Effect of 1-phenyl-3-methyl-5-pyrazolone labeling on the fragmentation behavior of asialo and sialylated N-linked glycans under electrospray ionization conditions

Comparison of Different Labeling Techniques for the LC-MS Profiling of Human Milk Oligosaccharides

Human milk oligosaccharides (HMOs) exhibit various biological activities for infants, such as serving as prebiotics, blocking pathogens, and aiding in brain development. HMOs are a complex mixture of hetero-oligosaccharides that are generally highly branched, containing multiple structural isomers and no intrinsic chromophores, presenting a challenge to both their resolution and quantitative detection. While liquid chromatography-mass spectrometry (LC-MS) has become the primary strategy for analysis of various compounds, the very polar and chromophore-free properties of native glycans hinder their separation in LC and ionization in MS. Various labeling approaches have been developed to achieve separation of glycans with higher resolution and greater sensitivity of detection. Here, we compared five commonly used labeling techniques [by 2-aminobenzamide, 2-aminopyridine, 2-aminobenzoic acid (2-AA), 2,6-diaminopyridine, and 1-phenyl-3-methyl-5-pyrazolone] for analyzing HMOs specifically under hydrophilic-interaction chromatography-mass spectrometry (HILIC-MS) conditions. The 2-AA labeling showed the most consistent deprotonated molecular ions, the enhanced sensitivity with the least structural selectivity, and the sequencing-informative tandem MS fragmentation spectra for the widest range of HMOs; therefore, this labeling technique was selected for further optimization under the porous graphitized carbon chromatography-mass spectrometry (PGC-MS) conditions. The combination strategy of 2-AA labeling and PGC-MS techniques provided online decontamination (removal of excess 2-AA, salts, and lactose) and resolute detection of many HMOs, enabling us to characterize the profiles of complicated HMO mixtures comprehensively in a simple protocol.

Simultaneous analyses of N-linked and O-linked glycans of ovarian cancer cells using solid-phase chemoenzymatic method

BACKGROUND

Glycans play critical roles in a number of biological activities. Two common types of glycans, N-linked and O-linked, have been extensively analyzed in the last decades. N-glycans are typically released from glycoproteins by enzymes, while O-glycans are released from glycoproteins by chemical methods. It is important to identify and quantify both N- and O-linked glycans of glycoproteins to determine the changes of glycans.

METHODS

The effort has been dedicated to study glycans from ovarian cancer cells treated with O-linked glycosylation inhibitor qualitatively and quantitatively. We used a solid-phase chemoenzymatic approach to systematically identify and quantify N-glycans and O-glycans in the ovarian cancer cells. It consists of three steps: (1) immobilization of proteins from cells and derivatization of glycans to protect sialic acids; (2) release of N-glycans by PNGase F and quantification of N-glycans by isobaric tags; (3) release and quantification of O-glycans by β-elimination in the presence of 1-phenyl-3-methyl-5-pyrazolone (PMP).

RESULTS

We used ovarian cancer cell lines to study effect of O-linked glycosylation inhibitor on protein glycosylation. Results suggested that the inhibition of O-linked glycosylation reduced the levels of O-glycans. Interestingly, it appeared to increase N-glycan level in a lower dose of the O-linked glycosylation inhibitor. The sequential release and analyses of N-linked and O-linked glycans using chemoenzymatic approach are a platform for studying N-glycans and O-glycans in complex biological samples.

CONCLUSION

The solid-phase chemoenzymatic method was used to analyze both N-linked and O-linked glycans sequentially released from the ovarian cancer cells. The biological studies on O-linked glycosylation inhibition indicate the effects of O-glycosylation inhibition to glycan changes in both O-linked and N-linked glycan expression.

Quantitative Glycomics: A Combined Analytical and Bioinformatics Approach

Glycosylation is one of the most common and essential protein modifications. Glycans conjugated to biomolecules modulate the function of such molecules through both direct recognition of glycan structures and indirect mechanisms that involve the control of protein turnover rates, stability, and conformation. The biological attributes of glycans in numerous biological processes and implications in a number of diseases highlight the necessity for comprehensive characterization of protein glycosylation. This chapter reviews cutting-edge methods and tools developed to facilitate quantitative glycomics. This chapter highlights the different methods employed for the release and purification of glycans from biological samples. The most effective labeling methods developed for sensitive quantitative glycomics are also described and discussed. The chromatographic approaches that have been used effectively in glycomics are also highlighted.

Reversed-phase separation methods for glycan analysis

Reversed-phase chromatography is a method that is often used for glycan separation. For this, glycans are often derivatized with a hydrophobic tag to achieve retention on hydrophobic stationary phases. The separation and elution order of glycans in reversed-phase chromatography is highly dependent on the hydrophobicity of the tag and the contribution of the glycan itself to the retention. The contribution of the different monosaccharides to the retention strongly depends on the position and linkage, and isomer separation may be achieved. The influence of sialic acids and fucoses on the retention of glycans is still incompletely understood and deserves further study. Analysis of complex samples may come with incomplete separation of glycan species, thereby complicating reversed-phase chromatography with fluorescence or UV detection, whereas coupling with mass spectrometry detection allows the resolution of complex mixtures. Depending on the column properties, eluents, and run time, separation of isomeric and isobaric structures can be accomplished with reversed-phase chromatography. Alternatively, porous graphitized carbon chromatography and hydrophilic interaction liquid chromatography are also able to separate isomeric and isobaric structures, generally without the necessity of glycan labeling. Hydrophilic interaction liquid chromatography, porous graphitized carbon chromatography, and reversed-phase chromatography all serve different research purposes and thus can be used for different research questions. A great advantage of reversed-phase chromatography is its broad distribution as it is used in virtually every bioanalytical research laboratory, making it an attracting platform for glycan analysis.

Graphical Abstract

Linkage and branch analysis of high-mannose oligosaccharides using closed-ring labeling of 8-aminopyrene-1,3,6-trisulfonate and p-aminobenzoic ethyl ester and negative ion trap mass spectrometry

A strategy based on negative ion electrospray ionization tandem mass spectrometry and closed-ring labeling with both 8-aminopyrene-1,3,6-trisulfonate (APTS) and p-aminobenzoic acid ethyl ester (ABEE) was developed for linkage and branch determination of high-mannose oligosaccharides. X-type cross-ring fragment ions obtained from APTS-labeled oligosaccharides by charge remote fragmentation provided information on linkages near the non-reducing terminus. In contrast, A-type cross-ring fragment ions observed from ABEE-labeled oligosaccharides yielded information on linkages near the reducing terminus. This complementary information provided by APTS- and ABEE-labeled oligosaccharides was utilized to delineate the structures of the high-mannose oligosaccharides. As a demonstration of this approach, the linkages and branches of high-mannose oligosaccharides Man(5)GlcNAc(2), Man(6)GlcNAc(2), Man(8)GlcNAc(2), and Man(9)GlcNAc(2) cleaved from the ribonuclease B were assigned from MS(2) spectra of ABEE- and APTS-labeled derivatives.

Detection and quantitation of afucosylated N-linked oligosaccharides in recombinant monoclonal antibodies using enzymatic digestion and LC-MS

The presence of N-linked oligosaccharides in the CH2 domain has a significant impact on the structure, stability, and biological functions of recombinant monoclonal antibodies. The impact is also highly dependent on the specific oligosaccharide structures. The absence of core-fucose has been demonstrated to result in increased binding affinity to Fcγ receptors and, thus, enhanced antibody-dependent cellular cytotoxicity (ADCC). Therefore, a method that can specifically determine the level of oligosaccharides without the core-fucose (afucosylation) is highly desired. In the current study, recombinant monoclonal antibodies and tryptic peptides from the antibodies were digested using endoglycosidases F2 and H, which cleaves the glycosidic bond between the two primary GlcNAc residues. As a result, various oligosaccharides of either complex type or high mannose type that are commonly observed for recombinant monoclonal antibodies are converted to either GlcNAc residue only or GlcNAc with the core-fucose. The level of GlcNAc represents the sum of all afucosylated oligosaccharides, whereas the level of GlcNAc with the core-fucose represents the sum of all fucosylated oligosaccharides. LC-MS analysis of the enzymatically digested antibodies after reduction provided a quick estimate of the levels of afucosylation. An accurate determination of the level of afucosylation was obtained by LC-MS analysis of glycopeptides after trypsin digestion.

One-pot nonreductive O-glycan release and labeling with 1-phenyl-3-methyl-5-pyrazolone followed by ESI-MS analysis

A novel one-pot procedure for the nonreductive release of O-linked glycans from glycoproteins and the simultaneous derivatization of released glycans with 1-phenyl-3-methyl-5-pyrazolone (PMP) is described. Unlike the traditional reductive β-elimination, which produces alditols, this new method employs PMP/ammonia aqueous solution as the reaction medium. The O-glycans are released from glycoproteins and derivatized with PMP nonreductively, specifically, and quantitatively. Samples can be easily purified from ammonia, excess PMP, and peptide residues by evaporation, chloroform extraction, and solid-phase extraction (SPE) column fractionation for HPLC, CE, or MS analysis. The procedure has been elaborated with two purified glycoproteins, porcine stomach mucin and bovine fetuin, and successfully applied to O-glycan profiling of a challenging biological specimen, healthy human plasma. This new procedure has shown methodological significance in O-glycan analysis.

A glycomics platform for the analysis of permethylated oligosaccharide alditols

This communication reports the development of an LC/MS platform for the analysis of permethylated oligosaccharide alditols that, for the first time, demonstrates routine online oligosaccharide isomer separation of these compounds before introduction into the mass spectrometer. The method leverages a high-resolution liquid chromatography system with the superior fragmentation pattern characteristics of permethylated oligosaccharide alditols that are dissociated under low-energy collision conditions using quadrupole orthogonal time-of-flight (QoTOF) instrumentation and up to pseudo MS(3) mass spectrometry. Glycoforms, including isomers, are readily identified and their structures assigned. The isomer-specific spectra include highly informative cross-ring and elimination fragments, branch position specific signatures, and glycosidic bond fragments, thus facilitating linkage, branch, and sequence assignment. The method is sensitive and can be applied using as little as 40 fmol of derivatized oligosaccharide. Because permethylation renders oligosaccharides nearly chemically equivalent in the mass spectrometer, the method is semiquantitative and, in this regard, is comparable to methods reported using high field NMR and capillary electrophoresis. In this postgenomic age, the importance of glycosylation in biological processes has become clear. The nature of many of the important questions in glycomics is such that sample material is often extremely limited, thus necessitating the development of highly sensitive methods for rigorous structural assignment of the oligosaccharides in complex mixtures. The glycomics platform presented here fulfills these criteria and should lead to more facile glycomics analyses.

Linkage and branch determination of N-linked oligosaccharides using sequential degradation/closed-ring chromophore labeling/negative ion trap mass spectrometry

A method based on sequential degradation, p-aminobenzoic ethyl ester (ABEE) closed-ring labeling, and negative ion electrospray ionization tandem mass spectrometry is presented for the study of linkage and branch determination for N-linked oligosaccharides. Closed-ring labeling provides greater linkage information than the more popular open-ring reductive amination approach. In addition, after high-performance liquid chromatography (HPLC) separation, closed-ring labeling allows for regeneration of the underivatized oligosaccharide, a requirement for alkaline sequential degradation. The analytical scheme presented here uses HPLC separation of closed-ring labeled oligosaccharides to resolve the mixture into individual forms that undergo subsequent structural analysis by negative ion tandem mass spectrometry. To facilitate complete structural analysis, particularly for larger sugars, the closed-ring labels are removed and the sugars are sequentially degraded by controlled alkaline hydrolysis. It is noteworthy that for sugars containing sialic acid moieties, a protecting group must be used to stabilize sialic acid groups during sequential alkaline degradation. This described approach was applied to two high mannose oligosaccharides M5G2, M6G2 cleaved from the ribonuclease B and a complex oligosaccharide A2 cleaved from transferrin.

The use of mass spectrometry for the proteomic analysis of glycosylation

Of all protein PTMs, glycosylation is by far the most common, and is a target for proteomic research. Glycosylation plays key roles in controlling various cellular processes and the modifications of the glycan structures in diseases highlight the clinical importance of this PTM. Glycosylation analysis remains a difficult task. MS, in combination with modern separation methodologies, is one of the most powerful and versatile techniques for the structural analysis of glycoconjugates. This review describes methodologies based on MS for detailed characterization of glycoconjugates in complex biological samples at the sensitivity required for proteomic work.

Mass spectrometry of oligosaccharides

Glycosylation is a common post-translational modification to cell surface and extracellular matrix (ECM) proteins as well as to lipids. As a result, cells carry a dense coat of carbohydrates on their surfaces that mediates a wide variety of cell-cell and cell-matrix interactions that are crucial to development and function. Because of the historical difficulties with the analysis of complex carbohydrate structures, a detailed understanding of their roles in biology has been slow to develop. Just as mass spectrometry has proven to be the core technology behind proteomics, it stands to play a similar role in the study of functional implications of carbohydrate expression, known as glycomics. This review summarizes the state of knowledge for the mass spectrometric analysis of oligosaccharides with regard to neutral, sialylated, and sulfated compound classes. Mass spectrometric techniques for the ionization and fragmentation of oligosaccharides are discussed so as to give the reader the background to make informed decisions to solve structure-activity relations in glycomics.

Profiling of N -linked oligosaccharides using phenylhydrazine derivatization and mass spectrometry

N-linked oligosaccharide standards obtained from commercial sources were derivatized with phenylhydrazine (PHN) and analyzed by on-line reversed-phase high performance liquid chromatography (HPLC)/electrospray ionization mass spectrometry (ESI-MS). This procedure was then applied to mixtures of N-glycans enzymatically released from hen ovalbumin. Under ESI-MS conditions, phenylhydrazones of asialylated oligosaccharide standards and ovalbumin glycans produced mainly [M + 2H]2+ molecular ions at low cone voltage values, while minimal fragmentation was observed. Reversed-phase HPLC/ESI-MS total and selected ion chromatograms obtained for derivatized N-glycans from ovalbumin showed partial but useful separation. Overall glycan profiles obtained by ESI-MS were compared with results obtained by matrix-assisted laser desorption/ionization (MALDI)-MS. Qualitatively, profiles were similar from one technique to the other in terms of relative abundance of glycans versus composition. Post-source decay (PSD) analysis of the [M + Na]+ ions of PHN-glycans showed dominant B, C and internal B/Y, C/Y cleavages. These patterns were helpful in relating fragmentation to proposed structures. Cross-ring cleavage fragment ions (A-type) were also observed in most cases. The PHN derivatization method is fast and simple. It produces abundant parent ions in both MALDI-MS and ESI-MS, while avoiding the presence of salt contaminants during the labeling procedure.

Derivatization of carbohydrates for chromatographic, electrophoretic and mass spectrometric structure analysis

Carbohydrates, either alone or as constituents of glycoproteins, proteoglycans and glycolipids, are mediators of several cellular events and (patho)physiological processes. Progress in the "glycome" project is closely related to the analytical tools used to define carbohydrate structure and correlate structure with function. Chromatography, electrophoresis and mass spectrometry are the indispensable analytical tools of the on-going research. Carbohydrate derivatization is required for most of these analytical procedures. This review article gives an overview of derivatization methods of carbohydrates for their liquid chromatographic and electrophoretic separation, as well as the mass spectrometric characterization. Pre-column and on-capillary derivatization methods are presented with special emphasis on the derivatization of large carbohydrates.

Application of the StrOligo algorithm for the automated structure assignment of complex N-linked glycans from glycoproteins using tandem mass spectrometry

Oligosaccharides associated with proteins are known to give these molecules specific conformations and functions. Analysis of proteins would not be complete without studying the glycans. However, high-throughput techniques in proteomics will soon overwhelm the current capacity of methods if no automation is incorporated into glycomics. New capabilities of the StrOligo algorithm introduced for this purpose (Ethier et al., Rapid Commun. Mass Spectrom., 2002; 16: 1743) will be discussed here. Experimental tandem mass spectra were acquired to test the algorithm using a hybrid quadrupole-time-of-flight (QqTOF) instrument with a matrix-assisted laser desorption/ionization (MALDI) source. The samples were N-linked oligosaccharides from monoclonal antibody IgG, beta interferon and fetuin, detached by enzymatic deglycosylation and labeled at the reducing end. Improvements to the program were made in order to reduce the need for user intervention. StrOligo strips the spectra down to monoisotopic peaks only. The algorithm first builds a relationship tree, accounting for each observed loss of a monosaccharide moiety, and then analyzes the tree and proposes possible structures from combinations of adducts and fragment ion types. A score, which reflects agreement with experimental results, is then given to each proposed structure. The program then decides which combination is the best one and labels relevant peaks in the experimental mass spectrum using a modified nomenclature. The usefulness of the algorithm has been demonstrated by assigning structures to several glycans released from glycoproteins. The analysis was completed in less than 2 minutes for any glycan, which is a substantial improvement over manual interpretation.

Determination of linkages of linear and branched oligosaccharides using closed-ring chromophore labeling and negative ion trap mass spectrometry

Linear as well as branched oligosaccharides were labeled with p-aminobenzoic ethyl ester (ABEE) using the glycosylamine closed-ring labeling approach and analyzed by negative-ion electrospray ionization mass spectrometry (ESI-MS). Linkage specific fragment ions of ABEE labeled linear oligosaccharides were proposed based on the MS2 and MS3 data for several ABEE labeled linear oligosaccharides with known linkage configurations. Fragmentation at the reducing end was similar to that observed for ABEE disaccharides whereas the fragmentation pattern not involving the reducing end was similar to underivatized disaccharides. Based on these ions, all the linkages of linear oligosaccharides could be unambiguously determined. The fragmentation pattern at the branched sugar was in general not quite the same as the linear one. However, many linkage specific fragment ions were also observed for linkages at the branched sugar. These ions along with the ions proposed for linear oligosaccharides were found to be quite useful for the determination of all the linkages of branched oligosaccharides.

Branching pattern and sequence analysis of underivatized oligosaccharides by combined MS/MS of singly and doubly charged molecular ions in negative-ion electrospray mass spectrometry

We previously reported that sequence and partial linkage information, including chain and blood-group types, of reducing oligosaccharides can be obtained from negative-ion electrospray CID MS/MS on a quadrupole-orthogonal time-of-flight instrument with high sensitivity and without derivatization (Chai, W.; Piskarev, V.; Lawson, A. M. Anal. Chem. 2001, 73, 651-657). In contrast to oligonucleotides and peptides, oligosaccharides can form branched structures that result in a greater degree of structural complexity. In the present work we apply negative-ion electrospray CID MS/MS to core-branching pattern analysis using nine 3,6-branched and variously fucosylated oligosaccharides based on hexasaccharide backbones LNH/LNnH as examples. The important features of the method are the combined use of CID MS/MS of singly and doubly charged molecular ions of underivatized oligosaccharides to deduce the branching pattern and to assign the structural details of each of the 3- and 6-branches. These spectra give complimentary structural information. In the spectra of [M - H]-, fragment ions from the 6-linked branch are dominant and those from the 3-linked branch are absent, while fragment ions from both branches occur in the spectra of [M - 2H]2-. This allows the distinction of fragment ions derived from either the 3- or 6-branches. In addition, a unique D2beta-3 ion, arising from double D-type cleavage at the 3-linked glycosidic bond of the branched Gal core residue, provides direct evidence of the branching pattern with sequence and partial linkage information being derived from C- and A-type fragmentations, respectively.

Automated structural assignment of derivatized complex N-linked oligosaccharides from tandem mass spectra

Glycoprotein function is controlled by several biological factors, one of them being the structure of carbohydrate chains (glycans) attached to specific amino acids of the protein backbone. Changes in glycan structures have been shown to modify the secondary and tertiary conformation of glycoproteins, thus their function. Powerful analytical tools are available for the characterization of sugar structures, and recently mass spectrometry (MS) has been increasingly useful for this purpose. Manual interpretation of tandem mass spectrum is possible but tedious. Automated interpretation should speed the analysis and enhance the results obtained. A new computer program for automated interpretation of tandem MS spectra of complex N-linked glycans oligosaccharides from mammals will be described. N-Linked oligosaccharides standards were derivatized with 1-phenyl-3-methyl-5-pyrazolone (PMP) and analyzed by matrix-assisted laser desorption/ionization (MALDI)-tandem MS. Simulated tandem mass spectra of other common glycans were also generated to test the algorithm. The MALDI-MS/MS spectra featured resolved isotopic distributions for the [M + H](+) and fragment ions of oligosaccharides. These isotopic distributions complicated the automated analysis of the spectra and were removed to leave only monoisotopic peaks. An algorithm was written for this purpose, yielding simplified tandem mass spectra. Another algorithm is then used to determine the structure of the oligosaccharide. A score is then given to each structure, depending on agreement with experimental results. The program successfully assigned the true structure in 24 out of the 28 cases (86%) and the true structure was among the three top scoring structures in all cases.

Investigation of different combinations of derivatization, separation methods and electrospray ionization mass spectrometry for standard oligosaccharides and glycans from ovalbumin

Derivatization procedures using 1-phenyl-3-methyl-5-pyrazolone (PMP) and 2-aminonaphthalene trisulfone (ANTS) were selected among a number of well known methods for labelling carbohydrates. PMP derivatives were selected owing to our laboratory's previous high-performance liquid chromatography/electrospray ionization mass spectrometry (HPLC/ESI-MS) experience with these, whereas the ANTS-labelled compounds were prepared for fluorophore-assisted carbohydrate electrophoresis (FACE) separation. ANTS-oligosaccharide standards were characterized to study their ionization patterns. Reversed-phase and normal-phase HPLC systems were coupled on-line with ESI-MS. Each necessitated its own mobile phase system which, in turn, imposed some important changes in the ionization conditions used and/or on the ionization patterns and spectra obtained. Following characterization of the intact glycoprotein ovalbumin with ESI-MS, its glycans were detached using the enzyme PNGase-F. The glycans were subjected to PMP and ANTS derivatization. It was very difficult to separate ANTS derivatives by reversed-phase HPLC owing to lack of retention, and normal-phase HPLC offered reasonable retention with limited separation. PMP compounds overall yielded better normal- and reversed-phase separations and improved sensitivity over the ANTS-labelled sugars, for which negative mode ESI had to be used. The combination of ESI of intact ovalbumin and ESI of PMP-glycans gave rise to the detection of over 20 different glycoforms, excluding the possible presence of structural isomers for each sugar composition detected.

Liquid chromatography ion trap mass spectrometric analysis of oligosaccharides using permethylated derivatives

Reversed phase liquid chromatography was combined with the multiple stage mass analysis capability of an ion trap mass spectrometer for the characterization of permethylated oligosaccharide mixtures. The new method was used to separate the components of an unlabeled permethylated maltooligomer ladder, a 2-aminobenzamide-labeled (2-AB) maltooligomer ladder, a complex mixture of 2AB-labeled bi- (B), tri- (T), and tetraantennary (Q) standards, and a mixture of recombinant glycoprotein carbohydrates from soluble CD4 with varying sialic acid (S) content. Using reversed phase HPLC, permethylated mixture components including alpha and beta anomers were separated based on their structures. Fluorescent labeling with 2-aminobenzamide prior to permethylation was employed for off-line method development, but was not necessarily required for mass spectral analysis, as permethylation alone improved the ionization and fragmentation characteristics of the molecules. Antennae composition of permethylated derivatives was determined in MS(2) where the fragmentation patterns of the Y- and B-ion series predominated, and then further evaluated in MS(3), which provided additional information on branching obtained from A and X cross-ring fragmentation.

Effect of the labeling group in structural analyses of malononitrile-labeled oligosaccharides by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and electrospray ionization mass spectrometry

Structural analyses of oligosaccharide-malononitrile derivatives were conducted by matrix-assisted laser desorption/ionization post-source decay (MALDI-PSD) analysis in positive ion mode, and by electrospray ionization collision-induced dissociation (ESI-CID) analysis in negative ion mode. The malononitrile derivatives of oligosaccharides, which were developed for separation and detection of multi-component oligosaccharides by negative ion electrospray ionization mass spectrometry (ESI-MS), were detected by positive ion MALDI with a detection limit at a 400 fmol level even without any purification procedure for the derivatized oligosaccharide mixture. The results of structural analyses of oligosaccharide-malononitrile derivatives by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) were affected drastically by the choice of matrix used, and gave much more structural information than the free oligosaccharide. The effects of the malononitrile labeling group on the structural analyses of oligosaccharide-malononitrile derivatives by positive-ion MALDI-PSD analysis in reflectron mode, and by MALDI time-of-flight mass spectrometry (MALDI-TOFMS) in linear mode, are compared with those of in-source ESI-CID analysis in negative ion mode. The malononitrile derivatization method could be a powerful tool for structural analyses of oligosaccharides. Copyright 1999 John Wiley & Sons, Ltd.