Giuseppe Impellizzeri

Amino acids and low-molecular-weight carbohydrates of some marine red algae

Abstract Amino acids and low-MW carbohydrates of 18 red algae have been analyzed. Several non-protein amino acids have been identified, including pyrrolidine-2,5-dicarboxylic acid ( 3 c) and N -methylmethionine sulfoxide ( 5 ), new natural products, and 13 known compounds, citrulline, β-alanine, γ-aminobutyric acid, baikiain ( 1 ), pipecolic acid ( 2 ), domoic acid ( 3 a), kainic acid ( 3 b), azetidine-2-carboxylic acid ( 4 ), methionine sulfoxide taurine, N -methyltaurine, N,N -dimethyltaurine and N,N,N -trimethyltaurine. Sugars present were mainly floridoside, isofloridoside and mannoglyceric acid. Details of the structural elucidation of new compounds are also given.

Copper(II) binding modes in the prion octapeptide PHGGGWGQ: a spectroscopic and voltammetric study.

The N-terminal octapeptide repeat region of human prion protein (PrPc) is known to bind Cu(II). To investigate the binding modes of copper in PrPc, an octapeptide Ac-PHGGGWGQ-NH2 (1), which corresponds to an octa-repeat sequence, and a tetrapeptide Ac-HGGG-NH2 (2) have been synthesised. The copper(II) complexes formed with 1 and 2 have been studied by circular dichroism (CD) and electron spin resonance (ESR) spectroscopy. Both peptides form 1:1 complexes with Cu(II) at neutral and basic pH. CD, ESR and visible absorption spectra suggest a similar co-ordination sphere of the metal ion in both peptides, which at neutral pH consists of a square pyramidal geometry with three peptidic nitrogens and the imidazole nitrogen as donor atoms. Cyclic voltammetric measurements were used to confirm the geometrical features of these copper(II) complexes: the observation of negative redox potentials are in good agreement with the inferred geometry. All these results taken together suggest that peptide 1 provides a single metal binding site to which copper(II) binds strongly at neutral and basic pH and that the binding of the metal induces the formation of a stiffened structure in the HGGG peptide fragment.

The Metal Loading Ability of β-Amyloid N-Terminus: A Combined Potentiometric and Spectroscopic Study of Copper(II) Complexes with β-Amyloid(1−16), Its Short or Mutated Peptide Fragments, and Its Polyethylene Glycol (PEG)-ylated Analogue

Alzheimers disease (AD) is becoming a rapidly growing health problem, as it is one of the main causes of dementia in the elderly. Interestingly, copper(II) (together with zinc and iron) ions are accumulated in amyloid deposits, suggesting that metal binding to Abeta could be involved in AD pathogenesis. In Abeta, the metal binding is believed to occur within the N-terminal region encompassing the amino acid residues 1-16. In this work, potentiometric, spectroscopic (UV-vis, circular dichroism, and electron paramagnetic resonance), and electrospray ionization mass spectrometry (ESI-MS) approaches were used to investigate the copper(II) coordination features of a new polyethylene glycol (PEG)-conjugated Abeta peptide fragment encompassing the 1-16 amino acid residues of the N-terminal region (Abeta(1-16)PEG). The high water solubility of the resulting metal complexes allowed us to obtain a complete complex speciation at different metal-to-ligand ratios ranging from 1:1 to 4:1. Potentiometric and ESI-MS data indicate that Abeta(1-16)PEG is able to bind up to four copper(II) ions. Furthermore, in order to establish the coordination environment at each metal binding site, a series of shorter peptide fragments of Abeta, namely, Abeta(1-4), Abeta(1-6), AcAbeta(1-6), and AcAbeta(8-16)Y10A, were synthesized, each encompassing a potential copper(II) binding site. The complexation properties of these shorter peptides were also comparatively investigated by using the same experimental approach.

Metal Loading Capacity of Aβ N-Terminus: a Combined Potentiometric and Spectroscopic Study of Zinc(II) Complexes with Aβ(1−16), Its Short or Mutated Peptide Fragments and Its Polyethylene Glycol−ylated Analogue

Aggregation of the amyloid beta-peptide (Abeta) into insoluble fibrils is a key pathological event in Alzheimers Disease (AD). There is now compelling evidence that metal binding to Abeta is involved in AD pathogenesis. The amino acid region 1-16 is widely considered as the metal binding domain of Abeta. In this work, we used a combined potentiometric, NMR, and electrospray ionization mass spectrometry (ESI-MS) approach to study the zinc(II) binding to a new polyethylene glycol (PEG)-conjugated peptide fragment encompassing the 1-16 amino acid sequence of Abeta (Abeta(1-16)PEG). Our results demonstrate for the first time that the Abeta(1-16) is able to coordinate up to three zinc ions, all the histidyl residues acting as independent anchor sites. The study was complemented by systematically investigating the zinc(II) complexes of a series of shorter peptide fragments related to the Abeta(1-16) sequence, namely, Abeta(1-4), Abeta(1-6), AcAbeta(1-6), AcAbeta(8-16)Y10A. The comparison of the whole results allowed the identification of the zinc(II) preferred binding sites within the longer Abeta(1-16) amino acid sequence. Unlike copper(II) that prefers the N-terminal amino group as the main binding site, the zinc(II) is preferentially placed in the 8-16 amino acidic region of Abeta(1-16).

Biosynthesis of indicaxanthin in opuntia ficus-indica fruits

Abstract The result of the incorporation of doubly labelled tyrosine into indicaxanthin in the fruits of Opuntia ficus-indica shows that the dihydropyridine moiety of the pigment originates from dopa by extradiol cleavage of the aromatic ring.

Coordination properties of 6-deoxy-6-[1-(2-amino) ethylamino]-β-cyclodextrin and the ability of its copper(II) complex to recognize and separate amino acid enantiomeric pairs

The functionalized cyclodextrin 6-deoxy-6-[1-(2-amino)ethylamino]-β-cyclodextrin was synthesized, and an NMR, EPR, pH-metric, and calorimetric investigation was carried out in aqueous solution in order to ascertain its behaviour towards protonation and copper(II) complex formation. The thermodynamic parameters of the ternary complex formation with alanine, phenylalanine and tryptophan enantiomeric pairs were also determined (25° C andI=0.1 mol dm−3). No thermodynamic enantioselectivity was observed in these systems, while a chiral, though poor, discrimination was observed in LEC: c.d. spectra also show enantiomeric stereoselectivity. The results of the present investigation, compared with previously reported results, suggest the occurence of acis-complex ⇆trans-complex equilibrium in such systems.

A re-investigation of copper coordination in the octa-repeats region of the prion protein.

An aqueous solution spectroscopic (Vis and EPR) study of the copper(II) complexes with the Ac-HGGG-NH2 and Ac-PHGGGWGQ-NH2 polypeptides (generically designated as L) suggests square base pyramids ascribable to [Cu(L)H(-2)] complex species, which contain three nitrogen donor atoms, arising from imidazole and peptide groups, in the equatorial plane and for a pseudo-octahedral geometry in the case of [CuLH-3]- and [Cu(L)H-4]2- which have four nitrogen donor atoms in their equatorial plane. The coordination sphere of the copper complex in the [Cu(L)H(-2)] species, which is present at neutral pH values, is completed by two oxygen donor atoms. ESI-MS spectra ascertained that water molecules are not present in the coordination equatorial plane of this latter species, in comparison with other copper(II) complexes with ligands bearing nitrogen and oxygen donor atoms and surely having equatorial water molecules. This indicates the coordination of a carbonyl oxygen atom in the equatorial plane has to be invoked. However, no direct proof about the involvement of a carbonyl group oxygen donor atom apically linked to copper was obtained, due to the flexibility of these structures at room temperature. Additionally, the low A(ll) value leads one to consider another oxygen atom of a carbonyl group being involved in the apical bond to copper in a fast exchange fashion. This apical interaction, which may also involve a water molecule, is more pronounced in the Cu-Ac-HGGG-NH2 than in the analogous Cu-Ac-PHGGGWGQ-NH2 system, probably because of the presence of tryptophan and proline in the polypeptide sequence.

Copper(II) complexes with chicken prion repeats: influence of proline and tyrosine residues on the coordination features.

The prion protein (PrPc) is a copper-binding glycoprotein that can misfold into a β-sheet-rich and pathogenic isoform (PrPsc) leading to prion diseases. The first non-mammalian PrPc was identified in chicken and it was found to keep many structural motifs present in mammalian PrPc, despite the low sequence identity (approximately 40%) between the two primary structures. The present paper describes the synthesis and the coordination properties of some hexapeptide fragments (namely, PHNPGY , HNPGYP and NPGYPH) as well as a bishexapeptide (PHNPGYPHNPGY), which encompasses two hexarepeats. The copper(II) complexes were characterized by means of potentiometric, UV–vis, circular dichroism and electron paramagnetic resonance techniques. We also report the synthesis of three hexapeptides (PHNPGF, HNPGFP and NPGFPH), in which one tyrosine was replaced by phenylalanine as well as two bishexapeptides in which either one (PHNPGFPHNPGY and PHNPGYPHNPGF), or two tyrosines were replaced by phenylalanine, in order to check whether tyrosine was involved in copper(II) binding. Overall, the results indicate that the major copper(II) species formed by the chicken PrP dodecapeptides are stabler than the analogous species reported for the peptide fragments containing two octarepeat peptides from the mammalian prion protein. It is concluded that the presence of four prolyl residues, that are break points in copper coordination, induces the metal-assisted formation of macrochelates as well as the formation of binuclear species. Furthermore, it has been shown that the phenolic group is directly involved in the formation of copper binuclear species.

Copper(II) and nickel(II) binding modes in a histidine-containing model dodecapeptide

The formation of complexes of HGGGHGHGGGHG (HG12) with copper(II) and nickel(II) have been studied in aqueous solution under various experimental conditions, including different pH and metal to ligand ratios. The study has been carried out using visible absorption, circular dichroism and electron paramagnetic resonance spectroscopic methods. Moreover, electrospray ionisation mass spectrometry has been used to directly determine the stoichiometry of the copper(II) complexes. The results indicate that HG12 can easily accommodate two metal ions in as many binding sites. The solution structure of the main complex species formed in the reaction of copper(II) with HG12 has been inferred by comparison with the copper(II) complexes formed with the shorter peptide fragments HGGGHG–NH2 (HG6), Ac–HGGGHG–NH2 (AcHG6) and Ac–HGGG–NH2 (AcHG4). With an equimolar metal to ligand ratio, the copper(II) ion binds preferentially in the N-terminal region of HG12. Conversely, Ni(II) ions form identical complexes regardless of whether the metal to ligand ratio is 1∶1 or 2∶1. Finally, the circular dichroism spectra indicate a significant modification of the peptide conformation upon metal binding.

A new betaxanthin from Glottiphyllum longum

Abstract The new betaxanthin (II) dopaxanthin was isolated from flowers of Glottiphyllum longum . Its structure was confirmed by chemical synthesis.