The pyranose ring of l-iduronic acid (IdoA), a major constituent of the anticoagulant heparin, is an equilibrium of multiple ring puckers that have evaded quantification by experiment or computation. In order to resolve this enigma, we have calculated the free energy landscape of IdoA and two related monosaccharides from extensive microsecond simulations. After establishing that the simulated puckers had reached equilibrium, hypotheses were confirmed that (a) IdoA 1 C 4- and 4 C 1-chair conformations exchange on the microsecond time scale, (b) C5 epimerization leads to a 4 C 1-chair, and (c) IdoA 2-O-sulfation (IdoA2S) stabilizes the 1 C 4 conformer. The IdoA and IdoA2S 1 C 4 conformers were isoenergetic and computed to be 0.9 and 2.6 kcal mol−1 lower in free energy than their respective 4 C 1-chair conformations. The simulations also predicted that the IdoA 2 S O-skew-boat was less populated than previously thought. Novel chemical synthesis and ultra-high-field NMR supported these observations, but slight discrepancies in observed and predicted NMR vicinal couplings implied that the simulation overestimated the population of the IdoA 4 C 1-chair with respect to 1 C 4-chair due to small force field inaccuracies that only manifest in long simulations. These free-energy calculations drive improvements in computational methods and provide a novel route to carbohydrate mimetic biomaterials and pharmaceuticals.

BackgroundHeparan sulfate (HS) is an important regulator of the assembly and activity of various angiogenic signalling complexes. However, the significance of precisely defined HS structures in regulating cytokine-dependent angiogenic cellular functions and signalling through receptors regulating angiogenic responses remains unclear. Understanding such structure-activity relationships is important for the rational design of HS fragments that inhibit HS-dependent angiogenic signalling complexes.Methodology/Principal FindingsWe synthesized a series of HS oligosaccharides ranging from 7 to 12 saccharide residues that contained a repeating disaccharide unit consisting of iduronate 2-O-sulfate linked to glucosamine with or without N-sulfate. The ability of oligosaccharides to compete with HS for FGF2 and VEGF165 binding significantly increased with oligosaccharide length and sulfation. Correspondingly, the inhibitory potential of oligosaccharides against FGF2- and VEGF165-induced endothelial cell responses was greater in longer oligosaccharide species that were comprised of disaccharides bearing both 2-O- and N-sulfation (2SNS). FGF2- and VEGF165-induced endothelial cell migration were inhibited by longer 2SNS oligosaccharide species with 2SNS dodecasaccharide activity being comparable to that of receptor tyrosine kinase inhibitors targeting FGFR or VEGFR-2. Moreover, the 2SNS dodecasaccharide ablated FGF2- or VEGF165-induced phosphorylation of FAK and assembly of F-actin in peripheral lamellipodia-like structures. In contrast, FGF2-induced endothelial cell proliferation was only moderately inhibited by longer 2SNS oligosaccharides. Inhibition of FGF2- and VEGF165-dependent endothelial tube formation strongly correlated with oligosaccharide length and sulfation with 10-mer and 12-mer 2SNS oligosaccharides being the most potent species. FGF2- and VEGF165-induced activation of MAPK pathway was inhibited by biologically active oligosaccharides correlating with the specific phosphorylation events in FRS2 and VEGFR-2, respectively.Conclusion/SignificanceThese results demonstrate structure-function relationships for synthetic HS saccharides that suppress endothelial cell migration, tube formation and signalling induced by key angiogenic cytokines.

A diastereomerically pure cyanohydrin, preparable on kilogram scale, is efficiently converted in one step into a novel L-iduronamide. A new regioselective acylation of this iduronamide and a new mild amide hydrolysis method mediated by amyl nitrite enables short, scalable syntheses of an L-iduronate diacetate C-4 acceptor, and also L-iduronate C-4 acceptor thioglycosides. Efficient conversions of these to a range of heparin-related gluco-ido disaccharide building blocks (various C-4 protection options) including efficient multigram access to key heparin-building block ido-thioglycoside donors are described. A 1-OAc disaccharide is converted into a heparin-related tetrasaccharide, via divergence to both acceptor and donor disaccharides. X-ray and NMR data of the 1,2-diacetyl iduronate methyl ester and the analogous iduronamide show that while both adopt (1)C(4) conformations in solution, the iduronate ester adopts the (4)C(1) conformation in solid state. An X-ray structure is also reported for the novel, (4)C(1)-conformationally locked bicyclic 1,6-anhydro iduronate lactone along with an X-ray structures of a novel distorted (4)C(1) iduronate 4,6-lactone. Deuterium labeling also provides mechanistic insight into the formation of lactone products during the novel amyl nitrite-mediated hydrolysis of iduronamide into the parent iduronic acid functionality.

Synthesis of the longest heparin-related oligosaccharide backbones is enabled by efficient iterative [4]n-mer homologations via solution-phase synthesis. Pure-Shift HSQC NMR provides a dramatic improvement in anomeric signal resolution.

Multigram-scale synthesis of a sulfation-site programmed dodecasaccharide is described. CXCL8- and CXCL12-mediated in vitro and in vivo biology is shown to be regulated by a single sulfation site change.

A concise chemical synthesis of a series of structurallydefined heparin-like oligosaccharides is described. This work provides an efficient entry to octa-, deca-, and 10 dodecasaccharides, including the first synthesis of (GlcNS6S-IdoA2S) 5 and (GlcNS6S-IdoA2S) 6 . Evaluation of the in vitro activity of these species against FGF2-and VEGF 165 -dependent endothelial cell proliferation and migration establishes that octa-and decasaccharides are more potent in 15 targeting FGF2-induced effects, where cell migration is affected more significantly than proliferation. These structure-activity relationships exemplify the significance of 6-O-sulfation in regulating the activity of angiogenic growth factors. 20Heparin and heparan sulphate (H/HS) are highly-charged, ubiquitous, naturally-occurring glycosaminoglycans (GAGs) which are involved in regulating a wide range of biologically important cellular signalling events that control a variety of 25 biological functions, including angiogenesis.1 Amongst these, angiogenic signalling pathways that control angiogenesis are regulated by pro-angiogenic and anti-angiogenic cytokines, many of which depend on H/HS for their biological activity.Fibroblast Growth Factor 2 (FGF2) and Vascular Endothelial demonstrating the potency of octa-and deca-saccharides. There is considerable interest in developing synthetic, structurally-defined H/HS sequences as tools to further probe these angiogenic signalling pathways and for other structural 40 interaction studies. Efficient synthetic routes, as well as access to a diversity of functionality, are essential to provide such agents to interrogate a range of biological targets and also with relation to potentially developing new anti-angiogenic therapies. 3A number of reports concerning the construction of various 45 H/HS architectures are known and address variation of sequence length and sulfation pattern. 4 The majority of these target the (IS) n repeating sequence, 5 with disaccharide-based strategies typically introducing the S-I anomeric linkage or employing iditol-based rather than iduronate donors. Noteworthy also are 50 recent approaches utilising chemoenzymatic methodologies 6 and efforts towards sequences containing mixed (GS/IS) oligomers. 60The synthesis of these novel deca-and dodecassaccharides compliments the synthesis of the alternative dodecasaccharide sequence (IS) 6 reported by the Bonnaffé group 1d and an (SI) 4 octasaccharide, similar to 26, reported by Martin-Lomas' group. 4d Furthermore, our optimized approach provides rapid iterative 65 access to multi-hundred mg quantities of octasaccharide 15, scalability which is pivotal to further elongations up to and including novel dodecasaccharide 19. The work was underpinned by developing a reliable 2+(2) n disaccharide iteration strategy for oligosaccharide chain elongation using stable thioglycoside

Graphical abstract

Heparin-like oligosaccharides mediate numerous important biological interactions, of which many are implicated in various diseases. Synthetic improvements are central to the development of such oligosaccharides as therapeutics and, in addition, there are no methods to elucidate the pharmacokinetics of structurally defined heparin-like oligosaccharides. Here we report an efficient two-cycle [4+4+4] tetrasaccharide-iteration-based approach for rapid chemical synthesis of a structurally defined heparin-related dodecasaccharide, combined with the incorporation of a latent aldehyde tag, unmasked in the final step of chemical synthesis, providing a generic end group for labelling/conjugation. We exploit this latent aldehyde tag for 3H radiolabelling to provide the first example of this kind of agent for monitoring in vivo tissue distribution and in vivo stability of a biologically active, structurally defined heparin related dodecasaccharide. Such studies are critical for the development of related saccharide therapeutics, and the data here establish that a biologically active, synthetic, heparin-like dodecasaccharide provides good organ distribution, and serum lifetimes relevant to developing future oligosaccharide therapeutics.