NMR Spectroscopy and Metal Ions in Life Sciences

Inhibition of toxic metal-alpha synuclein interactions by human serum albumin

Human serum albumin (HSA), the most abundant protein in plasma and cerebrospinal fluid, not only serves as a crucial carrier of various exogenous and endogenous ligands but also modulates the aggregation of amyloidogenic proteins, including alpha synuclein (αSyn), which is associated with Parkinson's disease and other α-synucleinopathies. HSA decreases αSyn toxicity through the direct binding to monomeric and oligomeric αSyn species. However, it is possible that HSA also sequesters metal ions that otherwise promote aggregation. Cu(ii) ions, for example, enhance αSyn fibrillization in vitro, while also leading to neurotoxicity by generating reactive oxygen species (ROS). However, it is currently unclear if and how HSA affects Cu(ii)-binding to αSyn. Using an integrated set of NMR experiments, we show that HSA is able to chelate Cu(ii) ions from αSyn more efficiently than standard chelators such as EDTA, revealing an unexpected cooperativity between the HSA metal-binding sites. Notably, fatty acid binding to HSA perturbs this cooperativity, thus interfering with the sequestration of Cu(ii) ions from αSyn. We also observed that glycation of HSA diminished Cu(ii)-binding affinity, while largely preserving the degree of cooperativity between the HSA metal-binding sites. Additionally, our results show that Cu(ii)-binding to HSA stabilizes the interactions of HSA with αSyn primarily at two different regions, i.e. the N-terminus, Tyr 39 and the majority of the C-terminus. Our study not only unveils the effect of fatty acid binding and age-related posttranslational modifications, such as glycation, on the neuroprotective mechanisms of HSA, but also highlights the potential of αSyn as a viable NMR-based sensor to investigate HSA-metal interactions.

Halogen Bonding Tripodal Metallo-Receptors for Phosphate Recognition and Sensing in Aqueous-Containing Organic Media

The anion recognition and electrochemical anion-sensing properties of halogen-bonding (XB) tripodal zinc(II) receptors strategically designed and constructed for tetrahedral anion guest binding are described. The XB tris(iodotriazole)-containing hosts exhibit high affinities and selectivities for inorganic phosphate over other more basic, mono-charged oxoanions such as acetate and the halides in a competitive CD3 CN/D2 O (9 : 1 v/v) aqueous solvent mixture. 1 H NMR anion binding and electrochemical voltammetric anion sensing studies with redox-active ferrocene functionalised metallo-tripodal receptor analogues, reveal each of the XB tripods as superior anion complexants when compared to their tris(prototriazole)-containing, hydrogen bonding (HB) counterparts, not only exemplifying the halogen bond as a strong alternative interaction to the traditional hydrogen bond for molecular recognition but also providing rare evidence of the ability of XB receptors to preferentially bind the "harder" phosphate oxoanion over the "softer" and less hydrated halides in aqueous containing media.

Studying Peptide-Metal Ion Complex Structures by Solution-State NMR

Metal chelation can provide structural stability and form reactive centers in metalloproteins. Approximately one third of known protein structures are metalloproteins, and metal binding, or the lack thereof, is often implicated in disease, making it necessary to be able to study these systems in detail. Peptide-metal complexes are both present in nature and can provide a means to focus on the binding region of a protein and control experimental variables to a high degree. Structural studies of peptide complexes with metal ions by nuclear magnetic resonance (NMR) were surveyed for all the essential metal complexes and many non-essential metal complexes. The various methods used to study each metal ion are presented together with examples of recent research. Many of these metal systems have been individually reviewed and this current overview of NMR studies of metallopeptide complexes aims to provide a basis for inspiration from structural studies and methodology applied in the field.

Not only manganese, but fruit component effects dictate the efficiency of fruit juice as an oral magnetic resonance imaging contrast agent

Several fruit juices are used as oral contrast agents to improve the quality of images in magnetic resonance cholangiopancreatography. They are often preferred to conventional synthetic contrast agents because of their very low cost, natural origin, intrinsic safety, and comparable image qualities. Pineapple and blueberry juices are the most employed in clinical practice due to their higher content of manganese(II) ions. The interest of pharmaceutical companies in these products is testified by the appearance in the market of fruit juice derivatives with improved contrast efficacy. Here, we investigate the origin of the contrast of blueberry juice, analyze the parameters that can effect it, and elucidate the differences with pineapple juice and manganese(II) solutions. It appears that, although manganese(II) is the paramagnetic ion responsible for the contrast, it is the interaction of manganese(II) with other juice components that modulates the efficiency of the juice as a magnetic resonance contrast agent. On these grounds, we conclude that blueberry juice concentrated to the same manganese concentration of pineapple juice would prove a more efficient contrast agent than pineapple juice.

Origin of the MRI Contrast in Natural and Hydrogel Formulation of Pineapple Juice

Magnetic resonance imaging (MRI) often requires contrast agents to improve the visualization in some tissues and organs, including the gastrointestinal tract. In this latter case, instead of intravascular administration, oral agents can be used. Natural oral contrast agents, such as fruit juice, have the advantages of better taste, tolerability, and lower price with respect to the artificial agents. We have characterized the relaxometry profiles of pineapple juice in order to understand the origin of the increase in relaxation rates (and thus of the MRI contrast) in reference to its content of manganese ions. Furthermore, we have characterized the relaxometry profiles of pineapple juice in the presence of alginate in different amounts; the interaction of the manganese ions with alginate slows down their reorientation time to some extent, with a subsequent increase in the relaxation rates. The relaxometry profiles were also compared with those of manganese(II) solutions in 50 mmol/dm³ sodium acetate solution (same pH of pineapple juice), which revealed sizable differences, mostly in the number of water molecules coordinated to the metal ion, their lifetimes, and in the constant of the Fermi-contact interaction. Finally, the fit of the transverse relaxivity shows that the increased viscosity in the hydrogel formulations can improve significantly the negative contrast of pineapple juice at the magnetic fields relevant for clinical MRI.

How Do Nuclei Couple to the Magnetic Moment of a Paramagnetic Center? A New Theory at the Gauntlet of the Experiments

The recent derivation, based on pure quantum chemistry (QC) first-principles, of the pseudocontact shifts (PCSs) caused by a paramagnetic metal center on far away nuclei has cast doubts on the validity of the semiempirical (SE) theory, predicting PCSs to arise from the metal magnetic susceptibility anisotropy. The SE theory has been used and applied countless times, especially in the last 2 decades, to obtain structural information on proteins containing paramagnetic metal ions. We show here that the QC and SE predictions can be directly tested against experiments, provided a suitable macromolecular system is used. The SE approach yields a good prediction of the experimental PCSs while the QC one does not. It appears that the classic theory is able to grasp satisfactorily the underlying physics.