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Characterization of High Molecular Weight Pneumococcal Conjugate by SEC-MALS and AF4-MALS. Polymers (Basel) 2022; 14:polym14183769. [PMID: 36145915 PMCID: PMC9501040 DOI: 10.3390/polym14183769] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/25/2022] [Revised: 09/06/2022] [Accepted: 09/06/2022] [Indexed: 11/25/2022] Open
Abstract
Infections by Streptococcus pneumoniae can cause serious pneumococcal diseases and other medical complications among patients. Polysaccharide-based vaccines have been successfully developed as prophylactic agents against such deadly bacterial infections. In the 1980s, PNEUMOVAX® 23 were introduced as the first pneumococcal polysaccharide vaccines (PPSV). Later, pneumococcal polysaccharides were conjugated to a carrier protein to improve immune responses. Pneumococcal conjugate vaccines (PCV) such as PREVNAR® and VAXNEUVANCE™ have been developed. Of the more than 90 pneumococcal bacteria serotypes, serotype 1 (ST-1) and serotype 4 (ST-4) are the two main types that cause invasive pneumococcal diseases (IPD) that could lead to morbidity and mortality. Development of a novel multi-valent PCV against these serotypes requires extensive biophysical and biochemical characterizations of each monovalent conjugate (MVC) in the vaccine. To understand and characterize these high molecular weight (Mw) polysaccharide protein conjugates, we employed the multi-angle light scattering (MALS) technique coupled with size-exclusion chromatography (SEC) separation and asymmetrical flow field flow fractionation (AF4). MALS analysis of MVCs from the two orthogonal separation mechanisms helps shed light on the heterogeneity in conformation and aggregation states of each conjugate.
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Deng JZ, Lancaster C, Winters MA, Phillips KM, Zhuang P, Ha S. Multi-attribute characterization of pneumococcal conjugate vaccine by Size-exclusion chromatography coupled with UV-MALS-RI detections. Vaccine 2022; 40:1464-1471. [DOI: 10.1016/j.vaccine.2022.01.042] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2021] [Revised: 01/08/2022] [Accepted: 01/20/2022] [Indexed: 01/06/2023]
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Bazhenova A, Gao F, Bolgiano B, Harding SE. Glycoconjugate vaccines against Salmonella enterica serovars and Shigella species: existing and emerging methods for their analysis. Biophys Rev 2021; 13:221-246. [PMID: 33868505 PMCID: PMC8035613 DOI: 10.1007/s12551-021-00791-z] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2020] [Accepted: 02/25/2021] [Indexed: 12/26/2022] Open
Abstract
The global spread of enteric disease, the increasingly limited options for antimicrobial treatment and the need for effective eradication programs have resulted in an increased demand for glycoconjugate enteric vaccines, made with carbohydrate-based membrane components of the pathogen, and their precise characterisation. A set of physico-chemical and immunological tests are employed for complete vaccine characterisation and to ensure their consistency, potency, safety and stability, following the relevant World Health Organization and Pharmacopoeia guidelines. Variable requirements for analytical methods are linked to conjugate structure, carrier protein nature and size and O-acetyl content of polysaccharide. We investigated a key stability-indicating method which measures the percent free saccharide of Salmonella enterica subspecies enterica serovar Typhi capsular polysaccharide, by detergent precipitation, depolymerisation and HPAEC-PAD quantitation. Together with modern computational approaches, a more precise design of glycoconjugates is possible, allowing for improvements in solubility, structural conformation and stability, and immunogenicity of antigens, which may be applicable to a broad spectrum of vaccines. More validation experiments are required to establish the most effective and suitable methods for glycoconjugate analysis to bring uniformity to the existing protocols, although the need for product-specific approaches will apply, especially for the more complex vaccines. An overview of current and emerging analytical approaches for the characterisation of vaccines against Salmonella Typhi and Shigella species is described in this paper. This study should aid the development and licensing of new glycoconjugate vaccines aimed at the prevention of enteric diseases.
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Affiliation(s)
- Aleksandra Bazhenova
- School of Biosciences, University of Nottingham, Sutton Bonington, Loughborough, LE12 5RD UK
| | - Fang Gao
- Division of Bacteriology, National Institute for Biological Standards and Control (NIBSC), Blanche Lane, South Mimms, Potters Bar, EN6 3QG UK
| | - Barbara Bolgiano
- Division of Bacteriology, National Institute for Biological Standards and Control (NIBSC), Blanche Lane, South Mimms, Potters Bar, EN6 3QG UK
| | - Stephen E. Harding
- School of Biosciences, University of Nottingham, Sutton Bonington, Loughborough, LE12 5RD UK
- Museum of Cultural History, University of Oslo, Postboks 6762 St. Olavs plass, 0130 Oslo, Norway
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MacCalman TE, Phillips-Jones MK, Harding SE. Glycoconjugate vaccines: some observations on carrier and production methods. Biotechnol Genet Eng Rev 2020; 35:93-125. [PMID: 32048549 DOI: 10.1080/02648725.2019.1703614] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
Glycoconjugate vaccines use protein carriers to improve the immune response to polysaccharide antigens. The protein component allows the vaccine to interact with T cells, providing a stronger and longer-lasting immune response than a polysaccharide interacting with B cells alone. Whilst in theory the mere presence of a protein component in a vaccine should be sufficient to improve vaccine efficacy, the extent of improvement varies. In the present review, a comparison of the performances of vaccines developed with and without a protein carrier are presented. The usefulness of analytical tools for macromolecular integrity assays, in particular nuclear magnetic resonance, circular dichroism, analytical ultracentrifugation and SEC coupled to multi-angle light scattering (MALS) is indicated. Although we focus mainly on bacterial capsular polysaccharide-protein vaccines, some consideration is also given to research on experimental cancer vaccines using zwitterionic polysaccharides which, unusually for polysaccharides, are able to invoke T-cell responses and have been used in the development of potential all-polysaccharide-based cancer vaccines.A general trend of improved immunogenicity for glycoconjugate vaccines is described. Since the immunogenicity of a vaccine will also depend on carrier protein type and the way in which it has been linked to polysaccharide, the effects of different carrier proteins and production methods are also reviewed. We suggest that, in general, there is no single best carrier for use in glycoconjugate vaccines. This indicates that the choice of carrier protein is optimally made on a case-by-case basis, based on what generates the best immune response and can be produced safely in each individual case.Abbreviations: AUC: analytical ultracentrifugation; BSA: bovine serum albumin; CD: circular dichroism spectroscopy; CPS: capsular polysaccharide; CRM197: Cross Reactive Material 197; DT: diphtheria toxoid; Hib: Haemophilius influenzae type b; MALS: multi-angle light scattering; Men: Neisseria menigitidis; MHC-II: major histocompatibility complex class II; NMR: nuclear magnetic resonance spectroscopy; OMP: outer membrane protein; PRP: polyribosyl ribitol phosphate; PSA: Polysaccharide A1; Sa: Salmonella; St.: Streptococcus; SEC: size exclusion chromatography; Sta: Staphylococcus; TT: tetanus toxoid; ZPS: zwitterionic polysaccharide(s).
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Affiliation(s)
- Thomas E MacCalman
- National Centre for Macromolecular Hydrodynamics, University of Nottingham, Nottingham, UK
| | - Mary K Phillips-Jones
- National Centre for Macromolecular Hydrodynamics, University of Nottingham, Nottingham, UK
| | - Stephen E Harding
- National Centre for Macromolecular Hydrodynamics, University of Nottingham, Nottingham, UK.,Kulturhistorisk Museum, University of Oslo, Oslo, Norway
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Hennessey JP, Costantino P, Talaga P, Beurret M, Ravenscroft N, Alderson MR, Zablackis E, Prasad AK, Frasch C. Lessons Learned and Future Challenges in the Design and Manufacture of Glycoconjugate Vaccines. CARBOHYDRATE-BASED VACCINES: FROM CONCEPT TO CLINIC 2018. [DOI: 10.1021/bk-2018-1290.ch013] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/04/2022]
Affiliation(s)
| | | | - Philippe Talaga
- Department of Analytical Research and Development, Sanofi Pasteur, Marcy l’Etoile 69280, France
| | - Michel Beurret
- Janssen Vaccines & Prevention B.V., Leiden, 2301 CA, The Netherlands
| | - Neil Ravenscroft
- Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
| | | | - Earl Zablackis
- Analytical Process Technology, Sanofi Pasteur, Swiftwater, Pennsylvania 18370, United States
| | - A. Krishna Prasad
- Pfizer Vaccines Research and Development, Pearl River, New York 10965, United States
| | - Carl Frasch
- Consultant, Martinsburg, West Virginia 25402, United States
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Abdelhameed AS, Morris GA, Almutairi F, Adams GG, Duvivier P, Conrath K, Harding SE. Solution conformation and flexibility of capsular polysaccharides from Neisseria meningitidis and glycoconjugates with the tetanus toxoid protein. Sci Rep 2016; 6:35588. [PMID: 27782149 PMCID: PMC5080625 DOI: 10.1038/srep35588] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2016] [Accepted: 09/21/2016] [Indexed: 12/13/2022] Open
Abstract
The structural integrity of meningococcal native, micro-fluidized and activated capsular polysaccharides and their glycoconjugates - in the form most relevant to their potential use as vaccines (dilute solution) - have been investigated with respect to their homogeneity, conformation and flexibility. Sedimentation velocity analysis showed that the polysaccharide size distributions were generally bimodal with some evidence for higher molar mass forms at higher concentration. Weight average molar masses Mw where lower for activated polysaccharides. Conjugation with tetanus toxoid protein however greatly increased the molar mass and polydispersity of the final conjugates. Glycoconjugates had an approximately unimodal log-normal but broad and large molar mass profiles, confirmed by sedimentation equilibrium "SEDFIT MSTAR" analysis. Conformation analysis using HYDFIT (which globally combines sedimentation and viscosity data), "Conformation Zoning" and Wales-van Holde approaches showed a high degree of flexibility - at least as great as the unconjugated polysaccharides, and very different from the tetanus toxoid (TT) protein used for the conjugation. As with the recently published finding for Hib-TT complexes, it is the carbohydrate component that dictates the solution behaviour of these glycoconjugates, although the lower intrinsic viscosities suggest some degree of compaction of the carbohydrate chains around the protein.
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Affiliation(s)
- Ali Saber Abdelhameed
- National Centre for Macromolecular Hydrodynamics, University of Nottingham, Sutton Bonington LE12 5RD, UK
- Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, P.O. Box 2457, Riyadh, 11451, Kingdom of Saudi Arabia
| | - Gordon A. Morris
- Department of Chemical Sciences, School of Applied Science, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK
| | - Fahad Almutairi
- Biochemistry Department, Faculty of Science, University of Tabuk, P.O. Box 741-Tabuk 71491 Saudi Arabia
| | - Gary G. Adams
- National Centre for Macromolecular Hydrodynamics, University of Nottingham, Sutton Bonington LE12 5RD, UK
- Insulin and Diabetes Experimental Research (IDER) Group, University of Nottingham, Faculty of Medicine and Health Science, Clifton Boulevard, Nottingham NG7 2RD, UK
| | - Pierre Duvivier
- GSK Vaccines, Rue de l’Institut 89, B1-330 Rixensart, Belgium
| | - Karel Conrath
- GSK Vaccines, Rue de l’Institut 89, B1-330 Rixensart, Belgium
| | - Stephen E. Harding
- National Centre for Macromolecular Hydrodynamics, University of Nottingham, Sutton Bonington LE12 5RD, UK
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Effects of solution conditions on characteristics and size exclusion chromatography of pneumococcal polysaccharides and conjugate vaccines. Carbohydr Polym 2016; 152:12-18. [PMID: 27516244 DOI: 10.1016/j.carbpol.2016.06.095] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/13/2016] [Revised: 06/23/2016] [Accepted: 06/24/2016] [Indexed: 11/20/2022]
Abstract
Molecular properties of bacterial polysaccharides and protein-polysaccharide conjugates play an important role in the efficiency and immunogenicity of the final vaccine product. Size exclusion chromatography (SEC) is commonly used to analyze and characterize biopolymers, including capsular polysaccharides. The objective of this work was to determine the effects of solution ionic strength and pH on the SEC retention of several capsular polysaccharides from S. pneumoniae bacteria in their native and conjugated forms. The retention time of the charged polysaccharides increased with increasing ionic strength and decreasing pH due to compaction of the polysaccharides associated with a reduction in the intramolecular electrostatic interactions. The calculated radius of gyration was in good agreement with model calculations based on the worm-like chain model accounting for the increase in chain stiffness and excluded volume of the charged polysaccharide at low ionic strength. These results provide important insights into the effects of solution ionic strength on physical properties and SEC behavior of capsular polysaccharides and their corresponding conjugates.
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Abdelhameed AS, Adams GG, Morris GA, Almutairi FM, Duvivier P, Conrath K, Harding SE. A glycoconjugate of Haemophilus influenzae Type b capsular polysaccharide with tetanus toxoid protein: hydrodynamic properties mainly influenced by the carbohydrate. Sci Rep 2016; 6:22208. [PMID: 26915577 PMCID: PMC4768162 DOI: 10.1038/srep22208] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2015] [Accepted: 02/03/2016] [Indexed: 11/08/2022] Open
Abstract
Three important physical properties which may affect the performance of glycoconjugate vaccines against serious disease are molar mass (molecular weight), heterogeneity (polydispersity), and conformational flexibility in solution. The dilute solution behaviour of native and activated capsular polyribosylribitol (PRP) polysaccharides extracted from Haemophilus influenzae type b (Hib), and the corresponding glycoconjugate made by conjugating this with the tetanus toxoid (TT) protein have been characterized and compared using a combination of sedimentation equilibrium and sedimentation velocity in the analytical ultracentrifuge with viscometry. The weight average molar mass of the activated material was considerably reduced (Mw ~ 0.24 × 10(6) g.mol(-1)) compared to the native (Mw ~ 1.2 × 10(6) g.mol(-1)). Conjugation with the TT protein yielded large polydisperse structures (of Mw ~ 7.4 × 10(6) g.mol(-1)), but which retained the high degree of flexibility of the native and activated polysaccharide, with frictional ratio, intrinsic viscosity, sedimentation conformation zoning behaviour and persistence length all commensurate with highly flexible coil behaviour and unlike the previously characterised tetanus toxoid protein (slightly extended and hydrodynamically compact structure with an aspect ratio of ~3). This non-protein like behaviour clearly indicates that it is the carbohydrate component which mainly influences the physical behaviour of the glycoconjugate in solution.
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Affiliation(s)
- Ali Saber Abdelhameed
- National Centre for Macromolecular Hydrodynamics, University of Nottingham, Sutton Bonington LE12 5RD, UK
- Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, P.O. Box 2457, Riyadh 11451, Saudi Arabia
| | - Gary G. Adams
- National Centre for Macromolecular Hydrodynamics, University of Nottingham, Sutton Bonington LE12 5RD, UK
- Insulin and Diabetes Experimental Research (IDER) Group, University of Nottingham, Faculty of Medicine and Health Science, Clifton Boulevard, Nottingham, NG7 2RD UK
| | - Gordon A. Morris
- National Centre for Macromolecular Hydrodynamics, University of Nottingham, Sutton Bonington LE12 5RD, UK
- Department of Chemical Sciences, School of Applied Science, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK
| | - Fahad M. Almutairi
- National Centre for Macromolecular Hydrodynamics, University of Nottingham, Sutton Bonington LE12 5RD, UK
| | - Pierre Duvivier
- GSK Vaccines, Rue de l’Institut 89, B-1330 Rixensart, Belgium
| | - Karel Conrath
- GSK Vaccines, Rue de l’Institut 89, B-1330 Rixensart, Belgium
| | - Stephen E. Harding
- National Centre for Macromolecular Hydrodynamics, University of Nottingham, Sutton Bonington LE12 5RD, UK
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Almutairi FM, Cifre JGH, Adams GG, Kök MS, Mackie AR, de la Torre JG, Harding SE. Application of recent advances in hydrodynamic methods for characterising mucins in solution. EUROPEAN BIOPHYSICS JOURNAL: EBJ 2015; 45:45-54. [DOI: 10.1007/s00249-015-1075-0] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/06/2015] [Revised: 07/31/2015] [Accepted: 08/13/2015] [Indexed: 10/22/2022]
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Hadidi M, Buckley JJ, Zydney AL. Ultrafiltration behavior of bacterial polysaccharides used in vaccines. J Memb Sci 2015. [DOI: 10.1016/j.memsci.2015.04.047] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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Kodali S, Vinogradov E, Lin F, Khoury N, Hao L, Pavliak V, Jones CH, Laverde D, Huebner J, Jansen KU, Anderson AS, Donald RGK. A Vaccine Approach for the Prevention of Infections by Multidrug-resistant Enterococcus faecium. J Biol Chem 2015; 290:19512-26. [PMID: 26109072 DOI: 10.1074/jbc.m115.655852] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2015] [Indexed: 12/19/2022] Open
Abstract
The incidence of multidrug-resistant Enterococcus faecium hospital infections has been steadily increasing. With the goal of discovering new vaccine antigens, we systematically fractionated and purified four distinct surface carbohydrates from E. faecium endocarditis isolate Tx16, shown previously to be resistant to phagocytosis in the presence of human serum. The two most abundant polysaccharides consist of novel branched heteroglycan repeating units that include signature sugars altruronic acid and legionaminic acid, respectively. A minor high molecular weight polysaccharide component was recognized as the fructose homopolymer levan, and a glucosylated lipoteichoic acid (LTA) was identified in a micellar fraction. The polysaccharides were conjugated to the CRM197 carrier protein, and the resulting glycoconjugates were used to immunize rabbits. Rabbit immune sera were evaluated for their ability to kill Tx16 in opsonophagocytic assays and in a mouse passive protection infection model. Although antibodies raised against levan failed to mediate opsonophagocytic killing, the other glycoconjugates induced effective opsonic antibodies, with the altruronic acid-containing polysaccharide antisera showing the greatest opsonophagocytic assay activity. Antibodies directed against either novel heteroglycan or the LTA reduced bacterial load in mouse liver or kidney tissue. To assess antigen prevalence, we screened a diverse collection of blood isolates (n = 101) with antibodies to the polysaccharides. LTA was detected on the surface of 80% of the strains, and antigens recognized by antibodies to the two major heteroglycans were co-expressed on 63% of these clinical isolates. Collectively, these results represent the first steps toward identifying components of a glycoconjugate vaccine to prevent E. faecium infection.
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Affiliation(s)
- Srinivas Kodali
- From Pfizer Vaccine Research and Early Development, Pearl River, New York 10654
| | | | - Fiona Lin
- From Pfizer Vaccine Research and Early Development, Pearl River, New York 10654
| | - Nancy Khoury
- From Pfizer Vaccine Research and Early Development, Pearl River, New York 10654
| | - Li Hao
- From Pfizer Vaccine Research and Early Development, Pearl River, New York 10654
| | - Vilo Pavliak
- From Pfizer Vaccine Research and Early Development, Pearl River, New York 10654
| | - C Hal Jones
- From Pfizer Vaccine Research and Early Development, Pearl River, New York 10654
| | - Diana Laverde
- the Division of Infectious Diseases, Department of Medicine, University Hospital, Hugstetter Strasse 55, 79106 Freiburg, Germany, and
| | - Johannes Huebner
- the Division of Infectious Diseases, Department of Medicine, University Hospital, Hugstetter Strasse 55, 79106 Freiburg, Germany, and the Department of Pediatrics, Dr. von Hauner Children's Hospital, Ludwig-Maximilians-University, Lindwurmstrasse 4, 80338 Munich, Germany
| | - Kathrin U Jansen
- From Pfizer Vaccine Research and Early Development, Pearl River, New York 10654
| | | | - Robert G K Donald
- From Pfizer Vaccine Research and Early Development, Pearl River, New York 10654,
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Harding SE, Adams GG, Almutairi F, Alzahrani Q, Erten T, Samil Kök M, Gillis RB. Ultracentrifuge Methods for the Analysis of Polysaccharides, Glycoconjugates, and Lignins. Methods Enzymol 2015; 562:391-439. [DOI: 10.1016/bs.mie.2015.06.043] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
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Harding SE, Abdelhameed AS, Gillis RB, Morris GA, Adams GG. Characterization of Capsular Polysaccharides and Their Glycoconjugates by Hydrodynamic Methods. Methods Mol Biol 2015; 1331:211-227. [PMID: 26169743 DOI: 10.1007/978-1-4939-2874-3_13] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Hydrodynamic methods are relevant for the characterization of carbohydrates such as capsular bacterial polysaccharides or glycoconjugates in solution. This chapter focuses on the following hydrodynamic methods: sedimentation velocity analytical ultracentrifugation (SV AUC), dynamic light scattering (DLS), sedimentation equilibrium analytical ultracentrifugation (SE AUC), size exclusion chromatography coupled to multi-angle light scattering (SEC-MALS), and capillary viscometry-intrinsic viscosity measurement. The chapter highlights the general principle of these five methods, describes experimental details, and specifies advances in the last years.
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Affiliation(s)
- Stephen E Harding
- National Centre for Macromolecular Hydrodynamics, University of Nottingham, Sutton Bonington Campus, Sutton Bonington, LE12 5RD, UK,
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Abdelhameed AS, Morris GA, Adams GG, Rowe AJ, Laloux O, Cerny L, Bonnier B, Duvivier P, Conrath K, Lenfant C, Harding SE. An asymmetric and slightly dimerized structure for the tetanus toxoid protein used in glycoconjugate vaccines. Carbohydr Polym 2012; 90:1831-5. [DOI: 10.1016/j.carbpol.2012.07.032] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2012] [Revised: 07/05/2012] [Accepted: 07/09/2012] [Indexed: 10/28/2022]
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