The solution structure of mucous glycoproteins: proton NMR studies of native and modified ovine submaxillary mucin

The solution structure of native and systematically modified ovine submaxillary mucin (OSM) has been probed by proton NMR spectroscopic methods. Most of the resonances in the spectra have been tentatively assigned to the peptide and O-linked disaccharide, alpha-N-acetylneuraminic acid 2----6 alpha-N-acetylgalactosamine protons. On the basis of the observed chemical shifts, spectral resolution, and behavior of the exchangeable protons it is concluded the mucin possesses internal segmental flexibility and exists in solution as a random coil peptide. No long-lived interresidue peptide or carbohydrate hydrogen bonds were detected. The removal of (i) the C8 and C9 carbons of the sialic acid residue, (ii) the entire sialic acid residue, and (iii) the complete disaccharide side chain resulted in no significant changes in peptide core conformation. A limited set of proton spin coupling constants and nuclear Overhauser enhancements has been obtained for the threonine glycopeptide side chains in native and modified mucin. The results are consistent with the previously reported conformations for the (1----6) linkage in oligosaccharides and the threonyl glycosidic linkage in glycopeptides. The OSM disaccharide may exist as a extended linear structure with rotational freedom about the GalNAc C5-C6 bond, while the threonine glycosidic linkage appears to be sterically constrained, although multiple conformations about the threonine C beta-O gamma bond may be allowed. The small chemical shift perturbations detected in the glycosylated threonine methyl protons and the GalNAc carbons upon removal of the terminal sialic acid residue are consistent with this model.     

Comparison by 1H-nmr spectroscopy of the conformation of the 2600 dalton antifreeze glycopeptide of polar cod with that of the high molecular weight antifreeze glycoprotein

The antifreeze glycopeptide (AFGP-8) from polar cod, B. saida, is a 14-amino acid polypeptide having alternating glycotripeptide sequences of Ala-[Gal(β1 → 3)GalNAc(β1 → O)]-Thr-Pro and Ala-[Gal(β1 → 3)GalNAc(β1 → O)]-Thr-Ala, with alanyl residues at amino and carboxy terminals. Conformational studies of AFGP-8 have been carried out by ¹H-nmr and empirical energy calculations to investigate the difference in its antifreeze behavior from that of the more active high-molecular weight AFGP 1-4 of P. borchgrevinki. The ¹H-nmr spectra, including the resonances of the exchangeable amide protons, were assigned by two-dimensional correlated spectroscopy (COSY), one-dimensional difference decoupling, and nuclear Overhauser effect (NOE) measurements. For the four threonyl residues, the amide proton coupling constants and the small coupling constants between H^α and H^β indicate similar conformations, despite significant chemical shift differences. The strong NOE between the α protons and the amide protons of the residue following together with large temperature coefficients of chemical shifts, indicate an extended conformation not consisting of α-helix, turns or bends. Energy computations indicate several low-energy conformations consistent with the observed coupling constants for ?. Among these, a left-handed helical conformation with three repeating residues per turn has been proposed, which is in accordance with the observed NOE between the methyl group of the α-GalNAc and Ala H^βs. While the observed Overhauser effects in the threonyl side chain suggest a certain amount of conformational averaging, the effect involving the acetmido methyl of α-GalNAc and H^βs of Ala indicate that it as is a major conformer. In view of the close similarity between the conformations of AFGP-8 and the more active antifreeze polymer, AFGP 1-4, we propose that the difference in their activities is due to the length of the regular repeating structure with glycosylation at every third amino acid residue, and not due to any fundamental difference in their conformations.     

Conformations of type 1 and type 2 oligosaccharides from ovarian cyst glycoprotein by nuclear Overhauser effect spectroscopy and T1 simulations.

To probe differences in conformation of the type 1 and type 2 linkages in blood group oligosaccharides, two-dimensional nuclear Overhauser effect spectroscopy (2D-NOESY) and 1H T1 data were obtained for two blood group A oligosaccharide alditols containing the type 1 and type 2 linkage. The NOE data were interpreted using a complete relaxation matrix approach. Simulations of NOE and T1 values were made using disaccharide and tetrasaccharide model conformations generated by a systemic variation of the glycosidic dihedral angles phi and psi. NOEs from the amide protons of GlcNAc and GalNAc in the type 1 pentasaccharide alditol were obtained, and simulated in a manner similar to those from carbon-bound protons. In addition to providing data for determining the conformation of the type 1 linkage from amide proton NOEs of GlcNAc and GalNAc to neighboring residues, amide proton NOEs also yield information on the orientation of the acetamido side chains. The amide NOE data indicated subtle differences in the orientation of the amide side chain of GlcNAc among the A type 1 pentasaccharide alditol and two previously studied blood group oligosaccharides, lacto-N-difucohexaose 1 and lacto-N-fucopentaose 1. From the NOE and 1H T1 data, and from simple rigid geometry energy calculations, it is concluded that the type 1 and type 2 linkages in the oligosaccharides studied have different conformations and that these conformations are relatively rigid in solution.     

N.m.r. study on conformation of Ac-Thr(alpha-GalNAc)-Ala-Ala-OMe as a model for mucin type glycoprotein

This study report on the results of high resolution 1H n.m.r. investigations on Ac-Thr(alpha-GalNAc)-Ala-Ala-OMe 1 as a mucin type model glycopeptide of antifreeze glycoprotein (AFGP) in both dimethyl sulfoxide (DMSO) and H2O. The temperature dependence of amide proton chemical shifts strongly suggested the presence of the intramolecular hydrogen bond between the amide proton of GalNAc and the carbonyl oxygen of the Thr residues. Due to this bond, the orientation of the sugar residue of 1 appears to be fairly restricted relative to its peptide backbone. Despite the lack of the clear evidence for such intramolecular hydrogen bond in H2O, 1H coupling constant data suggested the structural similarity of 1 in DMSO and H2O, indicating the presence of the intramolecular hydrogen bond even in H2O, which may play an important role in determining the orientation of the sugar moiety with respect to the peptide backbone in glycoprotein.     

Carbon-13 nuclear-magnetic-resonance spectroscopy of Forssman hapten.

The 13C n.m.r. spectrum of Forssman hapten was obtained at 25.16 MHz in [3H] chloroform/[2H] methanol (1:1, v/v), using purified glycosphinogolipid from canine intestinal mucosa (glycolipid I). All amide, olefin, anomeric, intersaccharide glycosidic ether, amide linkage, methyl and many methylene resonances were resolved and assigned. Analysis of the anomeric region reveals the following pentaglycosylceramide structure as originally proposed [Siddiqui & Hakomori (1971) J. Biol. Chem. 246, 5766-5769]: GalNAc (alpha 1 leads to 3) GalNAc (beta 1 leads to 3) Gal (alpha 1 leads to 4) Gal (beta 1 leads to 1) ceramide. Analysis of the amide, olefin and methylene regions reveals no alpha-hydroxy fatty acyl group and less than or equal to 6 mol% unsaturated fatty acyl groups are present. Chemical-shift assignments are reported for the anomeric and glycosidic ether carbon atoms of intersaccharide-linked alpha-galactose and N-acetyl-alpha-galactosamine residues. Two rules are proposed for the assignment of the anomeric form of 1 leads to 3 and 1 leads to 4 linkages of galactose and N-acetylgalactosamine residues present in the glycone of glyco-conjugates. The present study emphasizes the importance of the anomeric "window" (80-120 p.p.m.) in studies of glycone structure.     

Solution conformation of Forssman antigen probed by NOE and exchange interactions

The conformation of Forssman glycolipid, GalNAc alpha 1-3GalNAc beta 1-3Gal alpha 1-4Gal beta 1-4Glc beta 1-1ceramide, was analysed with the aid of the rotating frame NOE and Hartmann-Hahn spectroscopy. NOE contacts between C-, O-, and N-linked protons were used for distance mapping. The glycosidic bonds that are common to globotriaosylceramide and globoside showed a similar flexibility as found for these compounds [Poppe et al., (1990) Eur. J. Biochem. 189, 313-325; J. Am. Chem. Soc. 112, 7762-7771]. In contrast, the conformational mobility of the terminal GalNAc alpha 1-3GalNAc beta linkage appears to be restrained. A new approach, based on 2D exchange spectroscopy, was proposed for revealing of spatial proximities between exchangeable protons in Me2SO solution.     

SMS 201-995, an octapeptide somatostatin analogue. Assignment of the 1H 500 MHZ n.m.r. spectra and conformational analysis of SMS 201-995 in dimethylsulfoxide

The possible conformations of SMS 201-995, an active analogue of somastostatin, have been studied in dimethylsulfoxide solution by 500 MHz proton n.m.r. spectroscopy. The assignments have been made by use of 2D-correlated methods to detect long-range coupling connectivities in aromatic residues and between the alpha protons of consecutive residues. NOESY experiments enabled us to correlate amide and alpha protons of neighbouring amino acid residues, which indicate a less flexible situation than in water. Measurements of temperature coefficients of the amide protons, of NH-C alpha H coupling constants and NOE effects are in favour of one predominant conformation with a beta turn, of type II', involving amino acids Phe3 to Thr6.     

Conformation of the eosinophil chemotactic tetrapeptides and analogues in dimethyl sulfoxide. 1H n.m.r. studies

Proton nuclear magnetic resonance parameters are reported for DMSO-d6 solutions of the eosinophil chemotactic tetrapeptides, Val1-Gly2-Ser3-Glu4 and Ala1-Gly2-Ser3-Glu4, as well as three analogues of the Val1 tetrapeptide, D-Val1, Ala2 and Ala3. The synthesis of Val-(S)-[alpha-2 H1] Gly-Ala-Glu, in which the glycine has been stereospecifically deuterated in the H alpha 3 position, has allowed the assignment of the 1H resonances belonging to individual H alpha 2 and H alpha 3 glycine methylene protons. Simulation of the glycine ABX spin system yields two vicinal coupling constants which are consistent with a highly preferred conformation about the glycine HN-C alpha bond. The chemical shifts, coupling constants, temperature coefficients of amide proton chemical shifts and calculated side chain rotamer populations are reported for all peptides. The coupling constant analysis and temperature coefficients of amide proton chemical shifts together suggest that a type I beta-turn conformation is preferred by the Ala3 analogue. The 1H n.m.r. parameters of the other peptides suggest that these can also adopt a beta-turn conformation in DMSO. There are, however, considerable differences in the extent of conformational averaging undergone by the various peptides.     

Experimental and theoretical conformation analysis of the O-antigenic polysaccharide from Pseudomonas cepacia strain 3181. I. Disaccharide links

Nuclear Overhauser effects, with preirradiation of glycoside bond anomeric protons, coupling constants 3J C3, H1' and 3J C1', H4 and linkage optical rotations A were measured for L-Rha beta 1-3-L-Rha alpha 1-OMe and L-Rha alpha 1-3-L-Rha alpha 1-OMe which are the models of the disaccharide units of the Pseudomonas cepacia polysaccharide. Theoretical conformational analysis was carried out in terms of a mechanical molecular model approximation. The spatial structures of these disaccharides as well as of D-Rha alpha 1-2-D-Rha beta 1-OMe in aqueous solutions were discussed basing on the obtained results.     

Carbohydrate dynamics at a micellar surface: GD1a headgroup transformations revealed by NMR spectroscopy.

The conformational dynamics of the carbohydrate headgroup of ganglioside GD1a, NeuAc alpha 2-->3Gal beta 1-->3GalNAc beta 1-->4[NeuAc alpha 2-->3]Gal beta 1-->4Glc beta 1-->1Cer, anchored in a perdeuterated dodecylphosphocholine micelle in aqueous solution, were probed by high resolution NMR spectroscopy. The observed 1H/1H NOE interactions revealed conformational averaging of the terminal NeuAc alpha 2-->3Gal and Gal beta 1-->3GalNAc glycosidic linkages. The pronounced flexibility of this trisaccharide moiety was substantiated further by two-dimensional proton-detected 13C T1, T1 rho and 1H/13C NOE measurements. The anchoring effect of the micelle allowed the detection of conformational fluctuations of the headgroup on the time scale of a few hundred picoseconds. NMR experiments performed on the GD1a/DPC micelles in H2O at low temperatures permitted the observation of hydroxyl proton resonances, contributing valuable conformational information.     

Two gamma-bends in the backbone conformation of [D-Ala2]-leucine enkephalin in solution

The solution conformation of [D-Ala2]-leucine enkephalin in its zwitterionic form in DMSO-d6 has been monitored by one- and two-dimensional proton magnetic resonance spectroscopy at 500 MHz. The resonances from the labile amide protons and the nonlabile protons have been assigned from the shift correlated spectroscopy. The chemical shift of the amide and C-alpha protons are found to vary with temperature but in opposite directions, except the C-alpha proton of the terminal tyrosine residue. This behavior has been explained by the shifting of equilibrium between the zwitterionic and neutral forms of the [D-Ala2]-leucine enkephalin and probably conformational changes accompanying temperature variation. The low values of the temperature coefficients of leucine and glycine amide protons indicate that these protons are either intramolecularly hydrogen bonded or solvent shielded. The observation of sequential cross peaks in the nuclear Overhauser effect spectra obtained at various mixing times, tau m (200-900 ms), indicate an extended backbone, which does not corroborate with the presence of a folded structure, i.e., beta-bend type structure. The estimate of interproton distances in conjunction with the low values of temperature coefficients of the leucine and glycine amide protons and vicinal coupling constants 3JHN-C alpha H have been rationalized by the predominance of two gamma-bends in the backbone conformation of [D-Ala2]-leucine enkephalin. The gamma-bend around the D-Ala residue has phi = 80 degrees and psi = 270 degrees, while the one around Phe it has phi = 285 degrees and psi = 90 degrees.     

Use of amide 1H-nmr titration shifts for studies of polypeptide conformation

This paper shows that backbone amide proton titration shifts in polypeptide chains are a very sensitive manifestation of intramolecular hydrogen bonding between carboxylate groups and backbone amide protons. The population of specific hydrogen-bonded structures in the ensemble of species that constitutes the conformation of a flexible nonglobular linear peptide can be determined from the extent of the titration shifts. As an illustration, an investigation of the molecular conformation of the linear peptide H-Gly-Gly-L -Glu-L -Ala-OH is described. The proposed use of amide proton titration shifts for investigating polypeptide conformation is based on 360-MHz ¹H-nmr studies of selected linear oligopeptides in H₂O solutions. It was found that only a very limited number of amide protons in a polypeptide chain show sizable intrinsic intration shifts arising from through-bond interactions with ionizable groups. These are the amide proton of the C-terminal amino acid residue, the amide protons of Asp and the residues following Asp, and possibly the amide proton of the residue next to the N-terminus. Since the intrinsic titration shifts are upfield, the downfield titration shifts arising from conformation-dependent through-space interactions, in particular hydrogen bonding between the amide protons and carboxylate groups, can readily be identified.     

N.m.r. study of interaction between sugar and peptide moieties in mucin-type model glycopeptides.

In order to investigate the structural properties of the sugar and peptide linkage region in glycoprotein, some glycopeptides were synthesized as a model for AFGP (antifreeze glycoprotein), which is one of the mucin-type glycoproteins. The results from n.m.r. measurements in DMSO and aqueous conditions revealed that the glycopeptides form an intramolecular hydrogen bond between the amide proton of N-acetylgalactosamine (GalNAc) and the carbonyl oxygen of threonine (Thr) to which the GalNAc is covalently linked. This intramolecular hydrogen bond may play an important role in determining the orientation of the sugar moiety relative to the protein backbone. The roles for the activity of the proline (Pro) residue in AFGP were also discussed.     

Solution conformation of a pectin fragment disaccharide using molecular modelling and nuclear magnetic resonance

In the present study, the conformational behaviour of methylated pectic disaccharide 4-O-alpha-D-galactopyranurosyl 1-O-methyl-alpha-D-galactopyranuronic 6,6'-dimethyl diester 1 has been completely characterized through combined n.m.r. and molecular modelling studies. The 1H-1H n.O.e. across the glycosidic bond was measured by both steady-state and transient 1D and 2D experiments. In parallel, the complete conformational analysis of the disaccharide has been achieved with the MM3 molecular mechanics method. The conformation of the pyranose ring is confirmed by the excellent agreement between the experimental and calculated intracyclic scalar coupling constants. The iso-energy contours displayed on the 'relaxed' map indicate an important flexibility about the glycosidic linkage. There is no significant influence of the methoxyl group on the conformational behaviour of the disaccharide. The theoretical n.m.r. data were calculated taking into account all the accessible conformations and using the averaging methods appropriate for slow internal motions. 3JC-H coupling constants were calculated using an equation suitable for C-O-C-H segments. The agreement between experimental and theoretical data is excellent. Within the potential energy surface calculated for the disaccharide, several conformers can be identified. When these conformations are extrapolated to a regular polymer structure, they generate pectins with right- and left-handed chirality along with a two-fold helix. These different types of helical structure are the result of small changes in conformation, without any drastic variation of the fibre repeat.     

Influence of N-terminal residue stereochemistry on the prolyl amide geometry and the conformation of 5-tert-butylproline type VI beta-turn mimics.

The effects of N-terminal amino acid stereochemistry on prolyl amide geometry and peptide turn conformation were investigated by coupling both L- and D-amino acids to (2S, 5R)-5-tert-butylproline and L-proline to generate, respectively, N-(acetyl)dipeptide N'-methylamides 1 and 2. Prolyl amide cis- and trans-isomers were, respectively, favored for peptides 1 and 2 as observed by proton NMR spectroscopy in water, DMSO and chloroform. The influence of solvent composition on amide proton chemical shift indicated an intramolecular hydrogen bond between the N'-methylamide proton and the acetamide carbonyl for the major conformer of dipeptides (S)-1, that became less favorable in (R)-1 and 2. The coupling constant (3J(NH,alpha)) values for the cis-isomer of (R)-1 indicated a phi2 dihedral angle value characteristic of a type VIb beta-turn conformation in solution. X-ray crystallographic analysis of N-acetyl-D-leucyl-5-tert-butylproline N'-methylamide (R)-lb showed the prolyl residue in a type VIb beta-turn geometry possessing an amide cis-isomer and psi3-dihedral angle having a value of 157 degrees, which precluded an intramolecular hydrogen bond. Intermolecular hydrogen bonding between the leucyl residues of two turn structures within the unit cell positioned the N-terminal residue in a geometry where their phi2 and psi2 dihedral angle values were not characteristic of an ideal type VIb turn. The circular dichroism spectra of tert-butylprolyl peptides (S)- and (R)-1b were found not to be influenced by changes in solvent composition from water to acetonitrile. The type B spectrum exhibited by (S)-1b has been previously assigned to a type VIa beta-turn conformation [Halab L, Lubell WD. J. Org. Chem. 1999; 64: 3312-3321]. The type C spectrum exhibited by the (R)-lb has previously been associated with type II' beta-turn and alpha-helical conformations in solution and appears now to be also characteristic for a type VIb geometry.     

Experimental and theoretical conformation analysis of the O-antigenic polysaccharide from Pseudomonas cepacia strain 3181. II. The polysaccharide

The spatial structure of Pseudomonas cepacia 3181 polysaccharide in aqueous solution is discussed basing on the data of nuclear Overhauser effect, observed with preirradiation of anomeric protons of all 3 D-rhamnose residues in the repeat unit, and theoretical conformational analysis. It is shown that conformational states of the free disaccharides and corresponding disaccharide units of the polysaccharide are similar. All conformations of the polysaccharide may be described by one shape representing an extended structure with characteristic turns in the D-Rha alpha 1-2-D-Rha beta 1-3-D-Rha units.     

Conformations of cyclic peptides. V. A proton magnetic resonance study of evolidine, cyclo-ser-phe-leu-pro-val-asn-leu

The 220 MHz proton magnetic resonance spectrum of the cyclic heptapeptide evoli-dine, cyclo-Ser-Phe-Leu-Pro-Val-Asn-Leu, has been analyzed. From the temperature dependence of chemical shift of the peptide protons in dimethyl sulfoxide, it is concluded that the peptide protons of the Asn and Phe residues are shielded from the solvent. This observation and H-C_α-N-H dihedral angles, estimated from the corresponding coupling constants, are combined in a proposed conformation of the peptide backbone. The consistency of this conformation with other proton magnetic resonance observations is discussed.     

Conformational study of glycopeptides. Asn-containing peptides and their glycosylated derivatives

The conformational feature has been studied by n.m.r. spectroscopy on the compounds, Boc-Asn-NHMe, Boc-Asn-Gly-NHMe, Boc-Gly-Asn-NHMe, and their glycosylated derivatives. From the temperature dependence of the amide proton chemical shifts and vicinal coupling constants, little change was confirmed in the peptide conformation upon N-glycosylation. There is no particular intramolecular interaction between the peptide and carbohydrate moieties. Boc-Asn-Gly-NHMe takes, to some extent, a folded structure with a hydrogen bond involving the amide proton of N-methylamide group. This backbone conformation is also preferable in the corresponding glycopeptide.     

The conformational characteristics in solution of the synthetic cyclic hexapeptide 5L-ala-D-ala

An attempt to elucidate the solution conformation(s) of the synthetic cyclic hexapeptide 5L -ala·D-ala is described. Nuclear magnetic resonance (nmr) spectra are recorded for the purpose of measuring the vicinal coupling constant between the amide and α-protons in each residue and to observe the deuterium exchange rate and temperature dependence of the chemical shift of each amide proton. Low-energy cyclic conformations, whose individual residues are in conformations consistent with the observed amide to α-proton coupling constant, are searched for in an approximate theoretical treatment. The two lowest energy, all trans peptide bond conformations generated are distinguishable by the presence or absence of a single intramolecular hydrogen bond. The observed temperature independence of the chemical shift of one of the amide protons is consistent with the presence of a single intramolecular hydrogen bond, while the observation of similar deuterium exchange rates for each of the amide protons indicates their comparable availability to solvent. Consequently, it is concluded that 5L -ala·D-ala is in rapid equilibrium between conformations with and without a single internal hydrogen bond and possesses considerable conformational flexibility in solution.     

Effect of side chains on the conformational energy and rotational strength of the n-pi transition for some alpha-helical poly-alpha-amino acids

Calculations of the dependence of the conformational energy and the rotational strength of the amide n–π* electronic transition (in a series of α-helical polyhel-α- amino acids with different side chains) on conformation have been carried out. The conformational energies were computed by procedures developed in this laboratory; the computation of rotational strengths was carried out by the method of Schellman and Oriel, with a slight modification. Polyamino acids with both nonpolar and polar side chains were considered; in the latter case, it was assumed that the only influence of the polar side chain was on the backbone conformation and on the electrostatic field which perturbs the amide chromophore of the backbone. Only conformations in the range of backbone dihedral angles of the right- and left-handed a-helices were considered, and the assumption of regularity (i.e., uniformity of dihedral angles in every residue) was made. The rotational strength per residue was found to vary markedly with chain length (in oligomers of up to 40 residues long); both the conformational energy per residue and the rotational strength per residue were found to vary significantly with the backbone conformation, which in turn depends on the nature of the side chain. The geometry of the hydrogen bond in the α-helical backbone is the most important factor which influences the dependence of the rotational strength on conformation. The implications of these results, for the interpretation of experimental circular dichroism and optical rotatory dispersion data, are discussed.