X-ray scattering study of the effect of hydration on the cross-beta structure of amyloid fibrils

We have investigated the effect of sample hydration on the wide-angle X-ray scattering patterns of amyloid fibrils from two different sources, hen egg white lysozyme (HEWL) and an 11-residue peptide taken from the sequence of transthyretin (TTR105-115). Both samples show an inter-strand reflection at 4.7 A and an inter-sheet reflection which occurs at 8.8 and approximately 10 A for TTR105-115 and HEWL fibrils, respectively. The positions, widths, and relative intensities of these reflections are conserved in patterns obtained from dried stalks and hydrated samples over a range of fibril concentrations. In 2D scattering patterns obtained from flow-aligned hydrated samples, the inter-strand and inter-sheet reflections showed, respectively, axial and equatorial alignment relative to the fibril axis, characteristic of the cross-beta structure. Our results show that the cross-beta structure of the fibrils is not a product of the dehydrating conditions typically employed to produce aligned samples, but is conserved in individual fibrils in hydrated samples under dilute conditions comparable to those associated with other biophysical and spectroscopic techniques. This suggests a structure consisting of a stack of two or more sheets whose interfaces are inaccessible to bulk water.     

Conformational study of the natural iron chelator myo-inositol 1,2,3-trisphosphate using restrained/flexible analogues and computational analysis

myo-Inositol 1,2,3-trisphosphate [Ins(1,2,3)P₃], a component in mammalian cells, possesses the correct chemical properties of an intracellular iron transit ligand. Here we have examined the conformation of the Ins(1,2,3)P₃-Fe³⁺ complex. The synthesis and antioxidant properties of 4,6-carbonate-myo-inositol 1,2,3,5-tetrakisphosphate [4,6-carbonate Ins(1,2,3,5)P₄], which is locked in the unstable penta-axial chair conformation and 1,2,3-trisphosphoglycerol, a flexible acyclic analogue of Ins(1,2,3)P₃, are reported. 4,6-Carbonate Ins(1,2,3,5)P₄ caused complete inhibition of iron-catalysed hydroxyl radical (HO) formation at 100 μM, thereby resembling Ins(1,2,3)P₃ and supporting a penta-axial chair binding conformation. In contrast, 1,2,3-trisphosphoglycerol was shown to have incomplete antioxidant properties. In support of experimental observations, we have applied high-level density functional calculations to the binding of Ins(1,2,3)P₃ to iron. This study provides evidence that Fe³⁺ binds tightly to the less stable penta-axial conformation of Ins(1,2,3)P₃ using terminal and bridging phosphate oxygens, thought to also contain a tightly bound water molecule or hydroxyl ligand in the complex.     

The use of ESI-MS to probe the binding of divalent cations to calmodulin

Proteins have evolved with distinct sites for binding particular metal ions. This allows metalloproteins to perform a myriad of specialized tasks with conformations tailor-made by the combination of its primary sequence and the effect on this of the ligated metal ion. Here we investigate the selectivity of the calcium trigger protein calmodulin for divalent metal ions. This ubiquitous and highly abundant protein exists in equilibrium between its apo and its holo form wherein four calcium ions are bound. Amongst its many functions, calmodulin modulates the calcium concentration present in cells, but this functional property renders it a target for competition from other metal ions. We study the competition posed by four other divalent cations for the calcium binding sites in calmodulin using electrospray ionization mass spectrometry (ESI-MS). We have chosen two other group II cations Mg²⁺, Sr²⁺, and two heavy metals Cd²⁺, Pb²⁺. The ease with which each of these metals binds to apo and to holo CaM[4Ca] is described. We find that each metal ion has different properties with respect to calmodulin binding and competition with calcium. The order of affinity for apo CaM is Ca²⁺ ≫ Sr²⁺ ∼ Mg²⁺ > Pb²⁺ ∼ Cd²⁺. In the presence of calcium the affinity alters to Pb²⁺ > Ca²⁺ > Cd²⁺ > Sr²⁺ > Mg²⁺. Once complexes have been formed between the metal ions and protein (CaM:[xM]) we investigate whether the structural change which must accompanies calcium ligation to allow target binding takes place for a given CaM:[xM] system. We use a 20 residue target peptide, which forms the CaM binding site within the enzyme neuronal nitric-oxide synthase. Our earlier work (Shirran et al. 2005) [1] has demonstrated the particular selectivity of this system for CaM:4Ca²⁺. We find that along with Ca²⁺ only Pb²⁺ forms complexes of the form CaM:4M²⁺:nNOS. This work demonstrates the affinity for calcium above all other metals, but also warns about the ability of lead to replace calcium with apparent ease.     

Phenolic Compounds Recovery from Blood Orange Peels Using a Novel Green Infrared Technology Ired-Irrad®, and Their Effect on the Inhibition of Aspergillus flavus Proliferation and Aflatoxin B1 Production

The intensification of total phenolic compound (TPC) extraction from blood orange peels was optimized using a novel green infrared-assisted extraction technique (IRAE, Ired-Irrad^®) and compared to the conventional extraction using a water bath (WB). Response surface methodology (RSM) allowed for the optimization of ethanol concentration (E), time (t), and temperature (T) in terms of extracted TPC and their antiradical activity, for both WB extraction and IRAE. Using WB extraction, the multiple response optimums as obtained after 4 h at 73 °C and using 79% ethanol/water were 1.67 g GAE/100 g for TPC and 59% as DPPH inhibition percentage. IRAE increased the extraction of TPC by 18% using 52% ethanol/water after less than 1 h at 79 °C. This novel technology has the advantage of being easily scalable for industrial usage. HPLC analysis showed that IRAE enhanced the recovery of gallic acid, resveratrol, quercetin, caffeic acid, and hesperidin. IR extracts exhibited high bioactivity by inhibiting the production of Aflatoxin B1 by 98.9%.