Peptide dissociation in solution or bound to a polymer: comparative solvent effect

Comparative fibril formation of analogs corresponding to the (12-24) segment of the β-amyloid peptide

The (1-42) β-amyloid peptide is a main component of the plaques found in the brain of patients suffering from the Alzheimer's disease. As the single substitution of Glu for Gln at position 22 of this peptide seems to be responsible for the manifestation of the more severe amyloidosis (Dutch-type), we decided to evaluate the aggregation characteristics of peptide analogs interchanging Glu and Gln residues at positions 22 and also 15 in the minor (12-24) (VHHQ(15)KLVFFAE(22)DV) fragment. The Q15Q22, E15E22, E15Q22 and the native Q15E22 were compared to the (1-42) β-amyloid peptide in terms of fibril or structured aggregates formation propensity. In contrast to a rather similar solubility data measured of all analogs, fluorescence and light scattering methods indicated that only Q15E22 and Q15Q22 displayed relevant fibril formation capacity. Conversely, E15E22 and E15Q22 were not capable of the formation of this type of structure thus suggesting a key role for the Q(15) residue in the unique aggregation characteristic of the β-amyloid peptide.

New biotin derivatives for labeling and solubilizing IgG peptides

In this article, we describe the synthesis of a new class of oligoethylene-glycol based water-soluble biotin derivatives for labeling of peptides with limited solubility in aqueous solution. First 4,7,10-trioxa-1,13-tridecanediamine was mono-acetylated by succinic anhydride (Ttds) followed by the introduction of N-Fmoc-protecting group using Fmoc-N-hydroxysuccinimide ester. The resulting compound (Fmoc-Ttds) was used for the preparation of 4,7,10-trioxa-1,13-tridecanediamine di- and trimers on solid phase using Wang resin by carbodiimide coupling method. After attachment of Fmoc-Ttds to the solid support, the Fmoc-blocking group was removed and the Ttds-modified resin was repeatedly acylated by Fmoc-Ttds or by biotin using PyBOP/HOBt active ester reaction. Finally the product [Fmoc-(Ttds)(n) or biotinyl-(Ttds)(n) (where n = 1, 2 or 3)] was removed from the resin by trifluoroacetic acid in the presence of water. After appropriate HPLC purification and characterization (MS) biotinyl-(Ttds)(n) (where n = 1, 2 or 3) were introduced to the N-terminal of poorly soluble oligopeptides by solid phase peptide synthesis. We found that this new class of biotinylating reagent could be prepared easily and in good yield. Comparative solubility measurements suggest that the incorporation of these moieties-depending on the number of Ttds unit-could enhance water solubility.

Comparative investigation of the cleavage step in the synthesis of model peptide resins: implications for Nalpha-9-fluorenylmethyloxycarbonyl-solid phase peptide synthesis

Based on our studies of the stability of model peptide-resin linkage in acid media, we previously proposed a rule for resin selection and a final cleavage protocol applicable to the Nalpha-tert-butyloxycarbonyl (Boc)-peptide synthesis strategy. We found that incorrect choices resulted in decreases in the final synthesis yield, which is highly dependent on the peptide sequence, of as high as 30%. The present paper continues along this line of research but examines the Nalpha-9-fluorenylmethyloxycarbonyl (Fmoc)-synthesis strategy. The vasoactive peptide angiotensin II (AII, DRVYIHPF) and its [Gly8]-AII analogue were selected as model peptide resins. Variations in parameters such as the type of spacer group (linker) between the peptide backbone and the resin, as well as in the final acid cleavage protocol, were evaluated. The same methodology employed for the Boc strategy was used in order to establish rules for selection of the most appropriate linker-resin conjugate or of the peptide cleavage method, depending on the sequence to be assembled. The results obtained after treatment with four cleavage solutions and with four types of linker groups indicate that, irrespective of the circumstance, it is not possible to achieve complete removal of the peptide chains from the resin. Moreover, the Phe-attaching peptide at the C-terminal yielded far less cleavage (50-60%) than that observed with the Gly-bearing sequences at the same position (70-90%). Lastly, the fastest cleavage occurred with reagent K acid treatment and when the peptide was attached to the Wang resin.

Improvement of recovery and repeatability in liquid chromatography-mass spectrometry analysis of peptides

Poor repeatability of peak areas is a problem frequently encountered in peptide analysis with nanoLiquid Chromatography coupled on-line with Mass Spectrometry (nanoLC-MS). As a result, quantitative analysis will be seriously hampered unless the observed variability can be corrected in some way. Currently, labeling techniques or addition of internal standards are often applied for this purpose. However, these procedures are elaborate and error-prone and may render complex samples even more complex. Moreover, whenever poor repeatability results from variable recovery, not just quantification, but also sensitivity is affected. We have studied the parameters influencing the repeatability of chromatographic peak areas for a model set of proteolytic peptides (i.e., a cytochrome c tryptic digest) in nanoLC-MS analysis. It is demonstrated that repeatability issues are mainly due to poor recovery of peptides from the sample vial. Problems are largely resolved by addition of an organic modifier to the sample vial to improve solubility of the peptides, but care needs to be taken not to lose peptides due to reduced affinity for reversed-phase materials. Good results are obtained when applying dimethylsulfoxide (DMSO) for this purpose. When applying DMSO, repeatability increases, and the limit of detection (LOD) decreases. For the most hydrophobic peptides, a gain in LOD of at least an order of magnitude is obtained. In an aqueous sample containing 0.1% formic acid (FA), it is possible to detect 100-200 fmol of peptide, whereas +/-10 fmol can be detected in a sample containing 5% FA and 25% DMSO (10 microL injections).

Interpretation of the dissolution of insoluble peptide sequences based on the acid-base properties of the solvent

The dissolution process of model insoluble peptide sequences was investigated in view of the electron acceptor (AN) and electron donor (DN) solvent properties. The Alzheimer's disease-inducing (1-42) Abeta-amyloid peptide and its (1-21) fragment, the (66-97) transmembrane bradykinin B2 receptor sequence, and the strongly aggregated VVLGAAIV were selected as models of insoluble peptides. Solvents presenting similar AN and DN values failed, despite their polarities, to dissociate peptide chains (free in solution or bound to a polymer). The maximum solubility of these aggregated sequences was attained in solvents presenting the highest possible (AN-DN) values (in positive or negative mode). The AN-DN values ranged from approximately -20 to +80 and, notably, the lowest dissociation power was ascribed to solvents presenting values of approximately +40. The strong hydrogen bond donor water is located in this region, indicating that, for dissociation of specific insoluble segments, the solvent should appropriately combine its acid/base strength with the potential for van der Waals interactions. We also observed a sequence-dependent pH effect on peptide solubility confirmed through circular dichroism spectroscopy. This approach also revealed a complex but, in many cases, consistent influence of peptide conformation on its solubility degree, even when structure-inducing solvents were added. In conclusion, the random method of selecting solvents to dissolve insoluble and intractable peptide sequences, still in use by some, could be partially supplanted by the strategy described herein, which may be also applicable to other solute dissociation processes.

Determination of Site−Site Distance and Site Concentration within Polymer Beads: A Combined Swelling-Electron Paramagnetic Resonance Study

This work proposes a combined swelling-electron paramagnetic resonance (EPR) approach aiming at determining some unusual polymer solvation parameters relevant for chemical processes occurring inside beads. Batches of benzhydrylamine-resin (BHAR), a copolymer of styrene-1% divinylbenzene containing phenylmethylamine groups were, labeled with the paramagnetic amino acid 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amine-4-carboxylic acid (TOAC), and their swelling properties and EPR spectra were examined in DCM and DMF. By taking into account the BHARs labeling degrees, the corresponding swelling values, and some polymer structural characteristics, it was possible to calculate polymer swelling parameters, among them, the volume and the number of sites per bead, site-site distances and site concentration. The latter values ranged from 17 to 170 A and from 0.4 to 550 mM, respectively. EPR spectroscopy was applied to validate the multistep calculation strategy of these swelling parameters. Spin-spin interaction was detected in the labeled resins at site-site distances less than approximately 60 A or probe concentrations higher than approximately 1 x 10(-2) M, in close agreement with the values obtained for the spin probe free in solution. Complementarily, the yield of coupling reactions in different resins indicated that the greater the inter-site distance or the lower the site concentration, the faster the reaction. The results suggested that the model and the experimental measurements developed for the determination of solvation parameters represent a relevant step forward for the deeper understanding and improvement of polymer-related processes.