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I. Isolation and Synthesis of Naturally Occurring Carbohydrate/Polysaccharides
Recently, a new class of bacterial polysaccharides, that have the ability to modulate the cellular immune system by eliciting a T-cell response, have been characterized. It has long been known that carbohydrate processing in the immune system occurs through the mechanisms of the MHC I CD8+ pathway but never, until now, were they linked to the MHC II CD4+ pathway. One objective is to prepare well-defined zwitterionic polysaccharides (ZPS), in which the oligosaccharide is amenable to chemical modifications, for the development of bioprobes. Understanding this process, in which carbohydrates are involved, will assist in clarifying the mechanism and potentially give insight on immune responses directed toward carbohydrate processing.
Several immunomodulatory zwitterionic polysaccharides (ZPSs) have been identified from different bacterial species, including capsule polysaccharide from type 1 Streptococcus pneumoniae Sp1, PS A1 and PS B from Bacteroides fragilis strain 9343 and PS A2 from B. fragilis 638 (as illustrated above).
Although they share similar biological properties, these ZPSs have very different chemical structures. For clarity, PS A1 is composed of a branched tetrasaccharide repeating unit with three monosaccharides in the polymer backbone and one residue at the side chain. Its sequence consists of [®3)-b-D-Galp-(1®3)-a-D-Sugp-(1®4)[b-D-Galf -(1®3)]-a-D-GalpNAc-(1®], Sug is 2-acetamido-4-amino-2,4,6-trideoxygalactose. A pyruvate substituent spans O-4 and O-6 of the b-D galactopyranosyl residue. This is not a complete list of the ZPSs that exist in nature as other bacterial species synthesize polymers that have these motifs, however, there are relatively few bacterial polysaccharides that are capable of expressing zwitterionic charge character on their cell surface.
The reaction for the introduction of an oxime will take advantage of a highly a-stereoselective Lichtenthaler-Kaji glycosylation strategy while introducing the second amino group on the 2,4,6-trideoxy substituted D-galactose.
Publications
II. Carbohydrate Based Diversity-Oriented Synthesis
The current state of progress
in glycobiology has opened
a wide field for new
therapeutic undertakings. A diversity
oriented synthesis
(DOS) approach
toward the
construction
of skeletally diverse small molecules
containing
carbohydrates
as a “privileged” scaffold would
allow access to scarcely
inhabited chemical space.
The structures and functions of natural products suggest that structural complexity may be positively correlated with macromolecule-perturbing function and specificity of action. This correlation is particularly striking in small molecules known to disrupt protein–protein interactions. Therefore, one goal pertaining to DOS is to develop small molecules with complex molecular skeletons. Moreover, in contrast to the relatively flat structures often developed in medicinal and combinatorial chemistry that have a tendency to project appendages outward along the perimeter of a circle, one aim in DOS is to access globular or spherical skeletons to which substituents can be potentially appended.
DOS pathways aim to proceed in the direction of similar structures to diverse structures to gain access to broad regions of chemistry space efficiently. To achieve this requires planning a series of products-equals-substrates relationships, that is, the products of one diversity-generating process should share some common inherent chemical reactivity. This common reactivity serves as a keying element that makes the products collective substrates for a subsequent diversity-generating process. The goal of achieving diversity can be simplified by considering distinct diversity elements: appendages, stereochemistry, and skeletons.
This endeavor seeks to examine a diversity oriented synthesis (DOS) approach toward the construction of a class of compounds containing carbohydrate scaffolds as potential therapeutic agents. Using palladium and ruthenium catalysis, reductive diyne cyclization and eneyne metatheses will give rise to various diene-containing skeletons. Subsequently, the Diels-Alder reaction will produce a library of polycyclic ether/zwitterionic small molecules.
Publications
III.
Carbohydrate-Based Vaccines
Current
carbohydrate-based vaccines, while
effective, do not protect against the
carrier protein and, therefore,
an immunogenic response
is heterogeneous in nature.
The objective is to prepare
well-known carbohydrate antigens, such as the
well-defined
polysaccharide
(Man9GlcNAc2-HIV gp120),
in which the
oligosaccharides are linked to a
T-cell inducing
ZPS.
Synthetic vaccine development is a very young field and structural requirements for a satisfactory synthetic vaccine are not currently all that clear. Through the chemical synthesis of a known carbohydrate epitope found on surface of the gp120 protein, the idea is centered on eliciting a humoral (antibody production) and cellular (cytotoxic lymphocytes) immune response by “piggy-backing” on ZPSs. This area of research is highly diverse and not only can these synthetic carbohydrate vaccines be prepared according to the figure illustrated above but other fundamental parameters about the immune system can be answered such as the size of the oligosaccharide hapten required for immunogenicity, the size of the ZPS required for eliciting an immune response and the overall geometry of the antigen. A synthetic carbohydrate vaccine without proteins as carriers and adjuvants as elicitors of immune responses should produce an antibody with Fab portions specific for carbohydrates exclusively. This will ensure specificity in binding as well as a strong binding affinity, vital for immunity against the HIV virus.
Publications
IV. Combinatorial
Carbohydrate Libraries and Carbohydrate-Based Micro Arrays
To determine whether an
alternating charge character
is necessary to elicit a T-cell response, the Andreana
group will take advantage of
the split-pool concept of
combinatorial chemistry
and develop a carbohydrate-based
library with electrostatic charge character.
Despite the difference in their helical compositions and monosaccharide sequences, Sp1, PS A1, PS A2 and PS B share a striking structural feature, i.e., they are all zwitterionic polymers that display a high density of positive and negative charges (refer to Section I). The zwitterionic charge motif, shared by these ZPSs, is rare among natural polysaccharides, leading to the hypothesis that charges play essential roles in immunological function. It would be of interest to determine, systematically, how the electrostatic charges of ZPSs interact with various modulators of APCs. This hypothesis could be supported by several studies in which chemical conversion of charged groups to neutral components (e.g., conversion of primary amines to secondary amines and carboxylic acids to esters) eliminates the activity of ZPSs. Also, adjacent monosaccharide charge character could be altered so that the positively and negatively charged groups would be separated by one or more neutral species. This concept could then be extended to examine the ZPS fragment composition required for T-cell activation (e.g. combining one charged tetrasaccharide core with other neutral fragments, while maintaining the structural integrity).
Publications
1. Yatawara, A. K.; Gopinath, T.; Bordenyuk, A. N.; Andreana, P. R.; Benderskii, A. B. "Carbohydrate Surface Attachment Characterized by Sum Frequency Generation Spectroscopy" Langmuir, 2009, 24, 1901-1904. pdf and S.I.
V. Small Molecule Synthesis
Chemical genetics is a method that uses small molecules to alter the way proteins interact in a natural biological setting. It can be used to identify particular proteins that regulate different biological processes, to understand in molecular detail how proteins perform their biological functions, and to identify small molecules that may be of medicinal value.
We are interested in the MHC II pathway of extracellular pathogen processing specifically with a special class of molecules, namely zwitterionic polysaccharides (as noted above). The MHC II pathway is relevant in immunological processes specifically for monitoring extracellular/exogenous pathogens or foreign invaders. The proposed small molecule development (as illustrated above) ties in extremely well with this platform for the development of modulators (Chemical Genetics) of the immune pathway to determine mechanistic insights of carbohydrate processing in the antigen presenting cell or dendrite cell. The tyrosine kinase p56 (lck) is present in T-cells and is known to be required to initiate the activation response from the T-cell receptor (TCR) intracellular domain to other signaling proteins. T-cells that lack lck are unable to respond to stimulation through the TCR.
Publications
1. Santra, S.; Andreana, P. R. "A One-Pot, Microwave-Influenced Synthesis of Diverse Small Molecules by Multicomponent Reaction Cascades" Organic Letters 2007, 9, 5035-5038. pdf and S.I.
see Faculty of 1000-Biology (Reviewed by Prof. Michael Pirrung - U.C. Riverside)
see Organic Chemistry Portal under Microwave Chemistry Highlights.
see ChemInform.
2. De Silva, R. A.; Santra, S.; Andreana, P. R. "A Tandem One-Pot, Microwave-Assisted Synthesis of Regiochemically Differentiated 1,2,4,5-Tetrahydro-1,4-benzodiazepin-3-ones" Organic Letters 2008, 10, 4541-4544. pdf and S.I.
see Organic Chemistry Portal under Microwave Chemistry Highlights.
see ChemInform.