Copper(I) Chelators for Alzheimer's Disease - The Journal of Physical

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Copper(I) chelators for Alzheimer’s disease Stanley Opare, and Arvi Rauk J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b10480 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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The Journal of Physical Chemistry

Copper(I) Chelators for Alzheimer’s Disease Stanley K. A. Opare and Arvi Rauk* Department of Chemistry, University of Calgary, 2500 University Dr. NW, Calgary, AB, Canada T2N 1N4 * Author to whom correspondence should be addressed: Tel. (403) 220-6247; Email: [email protected]

Abstract A component of the neurotoxicity of the beta amyloid peptide (A) of Alzheimer’s disease is its ability to generate superoxide, hydrogen peroxide and hydroxyl radicals by the reaction its reduced copper complex, A/Cu+ has with molecular oxygen. The objective of the present work was to devise compounds, L, that could remove Cu from A/Cu+, with the property that L/Cu+ itself would not be capable of reducing O2 or hydrogen peroxide. We show by density functional calculations that several pincer-type compounds with two imidazole rings and a sulphur or nitrogen, have the desired combination of Cu+ binding affinity and Cu2+ reduction potential.

Introduction Alzheimer’s disease (AD) is the most common neurodegenerative disease; it is characterized by neuronal cell loss and the presence, in the brain, of intracellular tangles of hyperphosphorylated tau protein, and extracellular plaques of the beta amyloid peptide (A). There are many hypotheses for this disease.1 In the amyloid hypothesis, smaller oligomers of A (generated by cleavages of the amyloid precursor protein (APP) by the - and -secretases2) are responsible for the disease. It has also been observed that A induces oxidative stress.2 This is expressed in increased levels of reactive oxidative species (ROS), lipid peroxidation and reduced levels of antioxidants.3 There are also elevated levels of Zn and the redox-active metals, Fe and Cu.4 A has three histidine amino acids. These form the region for binding copper. The closeness of the pKa of histidine to the physiological pH implies a great chance of protonation. Hence the binding affinity of Cu(II) to the histidine residues is dependent on the pH. A careful measure 1 ACS Paragon Plus Environment

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of the reduction potential of monomeric Cu(II)/A(1-42) puts the value at 0.28 V.5 Cu(II)/A is reduced to Cu(I)/A in the presence of endogenous reducing agents like ascorbate and glutathione. In the presence of oxygen, H2O2 can be produced by reaction of Cu(I)/A species with oxygen to yield hydrogen peroxide (H2O2). In Fenton-like chemistry, Cu(I)/A can also reduce H2O2 to hydroxyl radicals HO•. This leads to further generation of more radicals.5,6,7,8,9 Thus, the primary damaging species is the Cu+ complex with A. Cu+ is a d10 ion. Due to its large size and lower charge density compared to the Cu2+ ion, its enthalpy of hydration, -593 kJ mol-1 is not as stabilizing as that of Cu2+, -2100 kJ mol-1.10 Hence in aqueous environment Cu(I) tends to disproportionate to form Cu(II) and elemental copper, Cu(s). 2Cu+(aq) → Cu(s) + Cu2+(aq) Predominantly, aqueous copper exists in the +2 state under physiological conditions. The +1 state must be stabilized by chelation. A few ligands are known to bind Cu+ with high affinity.11 All protein-bound Cu(II) is bound to one or more histidine residues. Histidine has an imidazole ring bearing two nitrogen atoms. A has three histidine residues (H6, H13 and H14) which are all involved in copper complexation in a complex manner. At physiological pH, there are at least three Cu(II)A species in equilibrium, all of which involve H6, a pair of ligands from the N-terminus, H13 or H14, and a backbone amide or carbonyl.12,13,14 In its reduced form, Cu(I), copper binds to a pair of histidines, H6,H13, H6H14, or H13H14.15,16 We focus here on the last, since it has been shown that H13 and H14 bind Cu(I) in a linear coordination using the  – nitrogen atoms.17,18 The binding energy of Cu(I) complexed to any two of the His residues of A, including His13His14, is estimated experimentally to range from about -83 to -85 kJ mol-1 for full length A while the oxidized Cu(II) form ranges from -50 to -60 kJ mol-1 for A16.20 These data suggest that A binds more strongly to Cu(I) than Cu(II). This research aims at designing ligands that will bind Cu(I) with a similar or higher binding strength that will compete with H13 and H14 in A for Cu+. With care, Cu(I)-chelating ligands may be applied to alter the redox properties of A-bound Cu+ without disturbing overall copper homeostasis, i.e., remove copper from metalloenzymes needing it for their normal function. Metal-Protein Attenuating compounds (MPAC) are different from traditional chelators in the sense that they can release the metal back into the biological environment because of their moderate affinity for the metals they bind to. One such compound is PBT2, which was designed to affect the Cu(II) and Zn (II) mediated toxic 2 ACS Paragon Plus Environment

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oligomerization of A.21 It is a derivative of 8-hydroxyquinoline (8HQ) that showed therapeutic properties in APP transgenic mice.22 PBT2 is designed to enhance solubility and penetrability through the blood brain barrier in vivo.22 Like clioquinol, it binds to both Cu(II) and Zn(II).23 The binding affinity of PBT2 for copper has not been reported for either Cu(II) or Cu(I); the closest analog is clioquinol with a binding energy to Cu(II) at -58 kJ mol-1, estimated from Kd values.24 As part of the present investigation, we use computational means to estimate the binding affinities of Cu2+ and Cu+ with PBT2. The primary objective of this work is to design ligands that will compete with the Cu(I) binding site of A, modelled by (His)2, and reduce the copper toxicity in AD. An important part of the toxicity ensues from the reduction of oxygen by Cu(I) complex of A. There are two ways to achieve this goal. One way is to prevent the reduction of Cu(II)-A to Cu(I)-A using a ligand. The other is to design a ligand that will bind Cu(I) better than A. By doing this, there is also the danger of interfering with the normal function of Cu(II) metalloenzymes. The basic structural model is shown in Figure 1.

As Cu+ prefers linear or T-shaped

coordination, we model our ligands on the His13His14 sequence in A by providing two chelating sites (X) separated by a linker (Y) which can act as a third ligating site. The groups, X, are nitrogen based. The linkers, Y, are nitrogen- and sulphur-based. The nitrogen-based linker, Y, is a pyridine moiety while the sulphur-based Y is a thioether. For both the nitrogen and sulphur classes, the X’s are N(CH3)2 and imidazole. The first mimics one of the ligating sites of PBT2, the last mimics the His13His14 fragment of A. The linkers serve the purpose of allowing enough spacing between the chelating groups, X, to coordinate in a linear fashion with the Cu(I).

Figure 1. Names and generic chemical skeleton of ligands to bind Cu(I).

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One notes a caveat since the possibility of T-shaped coordination may accelerate the reoxidation of the Cu(I) species to Cu(II).17

Computational details Gaussian 16 suite of software25 was used to perform DFT calculations. Gaussview 4.126 and Molden 5.027,28 were used extensively for visualization. The hybrid density functional theory (DFT) procedure, CAM-B3LYP,29 was employed in this study coupled with either the small basis set, 6-31+G(d) (denoted SB), or a larger basis set, 6-311+G(2d,2p) (denoted LB). Geometry optimization and harmonic frequency analysis was performed in the gas phase on each molecule at CAM-B3LYP/SB level. Thermochemical values were also obtained at the same level of theory. Zero point energies were scaled by 0.980630. Single point energies were calculated at CAM-B3LYP/LB level in gas phase. Since calculations were done in the gas phase, the entropies were converted to a 1 M state by adding a change in volume term, Rln(1/24.46), where R is the gas constant and the 24.46 is the volume in litres of 1 mol of an ideal gas. An extra term Rln(n) was applied to estimate the entropic contribution of rotamer counting where n is the number possible populated conformers.31 This term ensures that entropy loss or gain as a result of making or breaking cyclic structures with the copper is accounted for in all structural models. The free energy of solvation was estimated by CAM-B3LYP/SB and a polarizable continuum model with a cavity defined by the molecular isodensity surface with isodensity set to 0.001. As the iterative procedure invoked by SCRF = IPCM32 failed to converge in a number of cases, only the results of the first step were used. The calculated change in free energy of solvent was added to the free energy in the gas phase. Experimental values for the free energy of solvation of H+ (-1107 kJ mol-1)33 and H2O (-16.2 kJ mol-1 for 55.6 M water)34 were adopted. Reduction potentials of Cu (II)/Cu (I) redox coupling The equation below represents the reduction of aqueous Cu(II) complexes to Cu(I). This reaction involves a single electron transfer process. Cu(II)[(L)(H2O)n]2+ + e- → Cu(I)(L) (H2O)m + + (n-m)H2O, ∆GCu L represents the ligand in Figure 1 that binds specifically to Cu2+ in a square planar arrangement, the fourth ligand being water. The reduced copper will lose some water molecules and may or may not be dicoordinated. The reduction potential of the Cu (II)/Cu (I) relative to the standard hydrogen electrode (SHE) is 4 ACS Paragon Plus Environment

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E° (Cu (II)/Cu (I)) = - (∆GCu - ∆GSHE- ∆GerrCu)/F where F is Faraday constant, F = 96.485 kJ mol-1 V-1, ∆GSHE is the free energy change for a standard hydrogen cell half reaction ½ H2(g) + e- → H+(aq),

∆GSHE = -418 kJ mol-1

The third term, ∆GerrCu , accounts for the systematic error incurred at the CAM-B3LYP/6311+G(2df,2p) level of theory in calculating the change in oxidation state of copper. Roy, et al.35 have concluded that, while no DFT method can accurately predict reduction potentials, there exists a good linear correlation between calculated and experimental values, and good results can be calculated if one uses a reference system. In the past, we have used the ionization potential of Cu+ for which the experimental value is 1958 kJ mol-1,36 as a reference.37 The CAM-B3LYP/6-311+G(2df,2p) calculated value is 2006 kJ mol-1, a discrepancy of about +48 kJ mol-1. We assume that for any redox reaction involving Cu(II)/Cu(I), ∆GerrCu = 48 kJ mol-1. The relationship between the potential of the half reaction, E and the standard potential, E° is in the Nernst equation. E = E° - (RT/F) ln Q, where Q is the quotient of the reaction.

Results and Discussion Hydrated copper systems The two oxidized forms of copper, Cu(I) and Cu(II), possess different coordination environments. The coordination number of aquo Cu(II) complexes has been much discussed, 38,39

with the most recent experimental work yielding an average coordination number of 4.5

± 0.6.39 At the above level of theory, Cu(H2O)42+ is more stable than Cu(H2O)52+ by 8 kJ mol1

, while Cu(H2O)3+ and Cu(H2O)2+ are equally stable in water, the difference being about 1

kJ mol-1 (Figure 2). The calculated reduction potential, E° = 0.21 V, is in satisfactory agreement with that of aqueous Cu2+, E° = 0.16 V, after applying the above-mentioned correction for the error in the ionization potential of gaseous Cu+ ion.



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Figure 2. Equations to establish the more stable Cu+ and Cu2+ aquated ions, Cu(H2O)2+ (≈ Cu(H2O)3+) and Cu(H2O)42+, respectively, and the reduction potential, E° = 0.21 V (Expt E° = 0.16 V).

(His)2, Cu(His)2+, and Cu(His)22+ structures The (His)2 peptide is shown in Figure 3a (center) together with the Cu(I)- and Cu(II)-bound complexes. Only the trans amide conformation was considered. The C-terminus was terminated with a methyl group. The optimized structures showed structural stabilization using intramolecular hydrogen bonding. In the more flexible (His)2 there was hydrogen bonding involving the backbone amide linkage and the two histidine residues. Upon the introduction of Cu(I) in between the imidazole rings, the structure became rigid forming a ring system involving the two histidine residues and the Cu(I). The cyclisation implied substantial loss in degrees of freedom and gain of new bonds which is accounted for in the Rln n entropic correction as stated above. The optimized geometry has a nearly linear NCu  N angle (173°) with bond distances of about 1.91Å each. The measured distance for the Cu(I) – O=C was 2.2Å, indicating a weak interaction. The geometry around the Cu+ is a distorted T shape. The imidazole rings were staggered in conformation separated by a planeto-plane angle of about 56˚. Like the Cu(I) complex, the Cu(II) complex led to the formation of two rings with three bridging atoms, Cu(II) and O=C between the two residues. The coordination pattern around the Cu(II) ion in the optimized geometry is a distorted square plane with the carbonyl oxygen atom and a water molecule as the third and fourth ligands. The two Ns are about 1.98 Å from the copper and make an N-Cu-N angle of 170˚. The two Cu-O distances are 1.93 Å and 2.05 Å, for O=C, and OH2, respectively. The water oxygen is 20˚ out of the coordination plane. The planes of the two imidazole rings are twisted out-of-plane by about 40˚.


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Figure 3. a (His)2 system; b PBT2 system. Free energy changes, Gaq, for addition of Cu(H2O)2+ (left side) and Cu(H2O)42+ (right side) are provided, as are the computed reduction potentials, E°, relative to SHE. Binding strength of copper in the (His)2 complex The binding affinities of the Cu+ and Cu2+ complexes of (His)2 are specified in Figure 3a. The values, 73 kJ mol-1 and 55 kJ mol-1, respectively, are in reasonable agreement with literature values.19,20 For Cu+, the values range from -83 to -85 kJ mol-1 for full length A and, and for Cu2+, from -50 to 60 kJ mol-1 for A16. Thus, the computations are also in agreement that Cu+ should bind more strongly than Cu2+ to A. The computed reduction potential serves as additional validation of the present methodology. The value for the process illustrated in Figure 3a, 0.39 V vs SHE, is in reasonable agreement with the value of Balland, et al., 0.30 V vs SHE, measured by cyclic voltammetry of A16.40

PBT2 and its Cu(I) and Cu(II) complexes. The structure of PBT2, a derivative of 8-hydroxyquinoline (8HQ) is shown in Figure 3b (center). The pKa of PBT2 has not been reported. A close analog is clioquinol which has an iodo and chloro group instead of two chlorines at the ortho and para positions of the phenolic 7 ACS Paragon Plus Environment


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ring. The pKa of clioquinol is 8.0541 which is close to physiological pH 7.4. The pKa of PBT2 should be lower due to the inductive effect of the extra chloro group, and the phenolic moiety will likely be deprotonated in aqueous solution at physiological pH. The structure of the Cu+ complex is shown in Figure 3b (left side). The N-Cu-O angle, 157°, is bent away from the pyridine N atom indicating a repulsive interaction. This is also reflected in the low binding affinity, -14 kJ mol-1. The structure of the Cu2+ complex is shown in Figure 3b (right side). The Cu(II) coordination sphere is square planar four-coordinated, stabilized by a second water molecule that bridges from the bound water to the phenolic oxygen.

Optimization from a

pentacoordinated Cu(II) structure (both waters on the Cu) always led to the structure shown. The computed binding affinity, -85 kJ mol-1, is substantially higher than the estimated experimental value for A and the calculated value for (His)2Cu(II), both about 55 kJ mol-1. PBT2 was designed to prevent extracellular metal-A interactions, but to bind weakly enough to Cu2+ so as to be able to release the Cu2+ back to enzymic sites within the cell that require it.42 The computed results confirm that PBT2 can out-compete A for Cu2+ ions. In addition, the very low reduction potential, -0.54 V vs SHE indicates that it will be less likely to be reduced by endogenous reducing agents to a toxic Cu(I) species.

New Copper binding ligands As stressed above, the primary purpose of this work was to design ligands that bind Cu+ more strongly than A itself. These can serve as a last line of defense against the neurotoxic effects of reduced copper. Our model for A, (His)2 (Figure 3a), binds Cu+ with an affinity of -73 kJ mol-1. One design consideration for new ligands is that the Cu+ can be bound in a linear fashion, with the availability of a third coordination site if such binding proves to be energetically beneficial. A second consideration is that the ligand binds Cu2+ even more strongly, so as to lower the reduction potential and thus reduce the likelihood that the ligand-Cu+ complex can itself generate ROS. In the (His)2 complexes, the atoms involved are two nitrogen atoms from the imidazole ring and an oxygen from the backbone. Better electron pair donors, nitrogen and sulphur were used in place of the oxygen atom.

Structural Models


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All structures described in Figure 1 were constructed using Gaussview 4.1,26 and optimized. Data for energy, zero-point energy as well as entropy and thermal correction to the enthalpy were obtained. The data for all structures discussed are shown in Tables S1 (coordinates) and S2 (primary computed data) of supporting information.

PD1, PI0, and PI1 (see Figures 1 and 4) PD1 has the same dimethylaminomethylene sidechain as PBT2 attached to the 2 and 6 positions of a pyridine ring. In PI1, the dimethylamino group has been replaced by an imidazole ring. In PI0, n = 0 such that the imidazole is attached directly to the 2 and 6 positions of the pyridine. The optimized structures of the parent compounds and their Cu+ (left side) and Cu2+ (right side) complexes are shown in Figure 4. The three systems differ widely in their predicted affinity for Cu+. In fact, PD1Cu+ is not predicted to be stable in water, Gaq = +3 kJ mol-1. This result is consistent with the low Cu+ binding affinity calculated for PBT2, Gaq = -14 kJ mol-1. On the other hand, PI1Cu+, which has a similar structure except that the “X” ligands are replaced by imidazoles, is stable by -65 kJ mol-1 relative to the separated species, PI1 and aqueous Cu+. A large part of the difference is attributable to the fact that the tertiary amino group is a poor ligand for Cu+. This point is discussed in more detail below. Removal of the linker methylene groups increases the binding affinity by a further 48 kJ mol-1; the binding affinity of PI0 for Cu+ is -103 kJ mol-1, which is 30 kJ mol-1 higher than that of (His)2.



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Figure 4. a PD1 system; b PI1 system; PI0 system. Free energy changes, Gaq, for addition of Cu(H2O)2+ (left side) and Cu(H2O)42+ (right side) are provided, as are the computed reduction potentials, E°, relative to SHE.

The Cu(II) complexes increase in stability in the sequence (kJ mol-1): PD1Cu(H2O)2+, -51; PI1Cu(H2O)2+, -55; PI0Cu(H2O)2+, -153. The last value probably represents the optimum binding affinity that can be achieved; the Cu(II) coordination sphere is perfectly planar, the angles at the Cu(II) centre are close to ideal 90°, and the two imidazole rings are ideal ligands for stabilizing the copper. The considerably lower stability of PI1Cu(H2O)2+ must be attributed to a combination of ring strain in the 6-membered rings containing the copper, steric hindrance to the fourth ligand due to the two ortho hydrogen atoms of the imidazole rings, and weaker pi backdonation from the twisted imidazole rings. Steric hindrance to coordination of the water is not a factor in the low binding affinity of PD1Cu(H2O)2+. The principal factor is the poor ligating ability of the tertiary amino group. Like its Cu+ counterpart, PI0Cu(H2O)2+, has a higher stability than the corresponding complex with (His)2 (Figure 3a) (and A). The calculated reduction potentials (Figure 4) of PD1Cu(H2O)2+ and PI0Cu(H2O)2+ are negative relative to SHE due to the substantially greater stability of the Cu2+ complex relative 10 ACS Paragon Plus Environment

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to the Cu+ complex. The opposite is true in the case of PI1Cu(H2O)2+, for which the reduction potential, 0.31 V vs SHE, is similar to the value calculated for the (His)2 complex, 0.39 V vs SHE, and found for A16, 0.30 V vs SHE.

SD2 and SD3 (See Figures 1 and 5) SD2 and SD3, as well as SI1 and SI2 possess a thioether linker with X being either a dimethylamino group (SD2, SD3) or an imidazole ring (SI1, SI2). The soft thioether moiety is a better ligand for the soft Cu+ ion than for the harder Cu2+ ion. As expected, both SD2 and SD3 have a higher affinity for Cu+ than for Cu2+, but the difference is small and the absolute values are low. The latter observation can be attributed to the fact that the dimethylamino moiety is a poor ligand for both copper ions, and to the flexibility of both parent species and the resultant loss of entropy upon copper coordination. The loss in entropy due to loss of conformational degrees of freedom is approximated as –Rln(n), where the values of n, based on simple counting of three-fold rotors, are 6561 and 729 for SD3 and SD2, respectively (Table S2). These values reduce the free energy of binding by 22 kJ mol-1 and 16 kJ mol-1, respectively. One notes that a factor of 2 error in the conformer count entails an error of less than 2 kJ mol 1

.

Figure 5. a SD2 system; b SD3 system. Free energy changes, Gaq, for addition of Cu(H2O)2+ (left side) and Cu(H2O)42+ (right side) are provided, as are the computed reduction potentials, E°, relative to SHE. SI1 and SI2 (See Figures 1 and 6) 11 ACS Paragon Plus Environment


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SI1 and SI2 have two imidazole moieties which are excellent ligands for Cu ions, and a thioether moiety as a favourable third ligand. Of the complexes considered here, it was expected that SI1 and SI2 would have the highest affinities for Cu+ and possibly for Cu2+. As can be seen by comparing the data in Figure 6 with Figures 4 and 5, this is indeed the case for SI1 and Cu+ for which the binding affinity, -115 kJ mol-1, is 12 kJ mol-1 higher than that of PI0 (Figure 4). The Cu+ affinity of SI2 is 23 kJ mol-1 lower, in large part due to its greater conformational flexibility. The loss in entropy due to loss of conformational degrees of freedom is approximated as –Rln(n), where the values of n, based on simple counting of threefold rotors, are 81 and 729 for SI1 and SI2, respectively (Table S2). These values reduce the free energy of binding by 11 kJ mol-1 and 16 kJ mol-1, respectively. The affinities of SI1 and SI2 for Cu2+ are identical, -116 kJ mol-1, similar to the value for SI1 and Cu+, -115 kJ mol-1.

Figure 6. a SI1 system; b SI2 system. Free energy changes, Gaq, for addition of Cu(H2O)2+ (left side) and Cu(H2O)42+ (right side) are provided, as are the computed reduction potentials, E°, relative to SHE.

It is immediately apparent from comparing the PD, SD systems (Figures 4a, 5) to the PI, SI systems (Figures 4b,4c and 6) that the dimethylamino group (D) is a poorer ligand for both Cu+ and Cu2+ than is the imidazole group (I). With the exception of PBT2 which has a relatively high affinity for Cu2+ (but not Cu+), all of the copper binding affinities for both oxidation states 12 ACS Paragon Plus Environment

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are significantly higher when the two imidazole groups are involved. Computed reduction potentials of the Cu2+ complexes range from a low of -0.54 V vs SHE to a high of +0.39 V vs SHE, and reflect the relative binding affinities of the Cu2+ and Cu+ complexes. The low value, -0.54 V, for the PBT2Cu(H2O)2+/PBT2Cu + 2H2O couple (Figure 2b) is due to the strong electron donor ability of the phenoxy group, but is not as low as the Cu(II) complex with albumin, -0.8 V,43 in which two coordinating backbone amide groups are deprotonated. The high value, 0.39 V is computed for the (His)2Cu(H2O)2+/(His)2Cu+ + H2O couple (Figure 3a) and is similar to that expected for Cu(II)/A (0.30 V measured for A16).40

Conclusions The A/Cu2+ complex has a positive, elevated reduction potential compared to aqueous Cu2+. It is readily reduced to A/Cu+ by endogenous reducing agents. The A/Cu+ complex is capable of reducing molecular oxygen to hydrogen peroxide, and further reducing the hydrogen peroxide to generate hydroxyl radicals. The production of these ROS and downstream free radicals is one of the mechanisms of neurotoxicity of A. A primary objective of the present work was to discover ligands that are able to remove Cu+ ions from A as a last line of defense against this aspect of its neurotoxicity. The ligands, PI0, SI1, and SI2, satisfy this criterion. Each has a higher binding affinity for Cu+ than A (≈ -83 kJ mol-1) or the model employed here, (His)2 (-73 kJ mol-1). A secondary criterion is that the ligand/Cu+ complex should itself not be capable of generating ROS. The reduction of molecular oxygen and of hydrogen peroxide by various Cu2+/Cu+ couples based on a variation of (His)2 has previously been discussed.44 The redox potential of molecular oxygen depends on its concentration. The physiologically relevant reduction potentials are E(O2/O2•– ) = 0.10 V (air-saturated), E(O2/O2•–) = –0.02 V (anoxia), E(O2•– + 2H+/H2O2) = –0.09 V and E(H2O2 + H+/H2O + HO•) = 0.32 V. Thus, the PI0Cu2+/PI0Cu+ couple which has a reduction potential of -0.32 V is incapable of reducing molecular oxygen under either condition, while the SI2 system may effect the reduction under anoxic conditions and the SI1 system under both conditions. None of the three is capable of reducing hydrogen peroxide which requires a reduction potential greater than or equal to 0.3 V. We note that in order to have a low reduction potential, the Cu2+ complex should be bound at least as strongly as the Cu+ complex. Each of PI0, SI1 and SI2, as well as PBT2, satisfy this criterion. As a consequence, each is also capable of removing Cu2+ from A, which may be considered as 13 ACS Paragon Plus Environment


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the first line of defense against ROS generated toxicity. While this work was in progress, a paper in the same spirit appeared from the Hureau group. They describe a triHis ligand which can also remove both Cu(I) and Cu(II) from A; the Cu(I) complex is inactive toward O2.45

In future work, we will report on compound ligands in which the PI0, SI1 and SI2 moieties are attached to a pseudopeptidic -sheet blocking ligand (PP) designed to attach to a specific region of A. Thus, the combinations PP-PI0, PP-SI1, and PP-SI2, should serve the dual functions of reducing the copper-induced toxicity of A as well as preventing its aggregation into toxic oligomers. Preliminary work on several classes of PP has appeared.46,47,48,49,50

Acknowledgements We are grateful for financial support from the Natural Sciences and Engineering Council of Canada (NSERC) and to Compute Canada and Westgrid for generous allocations of computer facilities. We also wish to acknowledge fruitful discussions with Banafsheh Mehrazma.

Supporting Information Table S1. Geometries of all species optimized at CAM-B3LYP/6-31+G(d) in the form of Gaussian Input files:

Table S2. Computed data for all structures listed in Table S1


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Table of Content Graphic for: Copper(I) Chelators for Alzheimer’s Disease Stanley K. A. Opare and Arvi Rauk* Department of Chemistry, University of Calgary, 2500 University Dr. NW, Calgary, AB, Canada T2N 1N4

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