The influence of Cu(2+) on the unfolding and refolding of intact and proteolytically processed beta(2)-microglobulin

Human beta(2)-microglobulin (beta(2)m) is an amyloidogenic protein in patients suffering from chronic kidney disease and especially in those patients that need intermittent hemodialysis for longer periods, e.g., when awaiting transplantation. While many in vitro conditions induce beta(2)m-amyloid formation from wild-type (wt) beta(2)m and while a number of structurally altered beta(2)m molecules are known to be conformationally unstable and amyloidogenic on their own, it is not known why beta(2)m-amyloid is generated in some dialysis patients. For many amyloid proteins it is known that divalent metal ions, especially Cu(2+), display strong binding and distinct destabilizing effects on protein conformation. The present study uses CE to assess conformational states of wt and cleaved beta(2)m (dK58-beta(2)m, beta(2)m cleaved at lysine-58, a modification found in the circulation of hemodialysis patients) in the presence of divalent metal ions. The experiments provide both qualitative and quantitative data showing the specific destabilizing effects of Cu(2+)-ions on the folding of wt beta(2)m. Both refolding after acid denaturation and solution structure of beta(2)m under otherwise native conditions are severely influenced by Cu(2+). An increased unfolding, aggregation, and induction of Congo red-reactive molecular species in Cu(2+)-incubated wt-beta(2)m could be demonstrated while the refolding kinetics of dK58-beta(2)m, already slower than the wt molecule, appeared not to be further decreased by Cu(2+). Given the interest in the actions of metal ions in other types of amyloidosis, including, e.g., Alzheimer's disease and the prion encephalopathies, the use of microelectrophoretic methods to monitor unfolding and refolding of biomolecules available in scarce amounts as shown in this study is an attractive option.     

Quantitative Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS) Imaging of Individual Vesicles to Investigate the Relation between Fraction of Chemical Release and Vesicle Size

We used correlative transmission electron microscopy (TEM) and nanoscale secondary ion mass spectrometry (NanoSIMS) imaging to quantify the contents of subvesicular compartments, and to measure the partial release fraction of ¹³C-dopamine in cellular nanovesicles as a function of size. Three modes of exocytosis comprise full release, kiss-and-run, and partial release. The latter has been subject to scientific debate, despite a growing amount of supporting literature. We tailored culturing procedures to alter vesicle size and definitively show no size correlation with the fraction of partial release. In NanoSIMS images, vesicle content was indicated by the presence of isotopic dopamine, while vesicles which underwent partial release were identified by the presence of an ¹²⁷I-labelled drug, to which they were exposed during exocytosis allowing entry into the open vesicle prior to its closing again. Demonstration of similar partial release fractions indicates that this mode of exocytosis is predominant across a wide range of vesicle sizes.     

Myoglobin modification by enzyme-generated dopamine reactive species

The generation of reactive quinone species (DAQ) from oxidation of dopamine (DA) is involved in neurodegenerative pathologies like Parkinson's disease (A. Borta, G. U. H?glinger, J. Neurochem. 2007, 100, 587-595). The oxidation of DA to DAQ can occur either in a single two-electron process or in two consecutive one-electron steps, through semiquinone radicals, giving rise to different patterns of reactions. The former type of reaction can be promoted by tyrosinase, the latter by peroxidases in the presence of H(2)O(2), which can be formed under oxidative stress conditions. Both enzymes were employed for the characterization of the thiol-catechol adducts formed by reaction of DA and cysteine or glutathione, and for the identification of specific amino acid residues modified by DAQs in two representative target proteins, human and horse heart myoglobin. Our results indicate that the cysteinyl-DA adducts are formed from the same quinone intermediate independently of the mechanism of DA oxidation, and that the hallmark of a radical mechanism is the formation of the cystine dimer. The reactivity of quinone species also controls the DA-promoted derivatization of histidine residues in proteins. However, for the modification of the cysteine residue in human myoglobin, a radical intramolecular mechanism has been proposed, in which the protein acts both as the catalyst and target of the reaction. Most importantly, the modification of myoglobins through DAQ linkages, and in particular by DA oligomers, has dramatic effects on their stability, as it induces protein unfolding and incorporation into insoluble melanic precipitates.     

A novel Ni(4) complex exhibiting microsecond quantum tunneling of the magnetization

A highly asymmetric Ni(II) cluster [Ni(4)(OH)(OMe)(3)(Hphpz)(4)(MeOH)(3)](MeOH) (1) (H(2)phpz=3-methyl-5-(2-hydroxyphenyl)pyrazole) has been prepared and its structure determined by means of single-crystal X-ray diffraction by using synchrotron radiation. Variable-temperature bulk-magnetization measurements show that the complex exhibits intramolecular-ferromagnetic interactions leading to a spin ground state S=4 with close-lying excited states. Magnetization and high-frequency EPR measurements suggest the presence of sizable Ising-type magnetic anisotropy, with zero-field splitting parameters D=-0.263 cm(-1) and E=0.04 cm(-1) for the spin ground state, and an isotropic g value of 2.25. The presence of both axial and transverse anisotropy was confirmed through low-temperature specific heat determinations down to 300 mK, but no slow relaxation of the magnetization was observed by AC measurements down to 1.8 K. Interestingly, AC susceptibility measurements down to temperatures as low as 23 mK showed no indication of slow relaxation of the magnetization in 1. Thus, despite the presence of an anisotropy barrier (U approximately 4.21 cm(-1) for the purely axial limit), the magnetization relaxation remains extremely fast down to the lowest temperatures. The estimated quantum tunneling rate, Gamma>0.667 MHz, makes this complex a prime candidate for observation of coherent tunneling of the magnetization.     

Determination of alpha-amylase inhibitor activity of phaseolamin from kidney bean (Phaseolus vulgaris) in dietary supplements by HPAEC-PAD

Some dietary supplements, so-called 'starch-blockers', used to control overweight, are based on the protein concentrate of the kidney bean, known to contain high levels of the alpha-amylase inhibitor phaseolamin, which may hinder the digestion of complex carbohydrates, thereby promoting or supporting weight loss. Currently, methods to determine the levels of alpha-amylase inhibitor are based on the measurement of alpha-amylase activity using colorimetric methods that cannot be applied to dietary supplements because they are complex mixtures of different ingredients that may interfere with the measurement. The aim of this study was to develop an alternative method to determine the level of phaseolamin in dietary supplements, using high-performance anion-exchange chromatography coupled with pulsed amperometric detection (HPAEC-PAD) to measure the amount of maltose resulting from the action of the enzyme porcine alpha-amylase on soluble starch in the presence and absence of the inhibitor. The assay described proved sensitive and accurate for use with both dietary supplements and raw materials.     

Electrophilic S‐Trifluoromethylation of Cysteine Side Chains in α‐ and β‐Peptides: Isolation of Trifluoro‐methylated Sandostatin® (Octreotide) Derivatives

The new electrophilic trifluoromethylating 1‐(trifluoromethyl)‐benziodoxole reagents A and B (Scheme 1) have been used to selectively attach CF₃ groups to the S‐atom of cysteine side chains of α‐ and β‐peptides (up to 13‐residues‐long; products 7 – 14 ). Other functional groups in the substrates (amino, amido, carbamate, carboxylate, hydroxy, phenyl) are not attacked by these soft reagents. Depending on the conditions, the indole ring of a Trp residue may also be trifluoromethylated (in the 2‐position). The products are purified by chromatography, and identified by ¹H‐, ¹³C‐, and ¹⁹F‐NMR spectroscopy, by CD spectroscopy, and by high‐resolution mass spectrometry. The CF₃ groups, thus introduced, may be replaced by H (Na/NH₃), an overall Cys/Ala conversion. The importance of trifluoromethylations in medicinal chemistry and possible applications of the method (spin‐labelling, imaging, PET) are discussed.     

Mononuclear manganese(III) complexes as building blocks for the design of trinuclear manganese clusters: study of the ligand influence on the magnetic properties of the [Mn3(mu3-O)](7+) core

The synthesis, crystal structure, and magnetic properties of three new manganese(III) clusters are reported, [Mn 3(mu 3-O)(phpzH) 3(MeOH) 3(OAc)] (1), [Mn 3(mu 3-O)(phpzMe) 3(MeOH) 3(OAc)].1.5MeOH (2), and [Mn 3(mu 3-O)(phpzH) 3(MeOH) 4(N 3)].MeOH (3) (H 2phpzH = 3(5)-(2-hydroxyphenyl)-pyrazole and H 2phpzMe = 3(5)-(2-hydroxyphenyl)-5(3)-methylpyrazole). Complexes 1- 3 consist of a triangle of manganese(III) ions with an oxido-center bridge and three ligands, phpzR (2-) (R = H, Me) that form a plane with the metal ions. All the complexes contain the same core with the general formula [Mn 3(mu 3-O)(phpzR) 3] (+). Methanol molecules and additional bridging ligands, that is, acetate (complexes 1 and 2) and azide (complex 3), are at the terminal positions. Temperature dependent magnetic susceptibility studies indicate the presence of predominant antiferromagnetic intramolecular interactions between manganese(III) ions in 1 and 3, while both antiferromagnetic and ferromagnetic intramolecular interactions are operative in 2.     

Front Cover: Katritzky Salts for the Synthesis of Unnatural Amino Acids and Late-Stage Functionalization of Peptides (Eur. J. Org. Chem. 12/2023)

Peptide drug discovery often benefits from the large structural diversity permitted by unnatural amino acids (UAAs). Indeed, numerous approved peptide drugs include UAAs in their sequences. Therefore, innovative chemical approaches either to synthesize UAAs or to allow late-stage functionalization of peptides are emerging themes in peptide drug discovery. Thanks to the recent advances in deaminative strategies using alkylpyridiniums salts, often referred to as Katritzky salts, a variety of radical alkylation methods have been developed. In recent years the use of Katritzky salts have become popular in peptide chemistry due to their ease of preparation from a primary amine, which is a predominant functional group in amino acids. This review highlights the progress that has been made by using Katritzky salts in the synthesis of UAAs, late-stage peptide functionalization, and peptide macrocyclization.