Shin Jung C. Lee

Endoplasmic Reticulum-Localized Iridium(III) Complexes as Efficient Photodynamic Therapy Agents via Protein Modifications

Protein inactivation by reactive oxygen species (ROS) such as singlet oxygen ((1)O2) and superoxide radical (O2(•-)) is considered to trigger cell death pathways associated with protein dysfunction; however, the detailed mechanisms and direct involvement in photodynamic therapy (PDT) have not been revealed. Herein, we report Ir(III) complexes designed for ROS generation through a rational strategy to investigate protein modifications by ROS. The Ir(III) complexes are effective as PDT agents at low concentrations with low-energy irradiation (≤ 1 J cm(-2)) because of the relatively high (1)O2 quantum yield (> 0.78), even with two-photon activation. Furthermore, two types of protein modifications (protein oxidation and photo-cross-linking) involved in PDT were characterized by mass spectrometry. These modifications were generated primarily in the endoplasmic reticulum and mitochondria, producing a significant effect for cancer cell death. Consequently, we present a plausible biologically applicable PDT modality that utilizes rationally designed photoactivatable Ir(III) complexes.

Host–Guest Chemistry from Solution to the Gas Phase: An Essential Role of Direct Interaction with Water for High-Affinity Binding of Cucurbit[n]urils

An investigation of the host-guest chemistry of cucurbit[n]uril (CB[n], n = 6 and 7) with α,ω-alkyldiammonium guests (H2N(CH2)xNH2, x = 4, 6, 8, 10, and 12) both in solution and in the gas phase elucidates their intrinsic host-guest properties and the contribution of solvent water. Isothermal titration calorimetry and nuclear magnetic resonance measurements indicate that all alkyldiammonium cations have inclusion interactions with CB[n] except for the CB[7]-tetramethylenediamine complex in aqueous solution. The electrospray ionization of mixtures of CB[n] and the alkyldiammonium guests reflects their solution phase binding constants. Low-energy collision-induced dissociations indicate that, after the transfer of the CB[n]-alkyldiammonium complex to the gas phase, its stability is no longer correlated with the binding properties in solution. Gas phase structures obtained from density functional theory calculations, which support the results from the ion mobility measurements, and molecular dynamics simulated structures in water provide a detailed understanding of the solvated complexes. In the gas phase, the binding properties of complexation mostly depend on the ion-dipole interactions. However, the ion-dipole integrity is strongly affected by hydrogen bonding with water molecules in the aqueous condition. Upon the inclusion of water molecules, the intrinsic characteristics of the host-guest binding are dominated by entropic-driven thermodynamics.

Probing conformational change of intrinsically disordered α-synuclein to helical structures by distinctive regional interactions with lipid membranes.

α-Synuclein (α-Syn) is an intrinsically disordered protein, whose fibrillar aggregates are associated with the pathogenesis of Parkinsons disease. α-Syn associates with lipid membranes and forms helical structures upon membrane binding. In this study, we explored the helix formation of α-Syn in solution containing trifluoroethanol using small-angle X-ray scattering and electrospray ionization ion mobility mass spectrometry. We then investigated the structural transitions of α-Syn to helical structures via association with large unilamellar vesicles as model lipid membrane systems. Hydrogen-deuterium exchange combined with electrospray ionization mass spectrometry was further utilized to understand the details of the regional interaction mechanisms of α-Syn with lipid vesicles based on the polarity of the lipid head groups. The characteristics of the helical structures were observed with α-Syn by adsorption onto the anionic phospholipid vesicles via electrostatic interactions between the N-terminal region of the protein and the anionic head groups of the lipids. α-Syn also associates with zwitterionic lipid vesicles and forms helical structures via hydrophobic interactions. These experimental observations provide an improved understanding of the distinct structural change mechanisms of α-Syn that originate from different regional interactions of the protein with lipid membranes and subsequently provide implications regarding diverse protein-membrane interactions related to their fibrillation kinetics.

Mechanistic Insights into Tunable Metal-Mediated Hydrolysis of Amyloid-β Peptides

An amyloidogenic peptide, amyloid-β (Aβ), has been implicated as a contributor to the neurotoxicity of Alzheimers disease (AD) that continues to present a major socioeconomic burden for our society. Recently, the use of metal complexes capable of cleaving peptides has arisen as an efficient tactic for amyloid management; unfortunately, little has been reported to pursue this strategy. Herein, we report a novel approach to validate the hydrolytic cleavage of divalent metal complexes toward two major isoforms of Aβ (Aβ40 and Aβ42) and tune their proteolytic activity based on the choice of metal centers (M = Co, Ni, Cu, and Zn) which could be correlated to their anti-amyloidogenic properties. Such metal-dependent tunability was facilitated employing a tetra-N-methylated cyclam (TMC) ligand that imparts unique geometric and stereochemical control, which has not been available in previous systems. Co(II)(TMC) was identified to noticeably cleave Aβ peptides and control their aggregation, reporting the first Co(II) complex for such reactivities to the best of our knowledge. Through detailed mechanistic investigations by biochemical, spectroscopic, mass spectrometric, and computational studies, the critical importance of the coordination environment and acidity of the aqua-bound complexes in promoting amide hydrolysis was verified. The biological applicability of Co(II)(TMC) was also illustrated via its potential blood-brain barrier permeability, relatively low cytotoxicity, regulatory capability against toxicity induced by both Aβ40 and Aβ42 in living cells, proteolytic activity with Aβ peptides under biologically relevant conditions, and inertness toward cleavage of structured proteins. Overall, our approaches and findings on reactivities of divalent metal complexes toward Aβ, along with the mechanistic insights, demonstrate the feasibility of utilizing such metal complexes for amyloid control.

An Iridium(III) Complex as a Photoactivatable Tool for Oxidation of Amyloidogenic Peptides with Subsequent Modulation of Peptide Aggregation

Aggregates of amyloidogenic peptides are involved in the pathogenesis of several degenerative disorders. Herein, an iridium(III) complex, Ir-1, is reported as a chemical tool for oxidizing amyloidogenic peptides upon photoactivation and subsequently modulating their aggregation pathways. Ir-1 was rationally designed based on multiple characteristics, including 1) photoproperties leading to excitation by low-energy radiation; 2) generation of reactive oxygen species responsible for peptide oxidation upon photoactivation under mild conditions; and 3) relatively easy incorporation of a ligand on the IrIII center for specific interactions with amyloidogenic peptides. Biochemical and biophysical investigations illuminate that the oxidation of representative amyloidogenic peptides (i.e., amyloid-β, α-synuclein, and human islet amyloid polypeptide) is promoted by light-activated Ir-1, which alters the conformations and aggregation pathways of the peptides. Additionally, their potential oxidation sites are identified as methionine, histidine, or tyrosine residues. Overall, our studies on Ir-1 demonstrate the feasibility of devising metal complexes as chemical tools suitable for elucidating the nature of amyloidogenic peptides at the molecular level, as well as controlling their aggregation.

Tuning Structures and Properties for Developing Novel Chemical Tools toward Distinct Pathogenic Elements in Alzheimer’s Disease

Multiple pathogenic factors [e.g., amyloid-β (Aβ), metal ions, metal-bound Aβ (metal-Aβ), reactive oxygen species (ROS)] are found in the brain of patients with Alzheimers disease (AD). In order to elucidate the roles of pathological elements in AD, chemical tools able to regulate their activities would be valuable. Due to the complicated link among multiple pathological factors, however, it has been challenging to invent such chemical tools. Herein, we report novel small molecules as chemical tools toward modulation of single or multiple target(s), designed via a rational structure-property-directed strategy. The chemical properties (e.g., oxidation potentials) of our molecules and their coverage of reactivities toward the pathological targets were successfully differentiated through a minor structural variation [i.e., replacement of one nitrogen (N) or sulfur (S) donor atom in the framework]. Among our compounds (1-3), 1 with the lowest oxidation potential is able to noticeably modify the aggregation of both metal-free Aβ and metal-Aβ, as well as scavenge free radicals. Compound 2 with the moderate oxidation potential significantly alters the aggregation of Cu(II)-Aβ42. The hardly oxidizable compound, 3, relative to 1 and 2, indicates no noticeable interactions with all pathogenic factors, including metal-free Aβ, metal-Aβ, and free radicals. Overall, our studies demonstrate that the design of small molecules as chemical tools able to control distinct pathological components could be achieved via fine-tuning of structures and properties.

Regulatory Activities of Dopamine and Its Derivatives toward Metal-Free and Metal-Induced Amyloid-β Aggregation, Oxidative Stress, and Inflammation in Alzheimer’s Disease

A catecholamine neurotransmitter, dopamine (DA), is suggested to be linked to the pathology of dementia; however, the involvement of DA and its structural analogues in the pathogenesis of Alzheimers disease (AD), the most common form of dementia, composed of multiple pathogenic factors has not been clear. Herein, we report that DA and its rationally designed structural derivatives (1-6) based on DAs oxidative transformation are able to modulate multiple pathological elements found in AD [i.e., metal ions, metal-free amyloid-β (Aβ), metal-bound Aβ (metal-Aβ), and reactive oxygen species (ROS)], with demonstration of detailed molecular-level mechanisms. Our multidisciplinary studies validate that the protective effects of DA and its derivatives on Aβ aggregation and Aβ-mediated toxicity are induced by their oxidative transformation with concomitant ROS generation under aerobic conditions. In particular, DA and the derivatives (i.e., 3 and 4) show their noticeable anti-amyloidogenic ability toward metal-free Aβ and/or metal-Aβ, verified to occur via their oxidative transformation that facilitates Aβ oxidation. Moreover, in primary pan-microglial marker (CD11b)-positive cells, the major producers of inflammatory mediators in the brain, DA and its derivatives significantly diminish inflammation and oxidative stress triggered by lipopolysaccharides and Aβ through the reduced induction of inflammatory mediators as well as upregulated expression of heme oxygenase-1, the enzyme responsible for production of antioxidants. Collectively, we illuminate how DA and its derivatives could prevent multiple pathological features found in AD. The overall studies could advance our understanding regarding distinct roles of neurotransmitters in AD and identify key interactions for alleviation of AD pathology.