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Synthesis and structural characterization of sialic acid-glutamic acid hybrid foldamers as conformational surrogates of alpha-2,8-linked polysialic acid.

ABSTRACT: Surface expression of alpha-(2,8)-linked polymers of sialic acid in adult tissues has been correlated with metastasis of several human cancers. One approach to chemotherapeutic intervention against the spread of these cancers involves the development of immunogenic molecules that elicit an antibody response against alpha-(2,8)-linked polysialic acids. Naturally occurring polysialic acids are not viable candidates because they are present during embryonic development and are recognized as self by the immune system. These natural polymers also have poor pharmacokinetic properties because they are readily degraded by neuraminidase enzymes. We have been interested in developing structural surrogates of polysialic acids in an effort to overcome these limitations. Reported herein are microwave-assisted solid-phase peptide syntheses and structural characterization studies of a series of alpha/delta hybrid peptides derived from Fmoc-Neu2en and Fmoc-Glu(OtBu)-OH. Conformational experiments including circular dichroism, NH/ND exchange, and ROESY in aqueous solution were performed to study the secondary structures of these hybrid foldamers. ROESY data were analyzed with the assistance of XPLOR-NIH that was modified to include parameter and topology files to accommodate unnatural amino acids and the delta amide linkages. The results indicate that stable secondary structure is dependent upon both the amino acid sequence and the configuration of Glu. The most stable foldamer was composed of a total of 6 residues beginning with L-Glu at the carboxy terminus and alternating Neu2en and L-Glu residues. In water, this foldamer adopts a right-handed helical conformation with 3.7 residues per turn, 7.4 A pitch, 5.8 A diameter, and a length of 18.5 A, which is stabilized by both classical C=O...H-N backbone interactions and by pyranose ring O and L-Glu HN H-bonding. These structural features orient the L-Glu carboxylates along the helical backbone with a periodicity that matches the carboxylate positions along the reported G2(+) left-handed helix of alpha-(2,8)-polysialic acid. However, the charge density of the foldamer is one-half that of the natural polymer. These findings provide a fundamental understanding of the factors that influence stable secondary structure in hybrid Neu2en/Glu systems, and the tools we have developed establish a viable platform for the rational design of alpha-(2,8)-polysialic acid surrogates.

SUBMITTER: Saludes JP

PROVIDER: S-EPMC4509571 | biostudies-literature | 2009 Apr

REPOSITORIES: biostudies-literature

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Synthesis and structural characterization of sialic acid-glutamic acid hybrid foldamers as conformational surrogates of alpha-2,8-linked polysialic acid.

Saludes Jonel P JP Ames James B JB Gervay-Hague Jacquelyn J

Journal of the American Chemical Society 20090401 15

Surface expression of alpha-(2,8)-linked polymers of sialic acid in adult tissues has been correlated with metastasis of several human cancers. One approach to chemotherapeutic intervention against the spread of these cancers involves the development of immunogenic molecules that elicit an antibody response against alpha-(2,8)-linked polysialic acids. Naturally occurring polysialic acids are not viable candidates because they are present during embryonic development and are recognized as self by ...[more]

PMID: 19323529

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Project description:Every cell in nature carries a rich surface coat of glycans, its glycocalyx, which constitutes the cell's interface with its environment. In eukaryotes, the glycocalyx is composed of glycolipids, glycoproteins, and proteoglycans, the compositions of which vary among different tissues and cell types. Many of the linear and branched glycans on cell surface glycoproteins and glycolipids of vertebrates are terminated with sialic acids, nine-carbon sugars with a carboxylic acid, a glycerol side-chain, and an N-acyl group that, along with their display at the outmost end of cell surface glycans, provide for varied molecular interactions. Among their functions, sialic acids regulate cell-cell interactions, modulate the activities of their glycoprotein and glycolipid scaffolds as well as other cell surface molecules, and are receptors for pathogens and toxins. In the brain, two families of sialoglycans are of particular interest: gangliosides and polysialic acid. Gangliosides, sialylated glycosphingolipids, are the most abundant sialoglycans of nerve cells. Mouse genetic studies and human disorders of ganglioside metabolism implicate gangliosides in axon-myelin interactions, axon stability, axon regeneration, and the modulation of nerve cell excitability. Polysialic acid is a unique homopolymer that reaches >90 sialic acid residues attached to select glycoproteins, especially the neural cell adhesion molecule in the brain. Molecular, cellular, and genetic studies implicate polysialic acid in the control of cell-cell and cell-matrix interactions, intermolecular interactions at cell surfaces, and interactions with other molecules in the cellular environment. Polysialic acid is essential for appropriate brain development, and polymorphisms in the human genes responsible for polysialic acid biosynthesis are associated with psychiatric disorders including schizophrenia, autism, and bipolar disorder. Polysialic acid also appears to play a role in adult brain plasticity, including regeneration. Together, vertebrate brain sialoglycans are key regulatory components that contribute to proper development, maintenance, and health of the nervous system.

Project description:Glutamic acid is a commonly used linker to form dimeric peptides with enhanced binding affinity than their corresponding monomeric counterparts. We have previously labeled NOTA-Bn-NCS-PEG3-E[c(RGDyK)]2 (NOTA-PRGD2) [1] with [(18)F]AlF and (68)Ga for imaging tumor angiogenesis. The p-SCN-Bn-NOTA was attached to E[c(RGDyK)]2 [2] through a mini-PEG with a thiourea linkage, and the product [1] was stable at radiolabeling condition of 100 °C and pH 4.0 acetate buffer. However, when the same p-SCN-Bn-NOTA was directly attached to the ?-amine of E[c(RGDfK)]2 [3], the product NOTA-Bn-NCS-E[c(RGDfK)]2 [4] became unstable under similar conditions and the release of monomeric c(RGDfK) [5] was observed. The purpose of this work was to use HPLC and LC-MS to monitor the decomposition of glutamic acid linked dimeric peptides and their NOTA derivatives. A c(RGDyK) [6] and bombesin (BBN) [7] heterodimer c(RGDyK)-E-BBN [8], and a dimeric bombesin E(BBN)2 [9], both with a glutamic acid as the linker, along with a model compound PhSCN-E[c(RGDfK)] [10] were also studied. All the compounds were dissolved in 0.5 M pH 4.0 acetate buffer at the concentration of 1 mg/mL, and 0.1 mL of each sample was heated at 100 °C for 10 min and the more stable compounds were heated for another 30 min. The samples at both time points were analyzed with analytical HPLC to monitor the decomposition of the heated samples. The samples with decomposition were further analyzed by LC-MS to determine the mass of products from the decomposition for possible structure elucidation. After 10 min heating, the obvious release of c(RGDfK) [5] was observed for NOTA-Bn-NCS-E[c(RGDfK)]2 [4] and Ph-SCN-E[c(RGDfK)] [10]. Little or no release of monomers was observed for the remaining samples at this time point. After further heating, the release of monomers was clearly observed for E[c(RGDyK)]2 [2], E[c(RGDfK)]2 [3], c(RGDyK)-E-BBN [8], and E(BBN)2 [9]. No decomposition or little decomposition was observed for NOTA-Bn-NCS-PEG3-E[c(RGDyK)]2 [1], PEG3-E[c(RGDyK)]2 [11], NOTA-E[c(RGDyK)]2 [12], and NOTA-PEG3-E[c(RGDyK)]2 [13]. The glutamic acid linked dimeric peptides with a free ?-amine are labile due to the neighboring amine participation in the hydrolysis. The stability of peptides could be increased by converting the free amine into amide. The instability of thiourea derivatives formed from ?-amine was caused by participation of thiol group derived from thiourea.

Dataset Information

Synthesis and structural characterization of sialic acid-glutamic acid hybrid foldamers as conformational surrogates of alpha-2,8-linked polysialic acid.

Publications

Synthesis and structural characterization of sialic acid-glutamic acid hybrid foldamers as conformational surrogates of alpha-2,8-linked polysialic acid.

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