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118 Synthesis and Biological Applications of Glycoconjugates Reymond and Darbre reasonable prices, which renders the synthesis quite affordable and potentially scalable using the existing industrial infrastructure for SPPS. The split-and-mix protocol for assembling peptide libraries by SPPS [29-33], which was a founding experiment in combinatorial chemistry, can be applied to prepare the corresponding peptide dendrimer libraries on solid support [34-36]. In these so-called “one-bead-one-compound” (OBOC) libraries, each bead of the solid support carries only one possible sequence, yet a large number of different sequences are assembled simultaneously in the course of the synthesis. The number of different sequences accessible in a split-and-mix synthesis is limited by the number of beads used in the SPPS synthesis batch. A typical synthesis batch of 500 mg of 90 micrometer diameter beads contains approximately one million beads, which imposes a limit of 100,000 sequences for the library size to ensure 10-fold coverage of sequence space. Synthesis of Glycopeptide Dendrimers While the preparation of natural glycopeptides requires sophisticated chemical or enzymatic glycosylation chemistry, the decoration of synthetic macromolecules with carbohydrates is often more conveniently realized by reacting preassembled glycosidic building blocks by simpler coupling chemistries. Before its adaptation to combinatorial chemistry, we first investigated various glycosylation options for peptide dendrimers in a synthetic study aimed at the dendrimer mediated delivery of colchicine to cancer cells. Three different strategies were investigated to obtain glycopeptide dendrimers [37]. The first approach used oxime bond formation between a multivalent aldehyde-terminated peptide dendrimer and an oxyamine functionalized glycoside, as reported previously for the glycosylation of peptides [38, 39] and peptide dendrimers [40]. The second approach used reductive alkylation of the free N-terminus with lactose [41, 42], a method previously known for modifying whole proteins [43-45] and reported for the synthesis of glycodendrimers [46]. The third approach involved amide bond formation between an acetyl-protected glycosysidic carboxylic acid building block and the Ntermini of the peptide dendrimer directly on the solid support. Although this third approach required multigram quantities of carbohydrate building blocks, the synthesis was the most practical because it required only a single chromatographic purification and yielded products of excellent purity. For example the N-acetyl-glucosamine terminated peptide dendrimer colchicine conjugate A, which shows selective cytotoxicity to cancer cells, was obtained by this approach (Fig. 2). OH OH O HO AcNH O O O O HN H N O NH NH O N H N O OH O AcHN HO OH O S S N H O NH O MeO Figure 2: A cytotoxic colchicine glycopeptide dendrimer conjugate. NH OH OH O HO AcNH O HN H N N H NH O H N OMe O O OMe N O HN O O OH OMe A HN NH2 O NH O O HO AcNH O O O N H OH OH
Cyclopeptide-Based Glycoclusters Synthesis and Biological Applications of Glycoconjugates, 2011, 129-144 129 Olivier Renaudet * , Didier Boturyn and Pascal Dumy Olivier Renaudet and Nicolas Spinelli (Eds) All rights reserved - © 2011 Bentham Science Publishers CHAPTER 8 Département de Chimie Moléculaire, UMR-CNRS 5250 & ICMG FR 2607, Université Joseph Fourier, 38041 Grenoble Cedex 9, France Abstract: The emergence of glycomics has deeply stimulated the design of new bioactive glycoclusters over the past decade. Among the increasing number of multivalent synthetic structures reported so far, cyclopeptide-based glycoclusters (CBGs) have recently shown promising interest for diverse biological applications. This chapter aims at describing the major advances in this field, with a special focus on the inhibition of carbohydrate-protein interactions and the synthetic vaccines. Keywords: Cyclopeptide, glycocluster, oxime, combinatorial chemistry, synthetic vaccine, lectin. INTRODUCTION Decoding carbohydrate/protein interactions involved in biological events [1, 2] remains crucial for the design of bioactive molecules. The “glycoside cluster effect” introduced by Lee et al. in 1995 describes that these complex recognition processes are regulated by simultaneous and cooperative contacts between cluster of glycans and multimeric proteins [3]. While their kinetic and thermodynamic parameters are still not fully understood [4-6], diverse mechanisms such as chelating, proximity/statistical or clustering effects were proved to promote high selectivity and strong binding improvement, depending on the structural features of both ligand and protein [7-9]. These findings have inspired bioorganic chemists to develop diverse molecules capable of mimicking the multivalent display of carbohydrates expressed in living systems, namely glycoclusters, with the aim to study multivalency effects or to inhibit biological processes [10-12]. Although impressive subnanomolar affinities were sometimes achieved [13], most of studies reported so far clearly highlight that the design of an optimal structure for an amplified biological effect remains extremely difficult to rationalise. The usual design to develop biorecognizable glycoclusters consists in grafting multiple copies of carbohydrates onto a carrier platform, so that they bind several, or ideally, every protein binding sites simultaneously. Since the biological potency of glycoclusters closely depends on the tridimensional protein structure, a large variety of synthetic structures exhibiting variable topology, flexibility, valency and sugar distribution were explored. In particular, linear (e.g. polymers [7, 14], oligonucleotides [15-17]), cyclic (e.g. calixarenes [18, 19], cyclodextrins [20], fullerenes [21, 22]) and branched platforms (peptide dendrimers [23-25], poly(amidoamine) dendrimers [26]), liposomes or nanoparticles [27, 28] have been intensively investigated in different biological contexts. The use of cyclopeptides as carrier platforms for carbohydrates was more rarely described until recent studies have considerably renewed their interests [29]. This chapter focuses on the synthesis of several cyclopeptide-based glycoclusters (CBGs) that revealed promising biological properties. CYCLOPEPTIDE PLATFORM Pioneering studies by Mutter et al. on protein mimics have introduced an original concept that addresses crucial questions on peptide assembly, protein folding and function [30, 31]. For this purpose, conformationally restricted cyclopeptides were utilized as topological platforms to assemble and stabilize the secondary structure of peptide building blocks in a native-like folding [32]. Following a similar concept, other sophisticated platforms, such as the Regioselectively Addressable Functionalized Templates (RAFT) [33], were further used for the construction of biorecognizable compounds [29, 34]. As illustrated in Fig. 1, a typical cyclopeptide platform is composed of amide bonds in the trans configuration and contains generally two -turn inducers i.e. L-proline-glycine (Pro-Gly in green, Fig. 1A) that constrained the structure into an antiparallel -sheet (yellow rubber, Fig. 1B). *Address correspondence to Olivier Renaudet: Département de Chimie Moléculaire, UMR-CNRS 5250 & ICMG FR 2607, Université Joseph Fourier, 38041 Grenoble Cedex 9, France; E-mail: olivier.renaudet@ujf-grenoble.fr
Page 2 and 3: Synthesis and Biological Applicatio
Page 4 and 5: CONTENTS Foreword i Preface ii List
Page 6 and 7: FOREWORD Attachment of carbohydrate
Page 8 and 9: List of Contributors Didier Boturyn
Page 12 and 13: Bacterial Lectins and Adhesins Synt
Page 14 and 15: Ligands for FimH Synthesis and Biol
Page 16 and 17: 36 Synthesis and Biological Applica
Page 20 and 21: Solving Promiscuous Protein Carbohy
Page 26 and 27: Monovalent and Multivalent Glycocon
Page 28 and 29: 116 Synthesis and Biological Applic
Page 36 and 37: Oligonucleotide-Carbohydrate Conjug
Page 38 and 39: Glycoliposomes and Metallic Glycona
Page 42 and 43: Chemoselective Glycosylation Techni
Page 44 and 45: Glycosidases in Synthesis of Glycom
Page 52 and 53: A Synthesis and Biological Applicat
Page 54: Index Synthesis and Biological Appl

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