An easy α-glycosylation methodology for the synthesis and stereochemistry of mycoplasma α-glycolipid antigens

Abstract

Mycoplasma fermentans possesses unique α-glycolipid antigens (GGPL-I and GGPL-III) at the cytoplasm membrane, which carry a phosphocholine group at the sugar primary (6-OH) position. This paper describes a practical synthetic pathway to a GGPL-I homologue (C(16:0)) and its diastereomer, in which our one-pot α-glycosylation method was effectively applied. The synthetic GGPL-I isomers were characterized with (1)H NMR spectroscopy to determine the equilibrium among the three conformers (gg, gt, tg) at the acyclic glycerol moiety. The natural GGPL-I isomer was found to prefer gt (54%) and gg (39%) conformers around the lipid tail, while adopting all of the three conformers with equal probability around the sugar position. This property was very close to what we have observed with respect to the conformation of phosphatidylcholine (DPPC), suggesting that the Mycoplasma glycolipids GGPLs may constitute the cytoplasm fluid membrane together with ubiquitous phospholipids, without inducing stereochemical stress.

Keywords: cytoplasm membrane; glycolipid antigen; glycosylation; mycoplasma; stereochemistry.

Figure 1

Absolute chemical structures of M. fermentans α-glycolipid antigens, GGPL-I and GGPl-III (GGPL: Glycosyl-sn-glycerophospholipid).

Scheme 1

An established synthetic pathway to α-glycosyl-sn-glycerols 4a and 5a. A reagent combination of CBr₄ and Ph₃P (Appel–Lee reagent) is utilized in either CH₂Cl₂ or DMF as solvent.

Scheme 2

Syntheses of GGPL-I homologue I-a and its isomer I-b. Conditions: (a) K₂CO₃, CH₃OH; (b) cesium palmitate in DMF; (c) TBDMS chloride then palmitoyl chloride in pyridine + DMAP; (d) TFA in CH₃OH; (e) (i) 2-cyanoethyl-N,N,N’,N’-tetraisopropyl phosphorodiamidite, 1H-tetrazole and MS-4 Å in CH₂Cl₂; (ii) choline tosylate, 1H-tetrazole, (iii) mCPBA, (iv) aq. NH₃ in CH₃OH, (f) H₂, Pd(OH)₂/C in CH₃OH.

Figure 2

¹H NMR spectra of I-a and I-b (500 MHz, 25 °C, CDCl₃/CD₃OD 10:1). The assignment of sn-glycerol methylene protons (H_proR and H_proS) was performed on the basis of our preceding studies on deuterium-labeled glycerols [–37] and α(1→6)-linked disaccharides [–40].

Figure 3

Distributions of gg, gt and tg-conformers in 3-substituted sn-glycerols at 11 mM in solutions of CDCl₃ and CD₃OD (10:1) at 298 K.

Figure 4

Distributions of gg, gt and tg-conformers in 1-substituted sn-glycerols. In these sn-isomers, Φ1 and Φ2 represent the dihedral angles around the C–C single bond at the glycerol sn-2,3 and 1,2-position, respectively.

Figure 5

A common conformational property of GGPL-I and DPPC. The tail lipid moiety favors two gauche-conformers of gt and gg (gt > gg >> tg), while the head moiety takes three conformers in averaged populations (gt = gg = tg).