Actinoidin A

A series of glycopeptide antibiotics related to the vancomycin-ristocetin family have been successfully analyzed by fast atom bombardment mass spectrometry (FABMS). The FAB mass spectra of glycopeptides weighing up to 2,100 daltons exhibit intense molecular ions and fragment ions from which information concerning carbohydrate composition and sequence are readily obtained. Careful adjustment of the FABMS experimental conditions has enabled the accurate masses of the glycopeptides to be determined by high resolution FABMS with an accuracy of better than six ppm. Comparison of the observed molecular ion cluster pattern with calculated isotope distributions reveals the precise number of chlorine atoms in these molecules, which, together with the accurate mass data, can be used to restrict the number of possible elemental compositions to a meaningfully small value. These techniques have been used to characterize several glycopeptides of known structure including ristocetin, actinoidin, avoparcin, vancomycin and A35512B, as well as aridicins A, B and C which are three new, novel members of the vancomycin class.

The structural analysis of the intact glycopeptide antibiotics, actinoidins A (1a) and A2 (1b), by two-dimensional 1H NMR is described. The location of the single chlorine at the A3 position and the sites of attachment of the four carbohydrate substituents in actinoidin A are elucidated based on correlation spectroscopy (COSY), double quantum coherence experiments (DQCE), homonuclear Hartmann-Hahn experiments (HOHAHA) and nuclear Overhauser spectroscopy (NOESY). Similar 2D correlation and NOE NMR experiments are then performed on the novel analog, actinoidin A2, to determine its structure. The structural difference between actinoidins A and A2 is shown to reside in the presence of L-rhamnose in actinoidin A2 in place of L-acosamine in actinoidin A. All questions concerning the stereo-chemistry of the chiral centers in both the heptapeptide core and the carbohydrate moieties in each of these antibiotics could be successfully addressed with the exception of Gl', the alpha-carbon on the N-terminal amino acid which is known to have the R-configuration from previous studies.

An unidentified Nocardia sp. (SK&F-AAJ-193) was isolated and found to produce actinoidin A and a novel analog which we have named actinoidin A2. This new glycopeptide antibiotic differs from actinoidin A by the presence of rhamnose instead of acosamine. This analog was isolated using Dianion HP-20 resin followed by a specific glycopeptide affinity column (Affigel-10-D-Ala-D-Ala). The purification was accomplished using preparative ion-pairing chromatography. Actinoidin A2 is active against Staphylococcus aureus and coagulase-negative Staphylococci although it is less potent than actinoidin A.