Interterminal Distance and Flexibility of a Triantennary Glycopeptide As Measured by Resonance Energy Transfer

Abstract

Three geometric isomers of a single triantennary glycopeptide, each containing two fluorophores attached to terminal positions in the molecule, were used to probe distance and flexibility of the oligosaccharide in solution. A dansyl group (energy acceptor) was attached to the C6 of Gal at either position 6′, 6, or 8, and a naphthyl-2-acetyl group (energy donor) was coupled to the N terminus of the Ala-Asn peptide. [formula-omited] Resonance energy-transfer measurements revealed an average distance of approximately 22, 18, and 17 Å between the donor and the acceptor attached to either the 6, 8, or 6′ Gal residue, respectively. The lifetime of the donor’s emission was nearly a single-exponential decay of 27 ns (96%), whereas the decay of the donor with proximally attached acceptor was fit by nonlinear least-squares analysis to a multiexponential for each glycopeptide probe. Fitting with a Lorentzian function revealed spatially distinct donor/acceptor distances presumably arising from glycopeptide branch flexibility. The results suggest that the acceptor located at Gal 8 is the most rigid relative to the donor with a single population of distances centered at 18.4 Å. In contrast, the acceptor attached to either Gal 6′ or 6 displayed two populations of different distances from the donor. The Gal 6 isomer contained a major population with average donor/acceptor separation distance of 21.7 Å and a minor population with average separation distance of 9.7 Å. Similarly, the Gal 6′ isomer showed a major population with donor/acceptor separation distance of 18.3 Å and a minor population with separation distance of 11.7 Å. These data support the earlier conclusions that the Manα(1→6)Man linkage found in the core pentasaccharide of all branched N-linked oligosaccharides is flexible. In addition, the data suggest that the branch containing Gal 6 is also flexible in the triantennary glycopeptide. © 1991, American Chemical Society. All rights reserved.