The structures of the capsular polysaccharides (CPSs) of the two clinical isolates Acinetobacter baumannii SMAL and MG1 were elucidated. Hot phenol/water extractions of the dry biomasses, followed by enzymatic digestions and repeated ultracentrifugations led to the isolation of polysaccharides that were negative in Western blot analysis utilizing an anti-lipid A antibody, thus proving that they were not the LPS O-antigens but CPSs. Their structures were established on the basis of NMR spectroscopy and GC-MS analyses. The A. baumannii MG1 CPS consisted of a linear aminopolysaccharide with acyl substitution heterogeneity at the N-4 amino group of QuipN4N: 4)-α-D-GlcpNAc-(1→4)-α-L-GalpNAcA-(1→3)-β-D-QuipNAc4NR-(1→ R=3-hydroxybutyrryl or acetyl. The repeating unit of the CPS produced by strain SMAL is a pentasaccharide, already reported for the O-antigen moiety from A. baumannii strain ATCC 17961: (formula, see text).

Acinetobacter baumannii, capsular polysaccharide, structural analysis, Western blot

13C NMR data:
Linkage	Residue	C1	C2	C3	C4	C5	C6
3,4,2	Ac
3,4	aDGlcpN	99.5	54.6	70.4	79.7	71.6	60.7
3,2	Ac
3	aLGalpNA	98.5	50.4	68.1	79.7	72.7	175.1
2	Ac
	bDQuipN4N	101.8	57.9	76.8	57.9	72.5	17.6
4	19%lS3HOBut	175.3	46.2	66.0	23.8
4	81%Ac


1H NMR data:
Linkage	Residue	H1	H2	H3	H4	H5	H6
3,4,2	Ac
3,4	aDGlcpN	4.93	3.91	3.90	3.67	4.09	3.63-3.72
3,2	Ac
3	aLGalpNA	5.21	4.16	3.91	4.34	4.10	-
2	Ac
	bDQuipN4N	4.60	3.80	3.87	3.80	3.52	1.17
4	19%lS3HOBut	-	2.33	4.19	1.24
4	81%Ac


1H/13C HSQC data:
Linkage	Residue	C1/H1	C2/H2	C3/H3	C4/H4	C5/H5	C6/H6
3,4,2	Ac
3,4	aDGlcpN	99.5/4.93	54.6/3.91	70.4/3.90	79.7/3.67	71.6/4.09	60.7/3.63-3.72
3,2	Ac
3	aLGalpNA	98.5/5.21	50.4/4.16	68.1/3.91	79.7/4.34	72.7/4.10	
2	Ac
	bDQuipN4N	101.8/4.60	57.9/3.80	76.8/3.87	57.9/3.80	72.5/3.52	17.6/1.17
4	19%lS3HOBut		46.2/2.33	66.0/4.19	23.8/1.24
4	81%Ac

There is only one chemically distinct structure:

SVGMWSMILES3D molIonize

 

*O[C@@H]1[C@@H](CO)O[C@H](O[C@H]2[C@@H](O)[C@H](NC(C)=O)[C@H](O[C@@H]3[C@@H](NC(C)=O)[C@H](*)O[C@H](C)[C@H]3NC(=O)C[C@H](C)O)O[C@H]2C(=O)O)[C@H](NC(C)=O)[C@H]1O

692.672 g/mol (C₂₈H₄₄R₂N₄O₁₆, R = next and previous repeats, not counted in mol weight)