Circular dichroism and the conformation of sugars having vicinal diacylamino substituents

Abstract

The circular dichroism spectra of carbohydrates vicinally substituted by acetamido groups differ fundamentally from those of the common 2-acetamido-2-deoxy sugars as a result of mutual coupling effects. The calculations presented in this paper follow a method due to Schellman and co-workers which has been widely used in peptide CD calculations and which treats both the exciton coupling and static field effects in a consistent manner. The calculations predict, in agreement with experiment, that the 189-nm amide X-R* transition is split into two oppositely signed CD bands of approximately equal magnitude near 180 and 200 nm. A small n-r* band is predicted near 210 nm. The sign and magnitude of the bands depend on the dihedral angles defining the orientation of the amides with respect to the hexapyranose ring. It is shown that the calculations yield two independent parameters which may be compared with experiment: the product of the splitting with the rotational strength for the exciton bands and the total rotational strength for the n-X* band. Comparison of the calculated results with the experimental spectra of 2,3-diacetamidoglucose and of the glycopeptide linkage compound, 2-acetamido-I-N(4-Iaspartyl)-2-deoxy-/3-D-glucopyranosylamine, leads to the conclusion that both amide residues of the latter compound are oriented such that the relationship between the amide proton and the pyranose CH proton is trans while in the former the relationship is cis.

Conformation of the glycopeptide linkage in asparagine-linked glycoproteins

“…The appearance of a strong negative CD minimum at 178 nm WAVELENTH, nm is in accord with our earlier assignment of these strong CD bands to an exciton pair (Coduti et al, 1977;Bush & Duben, 1978). We had previously identified the exciton coupling with interaction of the vicinal diacyl amino functions of GlcNAc-Asn on the basis of a rather weak dependence of the CD spectrum on pH.…”

“…The contributions to the CD in this spectral region of the amine and carboxylate chromophores of the amino acid are modest. The exocyclic amide dihedral angles t¡ and t2 must be close to 120°as deduced from CD calculations on models (Bush & Duben, 1978). These values of t¡ and t2 place the amide protons trans to their respective ring protons, a conformation for which large amide proton coupling constants are expected.…”

“…A second exocyclic amide dihedral angle (t2) describing the orientation of the C2 amide will be defined by Cl, C2, the amide 2 nitrogen and amide 2 carbonyl carbon, = 0 will be taken as the eclipsed conformation, and positive dihedral angles will correspond to right-handed screw rotations. In a previous paper (Bush & Duben, 1978) these angles were called and 2. The present notation will avoid confusion with the standard peptide backbone conformational angles and .…”

Conformational statistics of the nitrogen linkage in glycopeptides using umbrella sampling

“…NMR [5], circular dichroism [4], and X-ray diffraction [6] were the independent experimental techniques used to determine the conformation of glcNAc-Asn. All three experimental techniques yielded results consistent with trans orientation of the amide N-H bond and the sugar C-H bond for both z~ and zz.…”