Complexes Of Divalent Metal Ions Research Articles

4,5-Bis(diphenylphosphinoyl)-1,2,3-triazole ligand: Studies on metal complex formations in liquid–liquid distribution systems

The formation reactions of hydrophobic metal complexes of divalent typical element and transition metal ions with a novel chelating ligand containing N and O donor atoms, 4,5-bis(diphenylphosphinoyl)-1,2,3-triazole (L TH), were investigated by the liquid–liquid distribution method carried out on metal ions between chloroform and aqueous solutions. The liquid–liquid distribution reaction formulae of metal ions via the formation of hydrophobic metal complexes were revealed, along with their equilibrium constants. Three types of hydrophobic mononuclear and binuclear metal complexes distributed into chloroform solutions were found, namely, ML 2 (M = Mg 2+, Zn 2+, Pb 2+; L = L T−), ML 2(HL) (M = Cd 2+, Mn 2+), and M 2L 3(OH) (M = Co 2+, Ni 2+, Cu 2+). Linear free energy relationships were found between the equilibrium constants of the liquid–liquid distribution reactions and the stability constants of 1:1 complexes consisting of a divalent metal ion and a glycinate. These relationships suggest the chelate formation of N,O-coordination with a heterocyclic five-membered ring in the metal complexes with L TH.

Preparation and Characterization of some Divalent Metal Ion Complexes with Schiff Base Ligands Derived from Amino Acids

A new complexes of some transition metal ions (Co(II),Ni(II),Cu(II)) and some non transition metal ions (Zn(II),Cd(II)) with a number of Schiff bases obtained from the condensation of some amino acids (Methionine) and (3-acetyl pyridine, 4-acetyl pyridine, acetoacetaanilide) have been prepared. All the prepared complexes have been characterized by elemental analysis, molar conductance, magnetic susceptibility infrared and electronic spectral. The complexes were classified as: A-mononuclear complexes: Complexes with the formulas [ML(CH3COO) ] (H2O) ,[ML (H2O) ] and [ML(CH3COO) (H2O)2]. B-Di nuclear complexes: Complexes with the formulas [M2(L)2(CH3COO)2] .2H2O and [M2(L) (CH3COO)3] H2O and [M2(L) (CH3COO)2(H2O)] M= Co(II),Ni(II),Cu(II),Zn(II)Cd(II). L= 3-acetylpyridine Methionineimine, 4-acetylpyridine Methionineimine and acetoacetanilide Methionineimine. The physical measurements showed that the prepared complexes may have a tetra coordinated (tetrahedral or square planer) and hexa-coordinated (octahedral) structure and that all the prepared complexes were non electrolyte.

Divalent Metal Ion Complexes of S100B in the Absence and Presence of Pentamidine

As part of an effort to inhibit S100B, structures of pentamidine (Pnt) bound to Ca2+-loaded and Zn2+,Ca2+-loaded S100B were determined by X-ray crystallography at 2.15 Å (Rfree=0.266) and 1.85 Å (Rfree=0.243) resolution, respectively. These data were compared to X-ray structures solved in the absence of Pnt, including Ca2+-loaded S100B and Zn2+,Ca2+-loaded S100B determined here (1.88 Å; Rfree=0.267). In the presence and absence of Zn2+, electron density corresponding to two Pnt molecules per S100B subunit was mapped for both drug-bound structures. One Pnt binding site (site 1) was adjacent to a p53 peptide binding site on S100B (±Zn2+), and the second Pnt molecule was mapped to the dimer interface (site 2; ±Zn2+) and in a pocket near residues that define the Zn2+ binding site on S100B. In addition, a conformational change in S100B was observed upon the addition of Zn2+ to Ca2+–S100B, which changed the conformation and orientation of Pnt bound to sites 1 and 2 of Pnt–Zn2+,Ca2+–S100B when compared to Pnt–Ca2+–S100B. That Pnt can adapt to this Zn2+-dependent conformational change was unexpected and provides a new mode for S100B inhibition by this drug. These data will be useful for developing novel inhibitors of both Ca2+- and Ca2+,Zn2+-bound S100B.

Electron Capture Dissociation of Complexes of Diacylglycerophosphocholine and Divalent Metal Ions: Competition Between Charge Reduction and Radical Induced Phospholipid Fragmentation

Divalent metal complexes of phosphocholines, [Metal(II)(L)(n)](2+) (where Metal=Cu(2+), Co(2+), Mg(2+), and Ca(2+), L=1,2-dihexanoyl-sn-glycero-3-phosphocholine [6:0/6:0GPCho] and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine [16:0/18:1GPCho] and n=2-5), were formed upon electrospray ionization mass spectrometry (ESI/MS) of 8 mM solution of phosphocholine (L) with 4 mM metal salt (Metal). The electron capture dissociation (ECD) reactions of these [Metal(II)(L)(n)](2+) complexes were examined via Fourier-transform ion-cyclotron resonance mass spectrometry. A rich and complex chemistry was observed, including charge reduction and fragmentation involving losses of a methyl radical, trimethylamine, and the acyl chains. The predominant reaction channel was dependent on the size (n) of the complex, the metal and ligand used, and the size of the acyl chain. Thus charge reduction dominates the ECD spectra of the larger phosphocholine, 16:0/18:1GPCho, but is largely absent in the smaller 6:0/6:0GPCho. For complexes of 16:0/18:1GPCho, n=4-5, fragmentation from the head group mainly occurs via loss of the methyl radical and trimethylamine. At n=3, the relative abundance of fragments due to loss of acyl chain radicals increases. The abundances of ions arising from these radical losses increase further for the n=2 complexes, thereby providing information on the composition and position of the 16:0 and 18:1 acyl groups. Thus ECD of metal complexes provides structurally useful information on the phosphocholine, including the nature of the head group, the acyl chains, and the positions of the acyl chains.

Bis-tridentate chelates of an asymmetric ligand: X-ray structures and solution NMR characterization of divalent zinc triad metal ion complexes of N-(2-pyridylmethyl)- N-(2-(methylthio)ethyl)amine

Divalent zinc triad metal ion complexes of type M( L) 2(ClO 4) 2 ( L = N-(2-pyridylmethyl)- N-(2-(methylthio)ethyl)amine) with N 4S 2 metal coordination spheres were isolated and characterized by X-ray crystallography and variable temperature proton NMR. Although bis-tridentate chelates have nine geometric isomers, the crystallographically characterized complexes of all three metal ions had trans facial octahedral coordination geometry with C i symmetry. Despite the low coordination number and geometric preferences of d 10 metal ions, which facilitate inter- and intramolecular exchange processes, dilute solutions of these bis-tridentate chelates exhibited slow geometric isomerization. Symmetry, sterics and shielding arguments supported specific isomeric assignments for the major and minor chemical shift environments observed at low temperature. At elevated temperature, rapid intramolecular exchange occurred for all three complexes but slow intermolecular exchange on the coupling constant time scale was evidenced through detection of J HgH interactions for Hg ( L ) 2 2 + . These unusual observations are discussed in the context of the zinc triad metal ion coordination chemistry of related bis-tridentate chelates.

Eight‐Step Synthesis of Routiennocin

AbstractRoutiennocin is a member of a family of polycyclic pyrrole ether antibiotics that simultaneously uncouple oxidative phosphorylation and inhibit ATPase as a result of selective complexation of divalent metal ions. We describe a concise synthesis of routiennocin with the longest linear sequence of 8 steps. Our synthesis features a unique bi‐directional strategy, which entails a sequential ring‐opening/cross metathesis of a highly strained cyclopropenone acetal. This approach enables rapid and highly convergent assembly of the fully extended polyketide subunit of this natural product from readily available homoallylic alcohol precursors.

Thermodynamic studies on complexation of divalent transition metal ions with some zwitterionic buffers for biochemical and physiological research

The interaction between the zwitterionic buffers (3-[ N-bis(2-hydroxyethyl)amino]-2-hydroxy propane sulfonic acid, N-(2-actamido)-2-aminoethane sulfonic acid, and 3-[(1,1-dimethyl-2-hydroxyethyl)amino]-2-hydroxy propane sulfonic acid) with some divalent transition metal ions (Cu II, Ni II, Co II, Zn II, and Mn II) were studied at different temperatures (298.15 to 328.15) K at ionic strength I = 0.1 mol · dm −3 NaNO 3 and in the presence of 10%, 30%, and 50% (w/w) dioxene by using potentiometry. The thermodynamic stability constants were calculated as well as the free energy change for the 1:1 binary complexation. The protonation constants of the zwitterionic buffers were also determined potentiometrically under the above conditions.

Piperazine-Bridged Homodinuclear Transition Metal Complexes

Piperazine-bridged transition metal complexes of the type [M(etdtc)]2ppzdtc and [M1(etdtc)2]2ppzdtc [where M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II), M1 = Cr(III), Fe(III), etdtc = (S2NC5H10) and ppzdtc = [S2C(NCH2CH2)2CS2] have been synthesised and characterised by elemental analyses, magnetic susceptibility measurements, TGA/DSC, IR and electronic spectroscopy and conductivity measurements. The presence of only one (C–S) stretching frequency at around 1000 cm−1 supports symmetrical bonding of the dithiocarbamato-moiety. The low molar conductance value of the complexes in DMSO at 25 °C indicates their non-electrolytic behaviour. Thermogravimetric analysis showed only two-step, simple pyrolysis. The decomposition is generally exothermic, leading to the formation of metal sulfide as the final product. The complexes of divalent metal ions appear to be tetrahedral, excepting Cu(II) and Ni(II) which are square-planar, while those of trivalent metal ions are octahedral, which is also corroborated from their electronic spectra and magnetic moment data.

Gas‐Phase Ion‐Molecule Reactions of Divalent Metal Complex Ions: Toward Coordination Structure Analysis by Mass Spectrometry and Some Intrinsic Coordination Chemistry Along the Way

AbstractFor Abstract see ChemInform Abstract in Full Text.

Speciation of Cyclo(Pro-Gly) 3 and Its Divalent Metal-Ion Complexes by Electrospray Ionization Mass Spectrometry

Electrospray ionization mass spectrometry (ESI-MS) was used to study the binding of selected group II and divalent transition-metal ions by cyclo(Pro-Gly)3 (CPG3), a model ion carrier peptide. Metal salts (CatXn) were combined with the peptide (M) at a molar ratio of 1:10 M/Cat in aqueous solvents containing 50% vol/vol acetonitrile or methanol and 1 or 10 mM ammonium acetate (NH4Ac). Species detected include [M+H]+, [M+Cat-H]+, [M2+Cat]2+, [M+Cat+Ac]+, and [M+Cat+X]+. The relative stabilities of complexes formed with different cations (Mg2+, Ca2+, Sr2+, Mn2+, Co2+, Ni2+, Cu2+, and Zn2+) were determined from the abundance of 1:1 and 2:1 M/Cat species relative to that of the unbound peptide. The largest metal ions (Ca2+, Sr2+, and Mn2+) formed the most stable complexes. Reducing the buffer concentration increased the overall extent of metal binding. Results show that the binding specificity of CPG3 depends upon the size of the metal ion and its propensity for electrostatic interaction with oxygen atoms. Product ion tandem mass spectrometry of [M+H]+ and [M+Cu-H]+ confirmed the cyclic structure of the peptide, although the initial site(s) of metal attachment could not be determined.

Are gas-phase reactions of five-coordinate divalent metal ion complexes affected by coordination geometry?

Five-coordinate metal complex ions of the type [ML](2+) [where M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and L= 1,9-bis(2-pyridyl)-2,5,8-triazanonane (DIEN-(pyr)(2)) and 1,9-bis(2-imidazolyl)-2,5,8-triazanonane (DIEN-(imi)(2)] have been reacted with acetonitrile in the gas phase using a modified quadrupole ion trap mass spectrometer. The kinetics and thermodynamics of these reactions show that the reactivity of these complexes is affected by metal electronic structure and falls into three groups: Mn(II) and Ni(II) complexes are the most reactive, Fe(II) and Co(II) complexes exhibit intermediate reactivity, and Cu(II) and Zn(II) complexes are the least reactive. To help explain the experimental trends in reactivity, theoretical calculations have been used. Due to the relatively large size of the metal complexes involved, we have utilized a two-layered ONIOM method to perform geometry optimizations and single point energy calculations for the [ML](2+) and [ML + CH(3)CN](2+) systems. The calculations show that the reactant five-coordinate complexes ([ML](2+)) exhibit structures that are slightly distorted trigonal bipyramidal geometries, while the six-coordinate complexes ([ML + CH(3)CN](2+)) have geometries that are close to octahedral. The Delta G values obtained from the ONIOM calculations roughly agree with the experimental data, but the calculations fail to completely explain the trends for the different metal complexes. The failure to consider all possible isomers as well as adequately represent pi-d interactions for the metal complexes is the likely cause of this discrepancy. Using the angular overlap model (AOM) to obtain molecular orbital stabilization energies (MOSE) also fails to reproduce the experimental trends when only sigma interactions are considered but succeeds in explaining the trends when pi interactions are taken into account. These results indicate that the pi-donor character of the CH(3)CN plays a subtle, yet important, role in controlling the reactivity of these five-coordinate complexes. Also, the AOM calculations are consistent with the experimental data when the [ML](2+) complexes have high-spin trigonal bipyramidal configurations. Generally, these results suggest that ion-molecule reactions can be very sensitive to metal complex coordination geometry and thus may have some promise for providing gas-phase coordination structure.

Hydrolysis of p-nitrophenyl picolinate catalyzed by divalent metal ion complexes containing imidazole groups in micellar solution

The hydrolysis of p-nitrophenyl picolinate (PNPP) catalyzed by Zn(II), Cu(II) and Co(II) complexes of imidazole groups have been investigated kinetically in the pH range 6.0–8.0 at the presence of three kinds of surfactants and 25.0±0.01°C, respectively. The results indicated that Zn(II) complex exhibit a great catalytic function in micellar solution with higher pH value, but when reactions were performed in weak acid solutions, Cu(II) complex catalyzed the hydrolysis of PNPP more efficiently than Zn(II) and Co(II) complexes did, which may be attributable to the different ionization states of the corresponding complexes in micellar media with different pH values and the different nucleophilic ability of active species in different complexes. In addition, these complexes showed more reactivity in zwitterionic (LSS) and nonionic (Brij35) micelles than that in cationic (CTAB) micelle, which may be explained by the electrostatic interaction between metal ions of these complexes and the head groups of surfactants or the substrate. The relative kinetic and thermodynamic parameters were obtained by kinetic analysis employing ternary complex model for metallomicellar catalysis.

Quinoline‐Containing Calixarene Fluoroionophores: A Combined NMR, Photophysical and Modeling Study

AbstractThe 8‐alkoxy‐5‐chloroquinoline fluorophore was appended at the lower rim of a calix[4]arene triamide with two different orientations. In ligand 1 the quinoline part is linked to the calixarene skeleton through the pyridine C2 atom, while in 2 it is linked through the phenolic oxygen atom of the chromophore. The binding properties of both ligands, investigated in chloroform and methanol solutions, indicate that they are efficient fluoroionophores, with selectivity for sodium and strontium ions among alkali and alkaline earth metal ions. Combined NMR, photophysical, and modeling studies disclosed the peculiar conformational and coordination features of monovalent and divalent metal ion complexes. Lanthanide metal ion complexes were prepared and studied in acetonitrile solution showing good luminescence in the case of Nd3+, Yb3+, and Er3+ ions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Synthesis and characterization of new 13 and 14-membered macrocycles and their transition metal complexes

Two new macrocyclic ligands 1,4,7,9,12-pentaaza-10,11-dioxo-8,9,12,13-bis-(1′-oxo-3′-thio-2′-hydropyrimidine)-trideca-7,13-diene, (L1) and 1,4,7,9,12-pentaaza-10,12-dioxo-8,9,13,14-bis-(1′-oxo-3′-thio-2′-hydropyrimidine)-tetradeca-7,14-diene, (L2) and their complexes with CrIII, MnII, FeIII, CoII, NiII, CuII and ZnII have been synthesized, and characterized by elemental analysis, i.r., 1H-n.m.r., e.p.r., u.v.–vis. spectroscopy, magnetic susceptibility and conductance measurements. The conductivity measurements suggest that the complexes of divalent metal ions are 1:1 electrolytes whereas the trivalent metal ions are non-electrolytes. On the basis of electronic spectra and magnetic moment measurements the CrIII and FeIII complexes are octahedral, while the divalent metal complexes are tetrahedral except for the NiII and CuII complexes which are proposed to have square planar geometry. All the ligands and their complexes have been screened against gram-positive bacteria Staphylococcus aureus and gram-negative bacteria E. coli. The results show that they inhibit the growth of bacteria.

Complexes of divalent transition metal ions with bis(aminomethyl)phosphinic acid in aqueous solution and in the solid state

Bis(aminomethyl)phosphinic acid, (NH2CH2)2PO2H (HL), was synthesized using a new procedure. Its coordination ability towards Co(II)/(III), Ni(II), Cu(II) and Zn(II) was studied both in solution and in the solid state. Because of the presence of two nitrogen atoms the ligand exhibits a higher overall basicity than common (aminoalkyl)phosphinic acids. Consequently, the values of the determined stability constants are comparable with those found for (aminoalkyl)phosphonic acids. NMR titrations of Zn(II) point to the interaction of phosphinate with the metal ion in a strong acid solution. The X-ray structures show several coordination modes in the solid state. All-trans-[MCl2(H2L-O)2(H2O)2]Cl2, where M(II) are Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Ca(II), and Cd(II), crystallized from acid solutions. The central ion is octahedrally coordinated with two phosphinate oxygen atoms, two molecules of water and two chlorides in all-trans arrangement. Amine groups are protonated and non-coordinated. Participation of the donor groups in crystals isolated from neutral solutions depends on the metal ion. All donor atoms are coordinated in monomeric fac-N,N,O-trans-O,O′-[Co(L-N,N,O)2]+. On the other hand, in the zinc(II) complex, two phosphinate oxygen atoms and two amine nitrogen atoms (trans to each other) of two different ligand molecules are coordinated in an equatorial plane and two amino groups of the two other ligand molecules are bound in axial positions. Thus, each molecule of the amino acid forms a five-membered N,O-chelate to one zinc(II) ion and the other amino group is bound to the neighbouring ion creating an infinite chain. Nickel(II) forms a trans-O,O′-[Ni(H2O)2(L-N,N)2] complex in which the metal ion is chelated by four amine nitrogen atoms forming two six-membered chelates in an equatorial plane and the octahedron is completed with two water molecules at the apical positions. The phosphinate group is not coordinated. The above results point to a relatively low coordination ability of the phosphinate group; however, due to its low pKA, it is able to bind metal ions at lower pH than other coordinating groups do.

Synthesis of Diazacrown Ethers Containing Phenolic Side Arms and Their Complex with Divalent Metal Ions

The aminomethylation of phenols with para-substituents by the Mannich reaction has successfully been accomplished to produce the Mannich bases 2-6. The compounds 7-8 have also been synthesized in order to identify the effect of the side arms and t he macrocycle in the complex formation. Protonation constants and stability constants of the double armed diaza-18-crown-6 ethers 2-7 with metal ions have been determined by potentiometric method at 25 <TEX>$^{\circ}C$</TEX> in 95 % methanol solution. Under a basic condition (pH > 8.0), the double-armed crown ethers 2-6 revealed stronger interaction with divalent metal ions than the simple diazacrown ether 1. The stability constants with these metal ions were Co 2+ < Ni2+ < Cu2+ > Zn 2+ in increasing order, which are in accordance with the order of the Williams-Irving series. The stability constants with alkali earth metal ions were Ca 2+ < Sr 2+ < Ba 2+ in increasing order, which may be explained by the concept of size effect. It is noteworthy that the hosts 2-6, which have phenolic side arms and a macrocycle, bind stronger with metal ions than the hosts 1 and 7. On the other hand, the host 8, which has phenolic side arms with a pyperazine ring,provided comparable stability constants to those with the host 3. These facts demonstrate that phenolic side arms play a more important role than the azacrown ether ring in the process of making a complex with metal ions especially in a basic condition. In particular, the log KML values for complexation of divalent metal ions with the hosts 2-6 had the sequence, i.e., 2 (R=OCH3) < 3 (R=CH3) < 4 (R=H) < 5 (R=Cl) < 6 (R=CF3). The stability constants of the hosts 5 and 6 containing an electron-withdrawing group are larger than those of the hosts 2 and 3 containing an electron-donating group. This substituent effect is attributed to the solvent effect in which the aryl oxide with an electron-donating group has a tendency to be tied strongly with protic solvents.

Hydration of Polymer Complexes with Iminodiacetate-Type Chelating Resin

Abstract Hydration of polymer complexes of divalent and trivalent metal ions with an iminodiacetate (IDA)-type chelating resin (-LNa2) was studied. The hydration numbers of [(-LH)2MII] for Ca2+ and [(-LH)3MIII] for Y3+ and La3+, which are practically the same as those of the respective aqua ions, indicate simple electrostatic interactions between the carboxylates of monoprotonated species of iminodiacetate, (-LH)-, and hydrated metal ions. In contrast, an appreciably smaller hydration number of [(-LH)3MIII] for Sc3+ than that of the aqua ion suggests involvement of the carboxylates in the coordination sphere. The difference in their interaction modes is discussed.

Fragmentation and charge transfer in gas-phase complexes of divalent metal ions with acetonitrile

The development of electrospray has enabled generation of gas-phase multiply charged metal ion complexes with various solvent molecules. These species exhibit rich fragmentation chemistry, involving competition among neutral ligand loss, ligand cleavage, and dissociative electron and proton transfer. Acetonitrile is a common aprotic solvent. Here we present a comprehensive MS/MS study on acetonitrile complexes of divalent metal cations. We measured the critical sizes below which dissociation channels other than the trivial neutral evaporation become operative and minimum sizes at which dications remain stable against charge reduction. For all sizes between the two, low-energy fragmentation patterns have been elucidated in detail.

Role of Biologically Important Zwitterionic Buffer Secondary Ligands on the Stability of the Mixed-Ligand Complexes of Divalent Metal Ions and Adenosine 5‘-Mono-, 5‘-Di-, and 5‘-Triphosphate

Potentiometric equilibrium measurements have been performed at (25.0 ± 0.1) °C and ionic strength I = 0.1 mol dm-3 KNO3 for the interaction of the purine nucleotides adenosine 5‘-mono, 5‘-di, and 5‘-triphosphate and Cu(II), Co(II), Ni(II), Mn(II), Zn(II), Ca(II), and Mg(II) with the biologically important secondary ligand zwitterionic buffers 3-(N-morpholino)propanesulfonic acid, 3-[(1,1-dimethyl-2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid, N-(2-hydroxyethyl)piperazine-N‘-2-hydroxypropanesulfonic acid, piperazine-N,N‘-bis(2-ethanesulfonic acid), and piperazine-N,N‘-bis(2-hydroxypropanesulfonic acid) in a 1:1:1 ratio and the formation of various 1:1:1 normal and protonated mixed-ligand complex species was inferred from the potentiometric pH titration curves. The experimental conditions were selected such that self-association of the purine nucleotides and their complexes was negligibly small; i.e., the monomeric normal and protonated ternary complexes were studied. Initial estimates of the formatio...

Intrinsic Ca 2+ affinities of peptides: application of the kinetic method to analogs of calcium-binding site III of rabbit skeletal troponin C

We extended the kinetic method to determine the intrinsic affinities of nonvolatile organic molecules with divalent metal ions and then applied the amended method to determine the calcium affinities of peptide analogs of the calcium-binding site III of rabbit skeletal troponin C. Metal– bis(peptide) complexes of the composition ([H 2P i + H 2P ii] − H + Ca) +, where H 2P is a neutral peptide, were introduced into the gas phase by fast atom bombardment. The extended kinetic method recognizes that the dissociation characteristics of a singly charged, bis(peptide) complexes of divalent metal ions are determined by not only the metal–ion affinity but also the proton affinities of the neutral and deprotonated peptides. The modified method requires one to measure the relative abundances of [H 2P − H + Ca] +, [H 2P + H] +, and [H 2P − H] − ions that form upon collisional activation of mixed peptide/metal complexes, proton-bound peptide dimers, and deprotonated peptide dimers, respectively. We found, by using the modified method, that the set of peptides has a different affinity order than that in solution. Peptides with one aspartic acid have a higher intrinsic Ca 2+ affinity than those with two aspartates. The location of the aspartic acid (Asp) residues at various positions also affects the Ca 2+ affinity. Those peptides with one Asp in the middle of the chain have higher Ca 2+ affinities than those with Asp on the end because the former peptides offer greater polarizability to stabilize the charge. Peptides with two Asp’s located in close proximity have higher intrinsic calcium affinities than those with aspartates positioned further apart.