Effects of Blood Collection Tubes on the Quantification of Catecholamines and Their O-Methylated Metabolites by Liquid Chromatography Coupled With Tandem Mass Spectrometry.

BACKGROUND: Catecholamines (epinephrine; norepinephrine; and dopamine) and their O-methylated metabolites (metanephrine; normetanephrine; and 3-methoxytyramine) are biomarkers for pheochromocytoma and paraganglioma. Liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) was recommended by Endocrine Society for detecting these compounds. The influence of blood collection tubes on the analysis of the six analytes by LC-MS/MS was not thoroughly investigated, which we want to clarify in our study. METHODS: Blood samples of healthy individuals were collected into serum, lithium heparin, and K2EDTA plasma tubes separately. Samples were subjected to solid phase extraction and then analyzed by LC-MS/MS. The retention behavior and assay performance of the six analytes were assessed for samples from different collection containers. The impacts of potassium and sodium as the counter ions of EDTA on the retention time and matrix effect were also studied. RESULTS: Compared with O-methylated metabolites, the results for catecholamines were more affected by the collection tubes, especially for norepinephrine, which displayed severely suppressed signal and very low extraction efficiency in K2EDTA plasma. Changing the counter ion of EDTA from potassium to sodium dramatically changed the retention behavior and matrix effect of norepinephrine. CONCLUSIONS: It is necessary to evaluate blood collection tubes for catecholamines and their O-methylated metabolites analyzed by LC-MS/MS. In addition, attention should also be paid when the anticoagulant counter ion was changed.

MALDI-TOF-MS for Rapid Screening and Typing of ß-Globin Variant and ß-Thalassemia through Direct Measurements of Intact Globin Chains.

BACKGROUND: Traditional phenotype-based screening for ß-globin variant and ß-thalassemia using hematological parameters is time-consuming with low-resolution detection. Development of a MALDI-TOF-MS assay using alternative markers is needed. METHODS: We constructed a MALDI-TOF-MS-based approach for identifying various ß-globin disorders and classifying thalassemia major (TM) and thalassemia intermedia (TI) patients using 901 training samples with known HBB/HBA genotypes. We then validated the accuracy of population screening and clinical classification in 2 separate cohorts consisting of 16 172 participants and 201 ß-thalassemia patients. Traditional methods were used as controls. Genetic tests were considered the gold standard for testing positive specimens. RESULTS: We established a prediction model for identifying different forms of ß-globin disorders in a single MALDI-TOF-MS test based on δ- to ß-globin, γ- to α-globin, γ- to ß-globin ratios, and/or the abnormal globin-chain patterns. Our validation study yielded comparable results of clinical specificity (99.89% vs 99.71%), and accuracy (99.78% vs 99.16%) between the new assay and traditional methods but higher clinical sensitivity for the new method (97.52% vs 88.01%). The new assay identified 22 additional abnormal hemoglobins in 69 individuals including 9 novel ones, and accurately screened for 9 carriers of deletional hereditary persistence of fetal hemoglobin or δß-thalassemia. TM and TI were well classified in 178 samples out of 201 ß-thalassemia patients. CONCLUSIONS: MALDI-TOF-MS is a highly accurate, predictive tool that could be suitable for large-scale screening and clinical classification of ß-globin disorders.

Addition of 3-methoxytyramine or chromogranin A to plasma free metanephrines as the initial test for pheochromocytoma and paraganglioma: Which is the best diagnostic strategy.

OBJECTIVE: Measurements of plasma free metanephrines (MNs), including MN and normetanephrine, provide high sensitivity and specificity for the diagnosis of pheochromocytoma and paraganglioma (PPGL). 3-Methoxytyramine (3-MT) and chromogranin A (CgA) may allow the detection of dopamine-producing or biochemically silent PPGL. The aim of this study was to evaluate whether measurements of plasma 3-MT or CgA as a supplement of plasma MNs offer a better diagnostic strategy for initial testing of PPGL. PATIENTS AND DESIGN: We enroled 125 patients who underwent surgery from 2015 to 2016 for our study and identified 33 patients with PPGL and 92 patients with non-PPGL masses. MEASUREMENT: The levels of plasma free MNs and 3-MT were measured for all 125 patients using liquid chromatography-tandem mass spectrometry. Plasma CgA concentrations were determined using a radioimmunoassay. To evaluate the diagnostic performance of plasma free MNs, 3-MT and CgA, sensitivity and specificity were determined, and receiver operating characteristic curves were constructed. RESULTS: We found that combining 3-MT and MNs increased the diagnostic sensitivity from 93.9% (95% confidence interval [CI]: 78.4%-98.9%) to 97.0% (95% CI: 82.5%-99.8%). In contrast, addition of plasma CgA test reduced the diagnostic specificity significantly from 91.3% (95% CI: 83.1%-95.9%) to 75.0% (95% CI: 64.7%-83.2%). CONCLUSION: Here, we demonstrated that 3-MT represents a valuable supplementary test to plasma MNs, which can further enhance the sensitivity of the assay, while plasma CgA added no additional diagnostic value to MNs due to the lowered diagnostic specificity.

Quantitative MALDI-MS assay of steroid hormones in plasma based on hydroxylamine derivatization.

Measuring the concentrations of steroid hormones in plasma is critical for understanding their role in various vital physiological processes. The detection of underivatized steroid hormones in biofluids through mass spectrometry (MS) is typically hindered by low ionization efficiency. We described a novel matrix-assisted laser desorption/ionization-MS (MALDI-MS) approach based on hydroxylamine derivatization (HA-D) to analyze low-concentration steroid hormones in plasma. The ketonic carbonyl group containing steroid hormones could be derivatized using HA to form oxime derivatives, which considerably enhanced the MS sensitivity for detecting steroid hormones. By using the optimized conditions, estrone (E1), testosterone (T), and progesterone (Prog), could be simultaneously quantified in plasma with a limit of detection (LOD) from 0.019 to 0.031 nM, recoveries from 86% to 108%, and coefficient of variation (CV%) from 4.59% to 11.90%. HA-D/MALDI-MS exhibited higher sensitivity than those using Girard T (GT). To establish potential utility of our method, we characterized fatty liver patient plasmas to demonstrate that the HA-D/MALDI-MS procedure could generate quantitative results comparable to the current clinical liquid chromatography-electrospray ionization tandem MS (LC-ESI MS/MS) method. This approach facilitates the rapid and accurate characterization of plasma hormones, and renders the MALDI-MS approach for steroid hormones more adaptable for clinical research and use.

Bovine serum albumin affects N-glycoforms of murine IgG monoclonal antibody purified from hybridoma supernatants.

Immunoglobulin G (IgG) is a class of monoclonal antibodies (mAbs) commonly produced in mammalian cell lines. These cell lines are grown in finely adjusted culture media, which contain components that may impact glycoforms. As variation of N-glycoforms can impact the biological properties of IgGs, medium composition should be controlled. Here, we studied the effects on IgG N-glycoforms of different components in hybridoma culture media, specifically compared bovine serum albumin (BSA) with other small molecules, using a matrix-assisted laser desorption/ionization quadrupole ion trap time-of-flight multistage mass spectrometry (MALDI-QIT-TOF MSn)-based approach. We show that small molecular additives caused little change in glycan species, though a number of these reagents, especially glutamine, affected levels of glycosylation. In comparison, the addition of macromolecular protein BSA significantly changed IgG N-glycan patterns, not only in species but also in glycosylation levels. Together, our finding suggests that BSA increases the complexity of IgG N-glycoforms, thus raising the difficulty in maintaining glycoforms consistency during antibody production. Therefore, the effect of BSA on IgG N-glycans should be considered when designing optimal medium formulations for IgG production. KEY POINTS: • Small molecular medium additives only affect glycosylation levels of IgG N-glycans. • BSA significantly changes IgG N-glycoforms as a medium additive. • BSA's skewing of IgG N-glycoforms should be considered in IgG production.

FcγR-binding affinity of monoclonal murine IgG1s carrying different N-linked Fc oligosaccharides.

Glycosylation is one of the most common post-translational modifications which diversifies the structure and function of glycoproteins like immunoglobulin G (IgG). The effector function of IgG depends on N-glycan patterns located in the crystalline fragment (Fc). Fc gamma receptor (FcγR)-binding affinity is one of the most important effector functions in IgG, and it varies with different IgG isotypes. Murine IgG1 (mIgG1) triggers various immune effector functions via FcγRs, however, how N-glycans of mIgG1 impact interactions between mIgG1s and murine FcγRs remains largely unknown. Here, we generated mIgG1s with different N-glycan patterns by adding different types of N-glycan processing enzyme inhibitors to the hybridoma culture media, before comparing their FcγR-binding affinity using enzyme-linked immunosorbent assay (ELISA) analysis. We showed that N-glycans critically affect mIgG1 affinity to FcγRs. The removal of N-glycans nearly completely abolished mIgG1-FcγR binding. In comparison, when N-glycans are present, decreasing fucosylation levels enhanced the FcγR-binding affinity regardless of the types of N-glycans. Furthermore, high-mannose type and hybrid type N-glycans reduced FcγR-binding affinity, compared to complex type N-glycans. In conclusion, our findings clearly demonstrate that FcγR-binding affinity of mIgG1 is under the control of glycosylation. Importantly, we found that both the levels of specific glycosylation as well as the types of N-glycans affect FcγR-binding affinity. Together, these insights should greatly expand our understanding of N-glycans function in general, and assist in manipulating host immune responses by controlling antibody N-glycan patterns, which is important for designing therapeutic antibodies with improved characteristics.

Distribution of abnormal IgG glycosylation patterns from rheumatoid arthritis and osteoarthritis patients by MALDI-TOF-MSn.

Glycosylation is a post-translational modification essential for maintaining the structure and function of proteins. Abnormal N-glycan patterns have been found in various diseases compared to healthy controls. A decrease in terminal galactosylated N-glycans of serum IgG in rheumatoid arthritis (RA) and osteoarthritis (OA) may be involved in their immunopathogenesis. However, how glycan patterns differ between RA and OA remains unclear. Here, we identified 15 glycan forms of serum IgG from RA and OA using MALDI-TOF MS. We found that IgG galactosylation represented a suitable candidate for differentiating RA from healthy controls (AUC > 0.9). Then, we performed binary logistic regression to screen out three bisecting N-acetylglucosamine (GlcNAc) glycoforms for distinguishing between OA and RA. Combined ROC analysis of the selected glycans yielded an AUC of 0.81 between OA and RA and an AUC of 0.79 between OA and RF/ACPA negative RA. Similar results were found in the validation set. In conclusion, our analysis demonstrates that RA and OA are distinguished on the basis of their different IgG glycan patterns, which thus serve as suitable candidates as biomarkers for reliably identifying clinical conditions such as RA and OA.

Comparison of retention behavior of oligolysine and oligoarginine in ion-pairing chromatography using heptafluorobutyric acid.

This paper describes the retention behavior of oligolysine and oligoarginine peptides of different lengths as a function of heptafluorobutyric acid (HFBA) concentration in ion-pairing reversed-phase chromatography in isocratic elution. A mixture of oligolysine and a mixture of oligoarginine with number of amino acid residues (dp) from two to eight were conveniently prepared by one-pot protease-catalyzed synthesis. Analysis of the logarithm of the retention factor k as a function of [HFBA] for each oligopeptide component, using a closed pairing model, provided values for (1) number (n) of paired HFBA anions per peptide molecule, (2) equilibrium constant (K(ip,m)) for ion pairing between oligopeptides and HFBA anions, and (3) product of the phase ratio and the distribution constant of the paired oligopeptide between the mobile and stationary phases (ßK(d,ip)). We found that ßK(d,ip) of oligoarginine is larger compared with oligolysine having the same dp. A linear relationship was obtained for ln ßK(d,ip) as a function of n + g · dp. By optimizing constant g separately for oligolysine and oligoarginine, we determined that g is larger for oligoarginine, in agreement with the higher hydrophobicity of arginine residues. Plotting the fraction of paired oligoarginine and oligolysine as a function of [HFBA] shows that the cooperative effect in forming ion pairs is greater for oligoarginine than oligolysine.

Polymers from fatty acids: poly(ω-hydroxyl tetradecanoic acid) synthesis and physico-mechanical studies.

This Article describes the synthesis and physicomechanical properties of bioplastics prepared from methyl ω-hydroxytetradecanoic acid (Me-ω-OHC14), a new monomer available by a fermentation process using an engineered Candida tropicalis strain. Melt-condensation experiments were conducted using titanium tetraisopropoxide (Ti[OiPr](4)) as a catalyst in a two-stage polymerization (2 h at 200 °C under N(2), 4 h at 220 °C under 0.1 mmHg). Poly(ω-hydroxytetradecanoate), P(ω-OHC14), M(w), determined by SEC-MALLS, increased from 53K to 110K as the Ti(OiPr)(4) concentration increased from 50 to 300 ppm. By varying the polymerization conditions (catalyst concentration, reaction time, second-stage reaction temperature) a series of P(ω-OHC14) samples were prepared with M(w) values from 53K to 140K. The synthesized polyesters with M(w) ranging from 53K to 140K were subjected to characterization by DSC, TGA, DMTA, and tensile testing. Influences of P(ω-OHC14) molecular weight, melting point, and enthalpies of melting/crystallization on material tensile properties were explored. Cold-drawing tensile tests at room temperature for P(ω-OHC14) with M(w) 53K-78K showed a brittle-to-ductile transition. In contrast, P(ω-OHC14) with M(w) 53K undergoes brittle fracture. Increasing P(ω-OHC14) M(w) above 78K resulted in a strain-hardening phenomena and tough properties with elongation at break ~700% and true tensile strength of ~50 MPa. Comparisons between high density polyethylene and P(ω-OHC14) mechanical and thermal properties as a function of their respective molecular weights are discussed.

Intestinal microbiota modulates adrenomedullary response through Nod1 sensing in chromaffin cells.

The intestinal microbiota closely interacts with the neuroendocrine system and exerts profound effects on host physiology. Here, we report that nucleotide-binding oligomerization domain 1 (Nod1) ligand derived from intestinal bacteria modulates catecholamine storage and secretion in mouse adrenal chromaffin cells. The cytosolic peptidoglycan receptor Nod1 is involved in chromogranin A (Chga) retention in dense core granules (DCGs) in chromaffin cells. Mechanistically, upon recognizing its ligand, Nod1 localizes to DCGs, and recruits Rab2a, which is critical for Chga and epinephrine retention in DCGs. Depletion of Nod1 ligand or deficiency of Nod1 leads to a profound defect in epinephrine storage in chromaffin cells and subsequently less secretion upon stimulation. The intestine-adrenal medulla cross talk bridged by Nod1 ligand modulates adrenal medullary responses during the immobilization-induced stress response in mice. Thus, our study uncovers a mechanism by which intestinal microbes modulate epinephrine secretion in response to stress, which may provide further understanding of the gut-brain axis.

Biosynthesis of monomers for plastics from renewable oils.

Omega-hydroxyfatty acids are excellent monomers for synthesizing a unique family of polyethylene-like biobased plastics. However, ω-hydroxyfatty acids are difficult and expensive to prepare by traditional organic synthesis, precluding their use in commodity materials. Here we report the engineering of a strain of the diploid yeast Candida tropicalis to produce commercially viable yields of ω-hydroxyfatty acids. To develop the strain we identified and eliminated 16 genes encoding 6 cytochrome P450s, 4 fatty alcohol oxidases, and 6 alcohol dehydrogenases from the C. tropicalis genome. We also show that fatty acids with different chain lengths and degrees of unsaturation can be more efficiently oxidized by expressing different P450s within this strain background. Biocatalysis using engineered C. tropicalis is thus a potentially attractive biocatalytic platform for producing commodity chemicals from renewable resources.

Two-step biocatalytic route to biobased functional polyesters from omega-carboxy fatty acids and diols.

Biobased omega-carboxy fatty acid monomers 1,18-cis-9-octadecenedioic, 1,22-cis-9-docosenedioic, and 1,18-cis-9,10-epoxy-octadecanedioic acids were synthesized in high conversion yields from oleic, erucic and epoxy stearic acids by whole-cell biotransformations catalyzed by C. tropicalis ATCC20962. Maximum volumetric yields in shake-flasks were 17.3, 14.2, and 19.1 g/L after 48 h conversion for oleic acid and 72 h conversions for erucic and epoxy stearic acids, respectively. Studies in fermentor with better control of pH and glucose feeding revealed that conversion of oleic acid to 1,18-cis-9-octadecenedioic acid by C. tropicalis ATCC20962 occurred with productivities up to 0.5 g/L/h. The conversion of omega-carboxy fatty acid monomers to polyesters was then studied using immobilized Candida antarctica Lipase B (N435) as catalyst. Polycondensations with diols were performed in bulk as well as in diphenyl ether. The retension of functionality from fatty acid, to omega-carboxy fatty acid monomer and to corresponding polyesters resulted in polymers with with unsaturated and epoxidized repeat units and M(w) values ranging from 25000 to 57000 g/mol. These functional groups along chains disrupted crystallization giving materials that are low melting (23-40 degrees C). In contrast, saturated polyesters prepared from 1,18-octadecanedioic acid and 1,8-octanediol have correspondingly higher melting transitions (88 degrees C). TGA results indicated that all synthesized polyesters showed high thermal stabilities. Thus, the preparation of functional monomers from C. tropicalis omega-oxidation of fatty acids provides a wide range of new monomer building blocks to construct functional polymers.

Identification of carbohydrate peripheral epitopes important for recognition by positive-ion MALDI multistage mass spectrometry.

Carbohydrate sequences are important for various biological processes. It has recently been estimated to have 100,000-500,000 carbohydrate structures in mammalian glycome. However, the peripheral carbohydrate determinants on N- and O-glycoproteins, glycolipids, polysaccharides and secreted free sugars are limited in numbers. Among these blood-group-related antigens the ABO(H)- and Lewis-types are particularly important. Negative-ion MS/MS has been successfully used in assignment of these epitopes on free reducing sugars but cannot be applied to reduced sugars, e.g. O-glycans typically released from mucins as alditols, or in positive-ion detection of either reducing or reduced oligosaccharides. In the present study, we investigate the fragmentation features of permethylated reducing and reduced sugars under positive-ion conditions of multi-stage MALDI-MS, and propose the concept of epitope ion and epitope spectrum for determination of peripheral blood-group related epitopes on secreted human milk oligosaccharides and N-glycans as reducing sugars and O-glycans as reduced alditols in conjunction with MALDI-MS glycan profiling.

Linkage and sequence analysis of neutral oligosaccharides by negative-ion MALDI tandem mass spectrometry with laser-induced dissociation.

Mass spectrometry (MS) has become the primary method for high-sensitivity structural determination of oligosaccharides. Fragmentation in the negative-ion MS can provide a wealth of structural information and these can be used for sequence determination. However, although negative-ion MS of neutral oligosaccharide using the deprotonated molecule [M-H]- as the precursor has been very successful for electrospray ionization (ESI), it has only limited success for matrix-assisted laser desorption/ionization (MALDI). In the present study, the features of negative-ion MALDI primary spectra were investigated in detail and the product-ion spectra using [M-H]- and [M+Cl]- as the precursors were carefully compared. The formation of [M-H]- was the main difficulty for MALDI while [M+Cl]- was proved to be useful as alternative precursor anion for MALDI-MS/MS to produce similar fragmentation for sequencing of neutral oligosaccharides. N-(1-naphthyl)ethylenediamine dihydrochloride was then used as both the matrix and the Cl- dopant to evaluate the extent of structural information that can be obtained by negative-ion fragmentation from [M+Cl]- using laser-induced dissociation (LID)-MS/MS for linkage assignment of gluco-oligosaccharides and for typing of blood-group ABO(H) and Lewis antigens on either type 1 or type 2 backbone-chains.

Humicola insolens cutinase-catalyzed lactone ring-opening polymerizations: kinetic and mechanistic studies.

This paper explores reaction kinetics and mechanism for immobilized Humicola insolenscutinase (HIC), an important new biocatalyst that efficiently catalyzes non-natural polyester synthetic reactions. HIC, immobilized on Lewatit, was used as catalyst for epsilon-caprolactone (CL) and omega-pentadecalactone (PDL) ring-opening polymerizations (ROPs). Plots of percent CL conversion vs time were obtained in the temperature range from 50 to 90 degrees C. The kinetic plot of ln([M]0/[M]t) vs time (r2 = 0.99) for HIC-catalyzed bulk ROP of CL was linear, indicating that chain termination did not occur and the propagation rate is first order with respect to monomer concentration. Furthermore, linearity to 90% conversion for M(n) vs fractional CL conversion is consistent with a chain-end propagation mechanism. Deviation from linearity above 90% conversion indicates that a competition between ring-opening chain-end propagation and chain growth by steplike polycondensations takes place at high monomer conversion. HIC was inactive for catalysis of L-lactide and (R,S)-beta-butyrolactone ROP. HIC-catalyzed ROP of epsilon-CL and PDL in toluene were successfully performed, giving high molecular weight poly(epsilon-caprolactone) and omega-poly(pentadecalactone). In addition, the relative activities of immobilized Candida antarctica lipase B (CALB) and HIC for epsilon-CL and PDL polymerizations are reported herein.

Candida antarctica lipase B chemically immobilized on epoxy-activated micro- and nanobeads: catalysts for polyester synthesis.

Candida antarctica Lipase B (CALB) was covalently immobilized onto epoxy-activated macroporous poly(methyl methacrylate) Amberzyme beads (235 microm particle size, 220 A pore size) and nanoparticles (nanoPSG, diameter 68 nm) with a poly(glycidyl methacrylate) outer region. Amberzyme beads allowed CALB loading up to 0.16 g of enzyme per gram of support. IR microspectroscopy generated images of Amberzyme-CALB beads showed CALB is localized within a 50 microm thick loading front. IR microspectroscopy images, recorded prior to and after treatment of Amberzyme-CALB with DMSO/aqueous Triton X-100, are similar, confirming that CALB is largely chemically linked to Amberzyme. The activity of CALB immobilized on Amberzyme, Lewatit (i.e., Novozym 435 catalyst), and nanoPSG was assessed for lactone ring-opening and step-condensation polymerizations. For example, the percent conversion of -caprolactone using the same amount of enzyme catalyzed by Amberzym-CALB, Novozym 435, and nanoPSG-CALB for 20 min was 7.0, 16, and 65%, respectively. Differences in CALB reactivity were discussed based on resin physical parameters and availability of active sites determined by active site titrations. Regardless of the matrix used and chemical versus physical immobilization, -CL ring-opening polymerizations occur by a chain growth mechanism without chain termination. To test Amberzyme-CALB stability, the catalyst was reused over three reaction cycles for -CL ring-opening polymerization (70 degrees C, 70 min reactions) and glycerol/1,8-octanediol/adipic acid polycondensation reactions (90 degrees C, 64 h). Amberzyme-CALB was found to have far better stability for reuse relative to Novozym 435 for the polycondensation reaction.

Reversed phase ion-pairing chromatography of an oligolysine mixture in different mobile phases: effort of searching critical chromatography conditions.

Our earlier study [J. Chromatogr. A 1218 (2011) 7765] on separation of an oligolysine mixture consisting of chains with 2-8 lysine residues (number of lysine residues, dp=2-8) by ion-pairing reversed-phase chromatography using heptafluorobutyric acid (HFBA) as an ion pairing reagent at fixed mobile phase acetonitrile (ACN) content was extended to isocratic elution conditions with different ACN percentages. The present work explored how manipulating the mobile phase HFBA concentration ([HFBA]) and %-ACN content influences separations of the oligolysine mixture. The closed pairing model was used to analyze variation of the retention factor as a function of [HFBA]. The partition coefficient of the paired peptide decreased with increasing %-ACN. Pairing of HFBA to oligolysine was cooperative, and the effect increased when %-ACN in the mobile phase was lowered. A plot of the partition coefficient as a function of %-ACN for oligolysines varying in dp converged at one ACN content, indicating a critical condition in which components of different dp co-elute.

Enzyme-triggered hydrogelation via self-assembly of alternating peptides.

α-Chymotrypsin catalyzed oligomerization of the "dipeptide lego" KL-ethyl ester (OEt) in aqueous media triggers a rapid sol-gel transition due to formation of alternating (KL)x. Resulting mixed chain oligomers, at alkaline pH, self-assemble into ß-sheets. Thereafter, intermolecular backbone hydrogen bonding between peptides causes formation of physically entangled nanofibrillar networks.

Cooperative effect in ion pairing of oligolysine with heptafluorobutyric acid in reversed-phase chromatography.

The retention behavior of an oligolysine mixture, consisting of two to eight residues, was examined at different concentrations of heptafluorobutyric acid (HFBA) in the mobile phase using a C18 column. A single ion record (SIR) mode of the mass spectrometer produced a distinct retention time for each oligomer component. As the concentration of HFBA increased, the retention time of each oligomer increased. Furthermore, the increase in retention time is chain-length dependent such that, the longer the oligomer chain, the more rapid was the rate that retention time increased. A closed pairing model that presumes an equilibrium between the unpaired state and the paired state with a fixed number of HFBA molecules was used to analyze the retention factor as a function of [HFBA]. Curve fitting gave estimates of the ion-pairing equilibrium constant (K(ip,m)), the distribution constant of paired oligolysine (K(D,ip)), and the number of paired HFBA for each oligolysine (n). The plot of the fraction of paired oligolysine in the mobile phase, estimated from K(ip,m) and n as a function of [HFBA], revealed a cooperative effect. In contrast, an open pairing model that assumes independent pairing of HFBA with each residue failed to describe the observed retention behavior.