Methods of the site-selective solid phase synthesis of peptide-derived Amadori products

Microwave-assisted solid-phase synthesis of lactosylated peptides for food analytical application

The lactosylation of whey proteins affects their properties, especially their allergenicity and nutritional value, which matters in infant feeding. The quantification of lactosylated peptides requires analytically pure standards which are not commercially available. Herein, we proposed a fast, simple, and efficient protocol for the synthesis of lactosylated peptides on solid support based on microwave-assisted synthesis combined with boronate affinity chromatography utilizing the functionalized resin developed in our research group. We have used our method to synthesize identified modified peptides derived from β-lactoglobulin in tryptic digestion. Thus, the standard peptides contain the dabcyl moiety for determination of the amount in a sample and a cleavable linker to obtain tryptic analogs of β-lactoglobulin. Moreover, for the first time, we applied the functionalized resin to enrich the sample of human serum albumin lactosylated in vitro, into lactosylated peptides.

HPLC-free method of synthesis of isotopically labeled deoxyfructosylated peptides

The biomarker strategy, based on multiple specific glycation sites in plasma proteins, could essentially increase the efficiency of glycemic control and disease prediction. Besides glycated albumin being a potential biomarker of early states of diabetes mellitus and control of short-term, it has been shown that the glycation of fibrinogen may also impact the formation of the fibrin network, while quantification of glycation of the CD59 protein allows for early detection of glucose intolerance in pregnant women. A different level of glycation of individual lysine residues in proteins has a crucial influence on the stages of the disease. The quantification of new biomarkers of different stages of diabetes requires appropriate isotope-labeled analogs that may improve biomarker search by providing more accurate quantitative data and by more robust detection/quantitation of low-abundance biomarkers. In the presented work, we proposed a fast and simple protocol for the synthesis of isotopically labeled and bi-labeled deoxyfructosylated peptide based on a combination of microwave-assisted synthesis and boronic affinity chromatography using functionalized resin (PhB-Lys(PhB)-ChemMatrix® Rink resin) developed by us. Our method is focused on the synthesis of glycated peptides identified in glycated albumin (GA) after enzymatic hydrolysis catalyzed by trypsin after arginine residues. Thereby, the standard peptides comprised [¹³C₆]-deoxyfructose attached to lysine residue side chain, a dabcyl moiety for determination of standard amounts, and a cleavable linker. Moreover, we applied bi-labeled deoxyfructosylated peptide to determine the concentration of appropriate analog in a sample of human serum albumin glycated in vitro.

Solid-phase synthesis of D-fructose-derived Heyns peptides utilizing Nα-Fmoc-Lysin[Nε-(2-deoxy-D-glucos-2-yl),Nε-Boc]-OH as building block

Aldoses and ketoses can glycate proteins yielding isomeric Amadori and Heyns products, respectively. Evidently, d-fructose is more involved in glycoxidation than d-glucose favoring the formation of advanced glycation endproducts (AGEs). While Amadori products and glucation have been studied extensively, the in vivo effects of fructation are largely unknown. The characterization of isomeric Amadori and Heyns peptides requires sufficient quantities of pure peptides. Thus, the glycated building block N^α-Fmoc-Lys[N^ε-(2-deoxy-d-glucos-2-yl),N^ε-Boc]-OH (Fmoc-Lys(Glc,Boc)-OH), which was synthesized in two steps starting from unprotected d-fructose and Fmoc-l-lysine hydrochloride, was site-specifically incorporated during solid-phase peptide synthesis. The building block allowed the synthesis of a peptide identified in tryptic digests of human serum albumin containing the reported glycation site at Lys233. The structure of the glycated amino acid derivatives and the peptide was confirmed by mass spectrometry and NMR spectroscopy. Importantly, the unprotected sugar moiety showed neither notable epimerization nor undesired side reactions during peptide elongation, allowing the incorporation of epimerically pure glucosyllysine. Upon acidic treatment, the building block as well as the resin-bound peptide formed one major byproduct due to incomplete Boc-deprotection, which was well separated by reversed-phase chromatography. Expectedly, the tandem mass spectra of the fructated amino acid and peptide were dominated by signals indicating neutral losses of 18, 36, 54, 84 and 96 m/z-units generating pyrylium and furylium ions.

Intramolecularly stapled amphipathic peptides via a boron–sugar interaction

The intramolecular interactions between the fructosyl moiety and phenylboronic acid incorporated into various positions of the peptide chain were investigated using mass spectrometry (MS), circular dichroism (CD), and nuclear magnetic resonance (NMR).

Solid-phase synthesis of peptides containing aminoadipic semialdehyde moiety and their cyclisations

Pathological levels of oxidative stress (OS) have been implicated in many diseases including diabetes mellitus, neurodegenerative diseases, inflammatory diseases, atherosclerosis, and cancer. Studies of oxidative stress are however complicated by the low concentration of oxidation products. To resolve this problem, we tested a new derivative of aminoadipic semialdehyde (Fmoc-Aea-OH) in the solid-phase synthesis of carbonylated peptides. We prepared a series of peptides with free and acetylated N-terminal amino groups using the Fmoc-Aea-OH reagent. LC-MS, ESI-MS, and MS/MS spectra confirmed the sequences of the modified peptides, although the LC-MS and ESI-MS spectra were dominated by signals corresponding to dehydration products. NMR studies of acetylated products revealed that the dominant product formed in this reaction contains a 1,2,3,4-tetrahydropyridine-2-carboxylic acid residue. Another side reaction in this system was the cleavage of the amide bond between the Aea residue and the amino acid moiety preceding it resulting in the formation of a side product with a six-membered ring at the N-terminus (2,3,4,5-tetrahydropyridine-2-carboxylic acid residue). We found that, depending on the peptide sequence, one of those side products is predominant. Our work suggests new methods for the solid-state synthesis of peptides containing unnatural amino acids.

Maillard Proteomics: Opening New Pages

Protein glycation is a ubiquitous non-enzymatic post-translational modification, formed by reaction of protein amino and guanidino groups with carbonyl compounds, presumably reducing sugars and α-dicarbonyls. Resulting advanced glycation end products (AGEs) represent a highly heterogeneous group of compounds, deleterious in mammals due to their pro-inflammatory effect, and impact in pathogenesis of diabetes mellitus, Alzheimer's disease and ageing. The body of information on the mechanisms and pathways of AGE formation, acquired during the last decades, clearly indicates a certain site-specificity of glycation. It makes characterization of individual glycation sites a critical pre-requisite for understanding in vivo mechanisms of AGE formation and developing adequate nutritional and therapeutic approaches to reduce it in humans. In this context, proteomics is the methodology of choice to address site-specific molecular changes related to protein glycation. Therefore, here we summarize the methods of Maillard proteomics, specifically focusing on the techniques providing comprehensive structural and quantitative characterization of glycated proteome. Further, we address the novel break-through areas, recently established in the field of Maillard research, i.e., in vitro models based on synthetic peptides, site-based diagnostics of metabolism-related diseases (e.g., diabetes mellitus), proteomics of anti-glycative defense, and dynamics of plant glycated proteome during ageing and response to environmental stress.

Mass spectrometry for the elucidation of the subtle molecular structure of biodegradable polymers and their degradation products

Contemporary reports by Polish authors on the application of mass spectrometric methods for the elucidation of the subtle molecular structure of biodegradable polymers and their degradation products will be presented. Special emphasis will be given to natural aliphatic (co)polyesters (PHA) and their synthetic analogues, formed through anionic ring-opening polymerization (ROP) of β-substituted β-lactones. Moreover, the application of MS techniques for the evaluation of the structure of biodegradable polymers obtained in ionic and coordination polymerization of cyclic ethers and esters as well as products of step-growth polymerization, in which bifunctional or multifunctional monomers react to form oligomers and eventually long chain polymers, will be discussed. Furthermore, the application of modern MS techniques for the assessment of polymer degradation products, frequently bearing characteristic end groups that can be revealed and differentiated by MS, will be discussed within the context of specific degradation pathways. Finally, recent Polish accomplishments in the area of mass spectrometry will be outlined.

Site-selective solid phase synthesis of carbonylated peptides

The aim of our research was to design an efficient method for the solid phase synthesis of carbonylated peptides. For this purpose, we designed and synthesized a fully protected derivative Fmoc-amino(2,5,5-trimetyhyl-1,3-dioxolan-2-yl)acetic acid (Fmoc-Atda-OH) of a novel unnatural amino acid (Thr(O)-2-amino-3-oxo-butanoic acid). To obtain the mentioned derivative, two synthetic strategies were investigated using different reagents for carbonyl protection, ethane-1,2-diol and 2,2-dimethyl-propane-1,3-diol. The racemization of oxidized threonine was also analyzed and discussed. We successfully carried out the solid phase synthesis of peptides containing a Thr(O) moiety using Fmoc-Atda-OH according to the standard Fmoc strategy. The application of the designed building block allows the synthesis of peptides containing D,L-Thr(O) residue, which may be used as models of oxidatively modified peptides which occur in biological systems and are related to many diseases.

Comparison of modification sites in glycated crystallin in vitro and in vivo

Glycation of α-crystallin is responsible for age- and diabetic-related cataracts, which are the main cause of blindness worldwide. We optimized the method of identification of lysine residues prone to glycation using the combination of LC-MS, isotopic labeling, and modified synthetic peptide standards with the glycated lysine derivative (Fmoc-Lys(i,i-Fru,Boc)-OH). The in vitro glycation of bovine lens α-crystallin was conducted by optimized method with the equimolar mixture of [(12)C6]- and [(13)C6]D-glucose. The in vivo glycation was studied on human lens crystallin. The glycated protein was subjected to proteolysis and analyzed using LC-MS. The results of in vitro and in vivo glycation of α-crystallin reveal a different distribution of the modified lysine residues. More Amadori products were detected as a result of the in vitro reaction due to forced glycation conditions. The developed method allowed us to identify the glycation sites in crystallin from eye lenses obtained from patients suffering from the cataract. We identified K166 in the A chain and K166 in the B chain of α-crystallin as major glycation sites during the in vitro reaction. We found also two in vivo glycated lysine residues: K92 in the B chain and K166 in the A chain, which are known as locations for Amadori products. These modification sites were confirmed by the LC-MS experiment using two synthetic standards. This study demonstrates the applicability of the LC-MS methods combined with the isotopic labeling and synthetic peptide standards for analysis of post-translational modifications in the biological material.

Nonenzymatic modification of Ubiquitin under high-pressure and -temperature treatment: mass spectrometric studies

The effect of high-pressure and/or high-temperature on the glycation of a model protein (ubiquitin) was investigated by mass spectrometry. This paper reports the impact of high pressure (up to 1200 MPa) on the modification of a ubiquitin using ESI-MS measurements. The application of glucose labeled with stable isotope allows a quantitative assessment of modification under the conditions of high-pressure (HPG) and high-temperature (HTG) glycation. A higher degree of modification was observed for the sample heated at 80 °C for 25 min under atmospheric pressure than for sample treated under high pressure. In samples treated at pressure below 400 MPa an insignificant increase of glycation level was observed, whereas high pressure (>600 MPa) has only a minor effect on the number of hexose moieties (Fru) attached to the lysine residue side chain.

Selective detection of carbohydrates and their peptide conjugates by ESI-MS using synthetic quaternary ammonium salt derivatives of phenylboronic acids

We present new tags based on the derivatives of phenylboronic acid and apply them for the selective detection of sugars and peptide-sugar conjugates in mass spectrometry. We investigated the binding of phenylboronic acid and its quaternary ammonium salt (QAS) derivatives to carbohydrates and peptide-derived Amadori products by HR-MS and MS/MS experiments. The formation of complexes between sugar or sugar-peptide conjugates and synthetic tags was confirmed on the basis of the unique isotopic distribution resulting from the presence of boron atom. Moreover, incorporation of a quaternary ammonium salt dramatically improved the efficiency of ionization in mass spectrometry. It was found that the formation of a complex with phenylboronic acid stabilizes the sugar moiety in glycated peptides, resulting in simplification of the fragmentation pattern of peptide-derived Amadori products. The obtained results suggest that derivatization of phenylboronic acid as QAS is a promising method for sensitive ESI-MS detection of carbohydrates and their conjugates formed by non-enzymatic glycation or glycosylation.

Hydrogen scrambling in non-covalent complexes of peptides

RATIONALE

Mass spectrometry analysis combined with hydrogen-deuterium exchange (HDX-MS) is arising as a tool for quick analysis of native protein conformation. However, during collision-induced dissociation (CID) the spatial distribution of deuterium is not always conserved. It is therefore important to find out how hydrogen scrambling occurs--this study concentrates on the possibility of scrambling between amino acid residues spatially close together, but not connected by covalent bonds.

METHODS

Peptides used in this study were synthesized by Fmoc strategy. Deuteration occurred in ammonia formate solution in D(2)O. Non-covalent complexes consisting of a deuterated and a non-deuterated peptide were analyzed by electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FT-ICR-MS) with quadrupole mass filter. Low-energy CID was used for complex dissociation.

RESULTS

The complexes were isolated on a quadrupole and subjected to CID to cause dissociation. The deuterium distribution before and after the dissociation of a non-covalent complex to its components was measured. The study revealed that no significant scrambling occurred between the constituents of the complexes--the degree of scrambling did not exceed 10%.

CONCLUSIONS

The results obtained for the complexes should be similar to those for protein parts spatially close together--hydrogen scrambling between them should be negligible. The knowledge that almost all the scrambling occurs along peptide chains gives a better insight into the mechanism of HDX inside a protein.

Solid-phase route to Fmoc-protected cationic amino acid building blocks

Diamino acids are commonly found in bioactive compounds, yet only few are commercially available as building blocks for solid-phase peptide synthesis. In the present work a convenient, inexpensive route to multiple-charged amino acid building blocks with varying degree of hydrophobicity was developed. A versatile solid-phase protocol leading to selectively protected amino alcohol intermediates was followed by oxidation to yield the desired di- or polycationic amino acid building blocks in gram-scale amounts. The synthetic sequence comprises loading of (S)-1-(p-nosyl)aziridine-2-methanol onto a freshly prepared trityl bromide resin, followed by ring opening with an appropriate primary amine, on-resin N(β)-Boc protection of the resulting secondary amine, exchange of the N(α)-protecting group, cleavage from the resin, and finally oxidation in solution to yield the target γ-aza substituted building blocks having an Fmoc/Boc protection scheme. This strategy facilitates incorporation of multiple positive charges into the building blocks provided that the corresponding partially protected di- or polyamines are available. An array of compounds covering a wide variety of γ-aza substituted analogs of simple neutral amino acids as well as analogs displaying high bulkiness or polycationic side chains was prepared. Two building blocks were incorporated into peptide sequences using microwave-assisted solid-phase peptide synthesis confirming their general utility.

Hydrate of 2,3:4,5-di-O-isopropylidene-β-D-arabino-hexos-2-ulo-2,6-pyranose as new crystalline reagent in the preparation of Amadori derived peptides

We established, that crystalline hydrate of 2,3:4,5-di-O-isopropylidene-β-D-arabino-hexos-2-ulo-2,6-pyranose is a new, convenient and stable reagent for solid phase synthesis of peptide derived Amadori products. The structure of the title compound was studied by X-ray analysis, NMR spectroscopy, and high resolution ESI-MS. The crystal structure indicated the existence of two symmetry-independent molecules that were not connected with hydrogen bonds. A comparison with previously reported 2,3:4,5-di-O-isopropylidene-β-D-fructopyranose revealed, that these two compounds are isostructural.

Electrospray ionization mass spectrometric analysis of complexes between peptide-derived Amadori products and borate ions

Hexose-modified peptides, products of the enzymatic hydrolysis of glycated proteins, could be used as markers of diabetes mellitus, the aging process and other diseases. The main difficulty in this approach is the detection of glycated peptides in the complex mixtures of compounds. In this study we investigated the formation of borate complexes of the peptide-derived Amadori products by high-resolution mass spectrometry (HRMS) and tandem mass spectrometry (MS/MS) experiments. It was found that the formation of a complex with the borate ion stabilizes the sugar moiety, resulting in the simplification of the fragmentation patterns of peptide-derived Amadori products. The level of dehydration, as well as the elimination of formaldehyde from the precursor ions of borate complexes, was lower as compared to the free peptide. On the other hand the intensity of the b- and y-type ions for borate complexes is significantly higher in comparison to the free peptide-derived Amadori product. Moreover, the elimination of a whole hexose moiety was not detected in the examined peptides.