Synthesis and Structural Characterization of Sialic Acid−Glutamic Acid Hybrid Foldamers as Conformational Surrogates of α-2,8-Linked Polysialic Acid

Abstract

Surface expression of α-(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 α-(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 α/δ 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 in order 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 δ 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 Å pitch, 5.8 Å diameter, and a length of 18.5 Å, 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 α-(2,8)-polysialic acid. However, the charge density of the foldamer is 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 α-(2,8)-polysialic acid surrogates.

Identifying selective inhibitors against the human cytosolic sialidase NEU2 by substrate specificity studies

“…24,36 The synthesis of 11 – 14 from 19 – 22 was achieved by following a reported method. 40,41 Briefly, compounds 19 – 22 were fully protected by esterification and acetylation in two steps 42 to produce peracetylated methyl esters 23 – 26 in excellent yields (92–99%). These esters then went through trimethylsilyl trifluoromethanesulfonate (TMSOTf)-catalyzed elimination 40 to produce 2,3-dehydro-2-deoxy products 27 – 30 in 80–92% yields.…”

One-pot SSA-catalyzed β-elimination: an efficient and inexpensive protocol for easy access to the glycal of sialic acid

“…2,4 Specifically, there is a great deal of interest in Neu5Ac derivatives related to 6 as influenza strains resistant to currently available antivirals have emerged, sparking increased interest in better neuraminidase inhibitors many of which are based on the Neu5Ac2en core ( 7 ). 5 Further, as part of our continuing research program on advancing sugar amino acid derivatives like 10 as building blocks for peptide foldamers, 6–8 we are interested in developing expedient and simple methods to access 7 .…”

“…In general, the synthesis of 9 is achieved in 2 to 3 steps from Neu5Ac1Me ( 2 ) and traditionally requires several days to complete (Scheme 1). 6,9–11 Many efforts have gone towards making Neu5Ac derivatives more accessible for large-scale use and several groups have reported microwave-assisted techniques that significantly reduced the time required to access these intermediates and allowed the use of more benign reagents. 12–16 Considering that both are esterification reactions, we hypothesized that both processes can be efficiently accomplished by utilizing solid supported acid catalyst coupled with microwave irradiation.…”

The Remarkable Stability of Chimeric, Sialic Acid‐derived α/δ‐Peptides in Human Blood Plasma

“…One source of unnatural amino acid is sialic acid (Neu5Ac), a δ‐sugar amino acid. We recently reported the solid phase peptide synthesis (SPPS) and conformational analysis of chimeric α/δ peptides (4) comprised of the Neu5Ac derivative Fmoc‐Neu2en ( 1 ), Fmoc‐ l ‐Glu ( 2 ), and Fmoc‐ d ‐Glu ( 3 ) building blocks (Figure 1). This research endeavor was aimed at the designing and synthesizing of structural surrogates of polysialic acid (PSA) that are capable of eliciting a cross‐reactive immune response to PSA.…”

“…A critical vaccine design feature is to incorporate metabolically stable functionality that gives rise to stable conformational surrogates. From the four synthetic designs of our chimeric α/δ peptides (4), one compound ( 8 ) was found to assume a stable helix in aqueous medium that possesses a similar conformation and periodicity of carboxylate groups with G2 + (left‐handed helix, n = 4), albeit with one‐half the charge distribution. It is possible that antibodies could be elicited by peptide 8 that may be cross‐reactive toward G2 + that could become a template for cancer vaccine development.…”