, the properties of levans with different branching degrees are not well understood at the molecular level. In two models of the generalized Born implicit solvent (GBHCT and GBOBC1), we employed replica-exchange molecular dynamics simulations to explore conformational spaces of 34-residue levans (L34) with branching degrees of zero (LFO34B0), one (LFO34B1), three (LFO34B3) and five (LFO34B5), as well as to elucidate their structural and molecular properties. To ensure a fair comparison of the effects of branching degree on these properties, we focused on analyzing the properties of the central 21-residue of the main chains of all systems. Our results show that all major representative conformations tend to form helix-like structures with kinks, where two-kink helix-like structures have the highest population. As branching degree increases, the population of helix-like structures with zero or one kink tends to increase slightly. As the number of kinks in the structures with the same branching degree increases, the average values of the lengths and angles among centers of masses of three consecutive turns of residue i, i+3, and i+6 tended to decrease. Due to its highest occurring frequencies, the O6 (i)—H3O (i+1) hydrogen bond could be important for helix-like structure formation. Moreover, hydrogen bonds forming among the branching residue (br), branching position (bp) and other residues of L34B1, L34B3 and L34B5 were identified. The O1(bp)—H3O(br), O1(br)—H3O(br) and O5(br)—H1O(br) hydrogen bonds were found in the first-, second- and third-highest occurrence frequencies, respectively. Our study provides novel and important insights into conformational spaces and the structural and molecular properties of 34-residue levans with various branching degrees, which tend to form helix-like structures with kinks.

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