Isomeric Amides Research Articles

Selective Bi‐directional Amide Bond Cleavage ofN‐Methylcysteinyl Peptide

AbstractA selective bi‐directional peptide bond cleavage mediated byN‐methylcysteine (MeCys) in Xaa‐MeCys‐Yaa peptides (Xaa and Yaa, non‐cysteine residues) leading to thioesters and thiolactones is described. Rate and product analyses showed that an Nα‐amide bond cleavage occurred at the Xaa‐MeCys bond by an N–S acyl shift to generate an Xaa‐S‐(MeCys‐Yaa) thioester at pH 1–5, whereas under strongly acidic conditions ofH0= –5, the MeCys‐Yaa bond underwent a Cα‐amide bond cleavage via an oxazolone intermediate, which was trapped by thiocresol (TC) as an Xaa‐MeCys‐TC thioester. This thioester was then transformed into an Xaa‐MeCys‐β‐thiolactone at pH 4–5. Replacing MeCys by a Cys residue did not result in significant bi‐directional peptide bond cleavage, which suggests thatN‐methylation in a MeCys residue is important for the N–S acyl shift reaction and formation of oxazolone. The isomerization of amides and thioesters was successfully used to prepare cyclic peptides.

Ruthenium Hydride/Brønsted Acid-Catalyzed Tandem Isomerization/N-Acyliminium Cyclization Sequence for the Synthesis of Tetrahydro-β-carbolines

This paper describes an efficient tandem sequence for the synthesis of 1,2,3,4-tetrahydro-β-carbolines (THBCs) relying on a ruthenium hydride/Brønsted acid-catalyzed isomerization of allylic amides to N-acyliminium ion intermediates which are trapped by a tethered indole nucleophile. The methodology provides not only a convenient "aldehyde-free" alternative to the classical Pictet-Spengler reaction but also attractive possibilities for total synthesis, including rapid generation of molecular complexity and formation of quaternary stereogenic centers. TBHCs can also be accessed by harnessing the Suzuki cross-coupling reaction to the isomerization/N-acyliminium cyclization sequence. Finally, diastereo- and enantioselective versions of the title reaction have been examined using substrate control (with dr >15: 1) and asymmetric catalysis (ee up to 57%), respectively.

Synthesis of Nitrogen‐Containing Derivatives of (18α,19β)‐19‐Hydroxy‐2,3‐secooleanane‐2,3,28‐trioic Acid 28,19‐Lactone

AbstractThe object of this study is the interaction of the cyclic anhydride 2 of (18α,19β)‐19‐hydroxy‐2,3‐secooleanane‐2,3,28‐trioic acid 28,19‐lactone (1) with primary and secondary amines. It was shown that the products of steric control (the corresponding 2‐amino‐2‐oxo‐3‐oic acids=2‐amides) were formed solely upon the opening of the anhydride cycle by secondary amines (Scheme 2), whereas the interaction with primary amines yielded a mixture of isomeric amides (Scheme 10). In the latter case, the solvent provided a noticeable effect on the reaction selectivity, which was demonstrated in the case of 4‐methoxybenzylamine. The interaction between the resulting 3‐amides and oxalyl chloride yielded the corresponding cyclic imides, whereas under these conditions, 2‐amides formed spiropyrrolidinetriones (Scheme 4).

Allenamides: a powerful and versatile building block in organic synthesis.

Allenamides: a powerful and versatile building block in organic synthesis.

Synthesis of Protected 2-Pyrrolylalanine for Peptide Chemistry and Examination of Its Influence on Prolyl Amide Isomer Equilibrium

Protected enantiopure 2-pyrrolylalanine was synthesized for application in peptide science as an electron-rich arylalanine (histidine) analog with π-donor capability. (2S)-N-(Boc)-N'-(Phenylsulfonyl)-, (2S)-N,N'-bis-(phenylsulfonyl)-, and (2S)-N,N'-bis-(Boc)-3-(2-pyrrolyl)alanines (10, 3, and 14, respectively) were made in 13-17% overall yields and six to seven steps from oxazolidine β-methyl ester 4. Homoallylic ketone 5 was prepared by a copper-catalyzed cascade addition of vinylmagnesium bromide to ester 4 and converted to pyrrolyl amino alcohol 7 by olefin oxidation and Paal-Knorr condensation. Protecting group shuffle and oxidation of the primary alcohol enabled the synthesis of pyrrolylalanines. The bis-Boc analog 14 proved useful in peptide chemistry and was employed to make N-acetyl-pyrrolylalaninyl-proline N''-methylamide 25. A study of the influence of the pyrrole moiety on the prolyl amide isomer equilibrium of 25 using (1)H NMR spectroscopy in chloroform, DMSO, and water demonstrated that the pyrrolylalanine peptide exhibited behavior and conformations different from those of other arylalanine analogs.

The stereochemistry of isoquinoline Reissert compounds: a unique platform for observation of steric and electronic interactions

Isoquinoline Reissert compounds (2-acyl-1,2-dihydroisoquinaldonitriles) with either 3-H (1) or 3-CH3 (2) substituents and various N-acyl groups have been examined in detail by 1H and 13C NMR spectroscopy and X-ray crystallography. In all cases the trans amide conformation, with reference to the carbonyl oxygen and the 3-position of the isoquinoline ring, predominates in solution. In the solid state the nitrile moieties are pseudo-axial and the amides exist almost exclusively in the trans form, except for the case of 2-isobutyryl-3-methyl-1,2-dihydroisoquinaldonitrile (2c), which exists exclusively as the cis amide form in the solid state. In N-aroyl 3-CH3 compounds with two ortho-aroyl substituents both amide isomerism and hindered aryl/carbonyl rotation are observed by 1H NMR spectroscopy. In other N-aroyl derivatives only hindered aryl/carbonyl rotations are observed by NMR and in N-alkanoyl compounds amide isomerism is observable only at very low temperatures. X-ray crystallography reveals the two rotamers in the solid state in four cases of ortho-substituted benzoyl compounds; with one exception, the rotamer with the larger ortho-aroyl substituent syn to the pseudo-axial cyano group is favored. Unusual solubility and reactivity patterns observed with these compounds are rationalized in terms of the interplay between steric and electronic factors.

Influence of a gamma amino acid on the structures and reactivity of peptide a3 ions

Collision-induced dissociation of protonated AGabaAIG (where Gaba is gamma-amino butyric acid, NH2–(CH2)3–COOH) leads to an unusually stable a3 ion. Tandem mass spectrometry and theory are used here to probe the enhanced stability of this fragment, whose counterpart is not usually observed in CID of protonated peptides containing only alpha amino acids. Experiments are carried out on the unlabelled and 15N-Ala labeled AGabaAIG (labeled separately at residue one or three) probing the b3, a3, a3-NH3 (a3*), and b2 fragments while theory is used to characterize the most stable b3, a3, and b2 structures and the formation and dissociation of the a3 ion. Our results indicate the AGabaA oxazolone b3 isomer undergoes head-to-tail macrocyclization and subsequent ring opening to form the GabaAA sequence isomer while this chemistry is energetically disfavored for the AAA sequence. The AGabaA a3 fragment also undergoes macrocyclization and rearrangement to form the rearranged imine–amide isomer while this reaction is energetically disfavored for the AAA sequence. The barriers to dissociation of the AGabaA a3 ion via the a3→b2 and a3→a3* channels are higher than the literature values reported for the AAA sequence. These two effects provide a clear explanation for the enhanced stability of the AGabaA a3 ion.

Conformational Study of the Hydroxyproline–O–Glycosidic Linkage: Sugar–Peptide Orientation and Prolyl Amide Isomerization in (α/β)–Galactosylated 4(R/S)–Hydroxyproline

Glycosylation is a frequent post-translational modification of proteins that has been shown to influence protein structure and function. Glycosylation of hydroxyproline occurs widely in plants, but is absent in humans and animals. Previous experimental studies on model amides have indicated that α/β-galactosylation of 4R-hydroxyproline (Hyp) has no measurable effect on prolyl amide isomerization, while a 7% increase in the trans isomer population, as well as a 25-50% increase in the isomerization rate, was observed for the 4S stereoisomer (hyp). In this work, molecular dynamics simulations in explicit water and implicit solvent DFT optimizations are used to examine the structure of the hydroxyproline-O-galactosyl linkage and the effect of glycosylation on the structure and cis/trans isomerization of the peptide backbone. The calculations show two major minima with respect to the glycosidic linkage in all compounds. The C(γ)-exo puckering observed in 4R compounds projects the sugar away from the peptide backbone, while a twisted C(γ)-endo/C(β)-exo pucker in the 4S compounds brings the peptide and sugar rings together and leads to an intramolecular hydrogen-bonding interaction that is sometimes bridged by a water molecule. This hydrogen bond changes the conformation of the peptide backbone, inducing a favorable n → π* interaction between the oxygen lone pair from the prolyl N-terminal amide and the C═O, which explains the observed increase in trans isomer population in α/β-galactosylated 4S-hydroxyproline. Our results provide the first molecular level information about this important glycosidic linkage, as well as provide an explanation for the previously observed increase in trans isomer population in 4S-hyp compounds. Moreover, this study provides evidence that sugar-mediated long-range hydrogen bonding between hydroxyl groups and the carbonyl peptide backbone can modify the properties of N-terminal prolyl cis/trans isomerization in peptides.

A Mechanistic and Structural Analysis of the Inhibition of the 90-kDa Heat Shock Protein by the Benzoquinone and Hydroquinone Ansamycins

The benzoquinone ansamycins inhibit the ATPase activity of the 90-kDa heat shock protein (Hsp90), disrupting the function of numerous client proteins involved in oncogenesis. In this study, we examine the role of NAD(P)H:quinone oxidoreductase 1 (NQO1) in the metabolism of trans- and cis-amide isomers of the benzoquinone ansamycins and their mechanism of Hsp90 inhibition. Inhibition of purified human Hsp90 by a series of benzoquinone ansamycins was examined in the presence and absence of NQO1, and their relative rate of NQO1-mediated reduction was determined. Computational-based molecular docking simulations indicated that the trans- but not the cis-amide isomers of the benzoquinone ansamycins could be accommodated by the NQO1 active site, and the ranking order of binding energies correlated with the relative reduction rate using purified human NQO1. The trans-cis isomerization of the benzoquinone ansamycins in Hsp90 inhibition has been disputed in recent reports. Previous computational studies have used the closed or cocrystallized Hsp90 structures in an attempt to explore this isomerization step; however, we have successfully docked both the trans- and cis-amide isomers of the benzoquinone ansamycins into the open Hsp90 structure. The results of these studies indicate that both trans- and cis-amide isomers of the hydroquinone ansamycins exhibited increased binding affinity for Hsp90 relative to their parent quinones. Our data support a mechanism in which trans- rather than cis-amide forms of benzoquinone ansamycins are metabolized by NQO1 to hydroquinone ansamycins and that Hsp90-mediated trans-cis isomerization via tautomerization plays an important role in subsequent Hsp90 inhibition.

Quantum-chemical studies of amide–iminol tautomerism for inhibitor of lactate dehydrogenase: Oxamic acid

Amide–iminol tautomerism was studied for oxamic acid (OA) in the gas phase using various quantum-chemical methods (HF, MP2, DFT, and G2). All possible twenty tautomers–rotamers, four amide and sixteen iminol forms, were considered for the neutral molecule. At each level of theory, the amidization process (amide←iminol) is favored for OA similarly as for the parent compound (formamide). Three amide and fourteen iminol rotamers of OA are found to be thermodynamically stable, among which the intramolecularly H-bonded amide structure has the lowest Gibbs free energy (G) in the gas phase. Low relative energies for the two other amide structures indicate that all of them may be present in the tautomeric mixture of OA. The G value of the most stable iminol structure is larger than those of the three amide isomers by ca. 10kcalmol−1. This suggests that the amide form(s) of OA may be responsible for binding with large biomolecules (proteins, enzymes, receptors) and particularly for bioseparation of lactate dehydrogenase (LDH).

Intramolecular Hydrogen Bond-Controlled Prolyl Amide Isomerization in Glucosyl 3(S)-Hydroxy-5-hydroxymethylproline Hybrids: A Computational Study

Peptide mimics containing spirocyclic glucosyl-(3(S)-hydroxy-5(S)-hydroxymethyl)proline (1) and glucosyl-(3(S)-hydroxy-5(R)-hydroxymethyl)proline (2) hybrids differing in the stereochemistry of the polar hydroxymethyl substitutent at the C-5- or (C(delta))-position have been investigated computationally. A computational "build and search" protocol of molecular mechanics systematic search/Monte Carlo search, followed by density functional theory (DFT), has been developed to ensure complete coverage of the large conformational space. Gas-phase DFT optimizations at the B3LYP level of theory lead to a strong preference for the cis conformation in the prolyl amide bond for both compounds 1 and 2. However, inclusion of the solvent water by means of continuum solvation (PCM) results in a reduction of the prolyl amide cis population in both compounds, leading to good agreement with previous experimental observations. Intramolecular hydrogen bonding involving the C-5-hydroxymethyl substitutent is seen to play a crucial role to tune the thermodynamics of prolyl amide cis/trans isomerization and is responsible for the high cis prolyl amide population in compound 2. Our results indicate that H-bond-forming substituents like the hydroxymethyl group at the C-5-position in proline can be used to control cis/trans prolyl amide isomerization. High cis prolyl amide conformer populations can be achieved by proper choice of the stereochemistry at the C-5-position.

The Implications of (2S,4S)‐Hydroxyproline 4‐O‐Glycosylation for Prolyl Amide Isomerization

The conformations of peptides and proteins are often influenced by glycans O-linked to serine (Ser) or threonine (Thr). (2S,4R)-4-Hydroxyproline (Hyp), together with L-proline (Pro), are interesting targets for O-glycosylation because they have a unique influence on peptide and protein conformation. In previous work we found that glycosylation of Hyp does not affect the N-terminal amide trans/cis ratios (K(trans/cis)) or the rates of amide isomerization in model amides. The stereoisomer of Hyp--(2S,4S)-4-hydroxyproline (hyp)--is rarely found in nature, and has a different influence both on the conformation of the pyrrolidine ring and on K(trans/cis). Glycans attached to hyp would be expected to be projected from the opposite face of the prolyl side chain relative to Hyp; the impact this would have on K(trans/cis) was unknown. Measurements of (3)J coupling constants indicate that the glycan has little impact on the C(gamma)-endo conformation produced by hyp. As a result, it was found that the D-galactose residue extending from a C(gamma)-endo pucker affects both K(trans/cis) and the rate of isomerization, which is not found to occur when it is projected from a C(gamma)-exo pucker; this reflects the different environments delineated by the proline side chain. The enthalpic contributions to the stabilization of the trans amide isomer may be due to disruption of intramolecular interactions present in hyp; the change in enthalpy is balanced by a decrease in entropy incurred upon glycosylation. Because the different stereoisomers--Hyp and hyp--project the O-linked carbohydrates in opposite spatial orientations, these glycosylated amino acids may be useful for understanding of how the projection of a glycan from the peptide or protein backbone exerts its influence.

Cis−Trans Proline Isomerization Effects on Collagen Triple-Helix Stability Are Limited

We investigated the effect of restricting cis-trans proline isomerization on collagen triple-helix stability. The Pro residues at the Xaa and Yaa positions of an (Xaa-Yaa-Gly) triplet were replaced by a Pro-trans-Pro alkene isostere in the host-guest peptide, H-(Pro-Pro-Gly)(10)-OH. The resulting alkene isostere peptide had a T(m) value 53.6 degrees C lower than that of the control peptide. The Pro-trans-Pro alkene isostere peptide had a T(m) value 3.9 degrees C higher than that of the previously reported Pro-trans-Gly alkene isostere peptide that did not involve cis-trans Pro isomerization (Jenkins, C. L.; Vasbinder, M. M.; Miller, S. J.; Raines, R. T. Org. Lett. 2005, 7, 2619-22). Thus, single cis-trans proline amide isomerization alone has limited contribution to the overall stability of the collagen triple helix. Since collagen has a high content of imino acid residues, the cumulative effects of cis-trans isomerization may be quite significant. The peptide containing the Pro-trans-Pro isostere was significantly less stable than the previously reported Gly-trans-Pro alkene isostere peptide that retained the backbone interchain hydrogen bond (Dai, N.; Wang, X. J.; Etzkorn, F. A. J. Am. Chem. Soc. 2008, 130, 5396-5397), which confirms that direct interchain backbone hydrogen bonding is a major force for stabilizing the collagen triple helix.

Thermodynamic cycle for the calculation of ab initio p Ka values for hydroxamic acids

A thermodynamic cycle to calculate p K a values (Minus log of acid dissociation constants) of hydroxamic acids is presented. Hydroxamic acids exist mainly as amide isomers in the aqueous medium. The amide form of hydroxamic acids has two deprotonation sites and may yield either an N-ion or an O-ion upon deprotonation. The thermodynamic cycle proposed includes the gas-phase N–H deprotonation of the hydroxamic acid, the solvent phase transformation of the N-ion to the O-ion and the solvation of the hydroxamic acid molecule and the O-ion in water. The CBS-QB3 method was employed to obtain gas-phase free energy differences between 12 hydroxamic acids and their respective anions. The aqueous solvation Gibbs free energy changes were calculated at the HF/6-31G(d)/CPCM and HF/6-31+G(d)/CPCM levels of theory using HF/6-31+G(d)/CPCM geometries. For the proton, literature values of the gas-phase free energy of formation and the solvation free energy change were used. The free energy change for the transformation of the N-ion to O-ion in the aqueous medium was calculated by employing CBS-QB3/CPCM in the aqueous medium. For this, the hydroxamic acids were divided in two classes according to the substituent at the carbonyl carbon. A common transformation free energy difference for aliphatic substituted hydroxamic acids and a separate common transformation free energy difference for aromatic substituted hydroxamic acids were obtained. The p K a calculation yielded a root mean square error of 0.32 p K a units.

Intramolecular Hydrogen Bond-Controlled Prolyl Amide Isomerization in Glucosyl 3′(S)-Hydroxy-5′-hydroxymethylproline Hybrids: Influence of a C-5′-Hydroxymethyl Substituent on the Thermodynamics and Kinetics of Prolyl Amide Cis/Trans Isomerization

Peptide mimics containing spirocyclic glucosyl-(3'-hydroxy-5'-hydroxymethyl)proline hybrids (Glc3'(S)-5'(CH(2)OH)HypHs) with a polar hydroxymethyl substituent at the C-5' position, such as C-terminal ester Ac-Glc3'(S)-5'(CH(2)OH)Hyp-OMe and C-terminal amide Ac-Glc3'(S)-5'(CH(2)OH)Hyp-N'-CH(3), were synthesized. C-Terminal esters exhibit increased cis population (23-53%) relative to Ac-3(S)HyPro-OMe (17%) or Ac-Pro-OMe (14%) in D(2)O. The prolyl amide cis population is further increased to 38-74% in the C-terminal amide form in D(2)O. Our study shows that the stereochemistry of the hydroxymethyl substituent at the C-5' position of proline permits tuning of the prolyl amide cis/trans isomer ratio. Inversion-magnetization transfer NMR experiments indicate that the stereochemistry of the hydroxymethyl substituent has a dramatic effect on the kinetics of prolyl amide cis/trans isomerization. A 200-fold difference in the trans-to-cis (k(tc)) isomerization and a 90-fold rate difference in the cis-to-trans (k(ct)) isomerization is observed between epimeric C-5' 3 and 4. When compared to reference peptide mimics Ac-Pro-OMe and Ac-3(S)Hyp-OMe, our study demonstrates that a (13-16)-fold decrease in k(tc) and k(ct) is observed for the C-5'(S), while a (5-24)-fold acceleration is observed for the C-5'(R) epimer. DFT calculations indicate that the pyrrolidine ring prefers a C(beta) exo pucker in both Ac-Glc3'(S)-5'(CH(2)OH)Hyp-OMe diastereoisomers. Computational calculations and chemical shift temperature coefficient (Delta delta/Delta T) experiments indicate that the hydroxymethyl group at C-5' in Ac-Glc3'(S)-5'(CH(2)OH)Hyp-OMe forms a stabilizing intramolecular hydrogen bond to the carbonyl of the N-acetyl group in both epimeric cis isomers. However, a competing intramolecular hydrogen bond between the hydroxymethyl groups in the pyrrolidine ring and pyran ring stabilizes the trans isomer in the C-5'(S) diastereoisomer. The dramatic differences in the kinetic properties of the diastereoisomeric peptide mimics are rationalized by the presence or absence of an intramolecular hydrogen bond between the hydroxymethyl substituent located at C-5' and the developing lone pair on the nitrogen atom of the N-acetyl group in the transition state.

Study of the Beckmann rearrangement of acetophenone oxime over porous solids by means of solid state NMR spectroscopy

The Beckmann rearrangement of acetophenone oxime using zeolite H-beta and silicalite-N as catalysts has been investigated by means of (15)N and (13)C solid state NMR spectroscopy in combination with theoretical calculations. The results obtained show that the oxime is N-protonated at room temperature on the acid sites of zeolite H-beta. At reaction temperatures of 423 K or above, the two isomeric amides, acetanilide and N-methyl benzamide (NMB) are formed, and interact with the Brønsted acid sites of zeolite H-beta through hydrogen bonds. The presence of residual water hydrolyzes the two amides, while larger amounts inhibit the formation of NMB and cause the total hydrolysis of the acetanilide. Over siliceous zeolite silicalite-N, containing silanol nests as active sites, the oxime is adsorbed through hydrogen bonds and only acetanilide is formed at reaction temperatures of 423 K or above. In the presence of water, the reaction starts at 473 K, still being very selective up to 573 K, and the amide is partially hydrolyzed only above this temperature .

Preparative argentation reversed-phase high performance liquid chromatography

In this study, a preparative chromatography method named preparative argentation reversed-phase high performance liquid chromatography (Ag-RP-HPLC) was developed by adding silver ion to the mobile phase of preparative HPLC. Firstly, an analytical Ag-RP-HPLC method was developed, and the effects of silver content and acid content in the mobile phase on the column efficiency were studied. Based on the method of linear amplification, a preparative Ag-RP-HPLC technique with optimized separation conditions was developed. The new technique was applied successfully to the separation of the unsaturated aliphatic acid amide isomers contained in Asarum forbesii Maxim. Compared with the commonly used technique of argentation normal phase chromatography, this method with little solvent consumption is simple, fast, efficient, and flexible for the isolation and purification of the unsaturated compounds.

Synthesis and antiproliferative evaluation of amide-containing flavone and isoflavone derivatives

Certain amide-containing flavone and isoflavone derivatives were synthesized and evaluated for their antiproliferative activities. These compounds were synthesized via alkylation of hydroxyl precursors followed by the reaction with H 2SO 4 and NaN 3 (Schmidt reaction). The preliminary assays indicated that the inhibitory activity against the growth of NCI-H661 decreased in an order of linked chromophore flavone-6-yl 16a– d > flavone-7-yl 17a– d > flavone-3-yl 15a– d and isoflavone-7-yl 18a– d. Among these flavone-6-yl derivatives, N-(4-methoxyphenyl)-2-(4-oxo-2-phenyl-4 H-chromen-6-yloxy)acetamide ( 16c) was the most potent with a GI 50 value of 0.84 μM. The inhibitory activity against the growth of NPC-TW01 decreased in an order of linked chromophore flavone-6-yl 16a– d > isoflavone-7-yl 18a– d > flavone-7-yl 17a– d > flavone-3-yl 15a– d. Flavone-6-yl derivatives 16a– d demonstrated significant inhibitory activities against the growth of NPC-TW01 cell with an average GI 50 value of 0.84 μM. The oxime derivatives 1 and 2 caused accumulation of NPC-TW01 cell in G 2/M phase which were distinct from that of their amide isomers 16b and 16c, respectively, which induced cell-cycle arrest in G 0/G 1 phase followed by apoptosis. Therefore, the antiproliferative mechanism of flavone derivatives was affected not only by the phenyl benzopyran-4-one pharmacophore but also by the peripheral substituents.

One‐Step RhCl3‐Catalyzed Deprotection of Acyclic N‐Allyl Amides.

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Effects of Glycosylation of (2S,4R)-4-Hydroxyproline on the Conformation, Kinetics, and Thermodynamics of Prolyl Amide Isomerization

Glycosylation (galactosylation) of (2S,4R)-4-hydroxyproline (Hyp) in the peptide mimic N-acetyl-Hyp-N‘-methylamide does not mediate the isomer equilibrium nor the rate of isomerization between the cis- and trans-prolyl amides in water. However, glycosylation of Hyp results in a downfield shift (6.5−8 ppm) of the Cγ atom of proline that is consistent with an enhanced inductive effect. Moreover, NOE experiments on the α-and β-linked glycosylated AcHypNHMe amides indicate proximity between the galactose and pyrrolidine rings. This study strongly suggests that glycosylation of Hyp will have important implications on peptide backbone conformation