Conformational changes of 1-4-glucopyranosyl residues of a sulfated C-C linked hexasaccharide

Abstract

This work describes the structure of a fully sulfated maltotriose alpha-beta C-C linked dimer, where a central glycosidic bond was substituted by a non natural, hydrolase-resistant C-C bond. Such compound shows anti-metastatic properties being an inhibitor of the heparanase enzymatic activity and of P-selectin-mediated cell-cell interactions. NMR spectroscopy was applied to investigate the structure and conformational properties of this C-C linked hexasaccharide. The presence of sulfate substituents and the internal C-C bond drives the two internal rings in an unusual (1)C(4) chair conformation, while the external rings linked by glycosidic bonds retain the typical (4)C(1) conformation. The NMR results were confirmed by molecular mechanics calculations using structure corresponding di- and tetrasaccharides as models.

Keywords: (1)C(4) chair conformation; Maltotriose; Molecular mechanics; NOESY spectroscopy; Sulfated CC oligosaccharides; Sulfated oligosaccharide-based drugs.

Figure 1

¹H NOESY spectrum of αβ-SMTC recorded at 600 MHz. The ¹H signals in accord with the HSQC spectrum are indicated by two types of label: a letter indicating the glycan ring of the sequence A–B–C–D–E–F, followed by a number corresponding to the position inside each residue. Red labels underline NOESY across the C—C bond. The insert is the enlargement of the NOESY spectrum from 5.70 to 3.90 ppm in F2 and from 5.80 to 5.45 ppm in F1, highlighting the H1–H4 and H1–H3 interglycosidic signals for the residues external to the C—C link.

Figure 2

Suggested conformation of the C—C linked residues (rings C and D) belonging to αβ-SMC in agreement with data reported in Figure 5. The picture shows the arrangement of central residues in ¹C₄ conformation and a torsional angle θ = 180° as obtained by molecular mechanics analysis and interpretation of the NOESY spectrum reported in Subsections 2.2.1 and 2.2.2. Arrows show protons correlated by NOESY signals as reported in Table 2.

Figure 3

Two possible conformations of the sulfated αβmaltose CAC linked dimer (αβSMC) considered in this study as a simplified model of the αβ-SMTC. Left: ⁴C₁–⁴C₁, where the four rings E, D, C, and B show the ⁴C₁ chair conformation. Right: ¹C₄–¹C₄ where the two central rings C and D are reported in ¹C₄ chairs, while the external rings E and B are represented in the ⁴C₁ conformation as usual for sulfated glucose units.

Figure 4

Potential energy maps for 2,3,6-hexasulfated maltose having the reducing residues in ⁴C₁–¹C₄ (left panel) and ⁴C₁–⁴C₁ (right panel) conformations, obtained by the torsional angle scan/energy minimization procedure. The energy scale is in Kcal mol⁻¹, while angles are expressed in degrees.

Figure 5

Potential energy profile (Kcal mol⁻¹) as a function of the θ torsional angle (H^C1–C^C1–C^D1–H^D1) for the sulfated-αβ-maltose C—C linked dimer (αβ-SMC, upper curves) and αβmaltose C—C linked dimer (αβ-MC, lower curves). For both αβ-SMC and αβ-MC models the red and black lines with filled dots correspond to the conformers with the C—C linked residues in ¹C₄–¹C₄ and ⁴C₁–⁴C₁ chair respectively. θ angle and potential energy values of interesting minima are reported in round brackets.

Scheme 1

Schematization of the synthetic procedure for αβ-SMTC. Letters A, B, C, D, E, and F indicate each hexasaccharide glucose ring.