Coordination chemistry of vitamin C. Part II. Interaction of L-ascorbic acid with Zn(II), Cd(II), Hg(II), and Mn(II) ions in the solid state and in aqueous solution

The reaction of L-ascorbic acid with the zinc group and manganese ions has been investigated in aqueous solution at pH 6-7. The solid salts of the type M (L-ascorbate)2.2H2O, where M = Zn(II), Cd(II) and Mn(II) were isolated and characterized by 13C NMR and Fourier Transform infrared (FT-IR) spectroscopy. Spectroscopic evidence showed that in aqueous solution, the bindings of the Zn(II) and Mn(II) ions are through the ascorbate anion O-3 and O(2)-H groups (chelation), while the Cd(II) ion binding is via the O-3 atom only. In the solid state, the binding of these metal ions would be through two acid anions via O-3, O-2 of the first and O-1, O-3 of the second anion as well as to two H2O molecules, resulting in a six-coordinated metal ion. The Hg(II) ion interaction leads to the oxidation of the ascorbic acid in aqueous solution.     

Interaction of deoxyguanylic acid with alkaline earth metal ions. Evidence for the deoxyribose C3'-endo/anti, O4'-endo/anti and C2'-endo/anti conformational transitions

The interaction of 2'-deoxyguanosine-5'-monophosphoric acid (H2-dGMP) with the alkaline earth metal ions has been investigated in aqueous solution at neutral pH. The solid salts of the type Mg-dGMP.5H2O Ca-dGMP.6H2O, Sr-dGMP.7H2O and Ba-dGMP.7H2O were isolated and characterized by FT-IR, proton-NMR and X-ray powder diffraction measurements. Two types of metal-nucleotide interactions have been identified: (a) the indirect metal-base and the direct metal-phosphate bindings (outer- and inner-sphere interaction), for the Mg(II) and Sr(II) ions and (b) the direct metal-base and direct metal-phosphate bindings (inner-sphere interaction), for the Ca(II) and Ba(II) ions. In aqueous solution, a dynamic equilibrium of the type base-metal-H2O. . .PO3 in equilibrium base. . .H2O-metal-PO3 is present. The deoxyribose moiety exhibits C3'-endo/anti conformation, in the Mg(II)-, Ca(II)-, Sr(II)- and Ba(II)-dGMP salts.     

Interaction of guanylic acid with the Mg(II), Ca(II), Sr(II), and Ba(II) ions in the crystalline solid and aqueous solution: evidence for the ribose C2'-endo/anti and C3'-endo/anti conformational changes.

The interaction of guanosine-5'-monophosphoric acid (H2-GMP) with the alkaline earth metal ions has been studied in aqueous solution at neutral pH. The crystalline salts of the type Mg-GMP.5H2O, Ca-GMP.6H2O, Sr-GMP.7H2O, and Ba-GMP.7H2O were isolated and characterized by Fourier transform ir, 1H-nmr and x-ray powder diffraction measurements. Two types of macrochelate complexes have been identified: (a) The direct metalbase and indirect metal-phosphate bindings (inner and outer sphere interaction) for the Mg(II), Ca(II), and Sr(II), ions; and (b) the indirect metal-base and direct metal-phosphate bindings (outer and inner sphere interaction) for the Ba(II) ion. In aqueous solution, an equilibrium exists between the base-metal-H2O...PO3 and base...H2O-M-PO3 interactions. The ribose moiety shows C3'-endo/anti conformation in the free acid; C2'-endo/anti in the Na2-GMP salt; C3'-endo/anti in the Mg(II)-, Ca(II)-, and Sr(II)-GMP salts; and C2'-endo/anti, in the Ba(II)-GMP salt.     

Interaction of adenylic acid with alkaline earth metal ions in the crystalline solid and aqueous solution. Evidence for the sugar C'2-endo/anti, C'3-endo/anti and C'4-exon/anti conformational changes

The reaction of adenosine 5'-monophosphoric acid (H2-AMP) with the alkaline earth metal ions has been investigated in aqueous solution at neutral pH. The solid salts of Mg-AMP.5H2O, Ca-AMP.6H2O, Sr-AMP.7H2O and Ba-AMP.7H2O were isolated and characterized by Fourier transform infrared, 1H-NMR spectroscopy and X-ray powder diffraction measurements. Spectroscopic and other evidence showed that the Sr-AMP.7H2O and Ba-AMP.7H2O are isomorphous, whereas the Mg-AMP.5H2O and Ca-AMP.6H2O are not similar. The Mg2+ binding is through the N-7 (inner-sphere) and the phosphate group (outer-sphere via H2O), while the Ca2+ binds to the phosphate group (inner-sphere) and to the base N-7 site (outer-sphere through H2O). The Sr2+ and Ba2+ bind to H2O molecules, H-bonding to the N-7, N-1 and the phosphate group (outer-sphere). In aqueous solution, an equilibrium between the inner- and outer-sphere metal ion bindings can be established. The sugar moiety exhibited C'2-endo/anti conformation, in the free H2-AMP acid and the magnesium salt, C'3-endo/anti in the calcium salt and unusual C'4-exo/anti, in the strontium and barium salts.     

Sugar adducts with alkaline earth metal ions. Interaction of L-arabinose with Sr(II) and Ba(II) ions and the effects of metal ion binding on the sugar anomeric configurations

The reaction between L-arabinose and hydrated Sr(II) or Ba(II) halide salts has been studied in H2O solution and adducts of the type M(L-arabinose)X(2).4H(2)O, where M = Sr(II) or Ba(II) and X = Cl- or Br- have been isolated and characterized by means of Fourier transform infrared spectroscopy, 1H-NMR spectroscopy, molar conductivity and X-ray powder diffraction measurements. Due to the marked spectral similarities with those of the structurally known Ca(L-arabinose)X2 . 4H2O (X= Cl- or Br-) compounds, the Sr(II) and the BA(II) ions are eight-coordinated, binding to two l-arabinose molecules via O1, O5 of the first and O3, O4 of the second sugar moiety and to four H2O molecules. 1H-NMR spectroscopy indicated that the free L-arabinose has the beta-anomer configuration in aqueous solution, whereas the alpha-anomer isomer is preferred by Mg(II), Ca(II), Sr(II) and Ba(II) ions, on complexation.     

Liquid crystalline phase behavior of high molecular weight DNA: a comparative study of the influence of metal ions of different size, charge and binding mode

The ability of Li(+), Na(+), K(+), Rb(+), Cs(+), Mg(2+), Ca(2+), Sr(2+), Ba(2+), Cu(2+), Cd(2+), Al(3+), V(4+), Hg(2+), Pd(2+), Au(3+), and Pt(4+) to provoke liquid crystalline (LC) phases in high molecular weight DNA was investigated. The alkali and alkaline earth metal ions provoked typical cholesteric/columnar structures, whereas transition metal ions precipitated DNA into solid/translucent gel-like aggregates. Heavy metal ions reduced viscosity of DNA solution, disrupting rigid, rod-like DNA structure necessary for LC textures. Three-layer quantum mechanical-molecular mechanical (QM/MM) studies of Li(+), Na(+), K(+), Mg(2+), and Ca(2+) binding DNA fragment suggested several possible binding modes of these ions to the phosphate groups. The dianion mode of metal binding, involving the phosphate groups of both strands of DNA, allowed for higher DNA binding affinity of the alkaline earth metal ions. These results have implications in understanding the biological role of metal ions and developing DNA-based sensors and nanoelectronic devices.     

Crystal structures and spectroscopic characterization of galactitol complexes of trivalent lanthanide and divalent alkaline earth chlorides

Crystal structures and FT-IR spectra of metal ion-galactitol (C6H14O6, the ligand here abbreviated as L) complexes: 2LaCl3*C6H14O6*10H2O and SrCl2*C6H14O6 complexes are reported. Crystal data of lanthanide chlorides (La3+, Nd3+, Sm3+, Eu3+, Tb3+)-galactitol complexes and alkaline earth chlorides (Ca2+, Sr2+)-galactitol complexes published earlier are summarized. Unlike other lanthanide ion-galactitol complexes (2MCl3*C6H14O6*14H2O), lanthanum ions give rise to two different structures: LaCl3*C6H14O6*6H2O (LaL1) and 2LaCl3*C6H14O6*10H2O (LaL2). Sr2+-galactitol complexes also crystallized with two structures: SrCl2*C6H14O6*4H2O (SrL1) and SrCl2*C6H14O6 (SrL2). These metal ions thus give different coordination structures with galactitol. The crystal structures and FT-IR spectra of lanthanide ion and alkaline earth ion-galactitol complexes were integrated to interpret the coordination modes of different metal ions. Similar IR spectra demonstrate the same coordination modes of the complexes.     

Sugar complexes with alkaline earth metal ions. Synthesis,structure, and spectroscopic studies of Mg(II), Sr(II), and Ba(II) complexes of d-glucur

Interaction between D-glucuronic acid and alkaline earth metal ions leads to the formation of the complexes such as M(D-glucuronate)X· nH₂O and M(D-glucuronate)₂ · nH₂O, where M = Mg(II), Sr(II), and Ba(II), X = Cl^? or Br^?, and n = 2–4. Owing to the distinct spectral similarities with the structurally known Ca(D-gluguronate)Br · 3H₂O compound, the metal cations bind to three sugar moieties (through O₆, O₅ of the first, O₆', O₄ of the second, and O₁, O₂ of the third residue) and to two H₂O molecules, forming an eight-coordination geometry around each metal ion, in M(D-glucuronate)X · nH₂O (except for Mg(II) ion, which is six-coordination). The metal ions in M(D-glucuronate)₂-nH₂O show six-coordination in different structural environments. The strong hydrogen bonding network of the free acid is weakened upon metalation and the sugar moiety crystallizes as α-anomer, in these series of metal-sugar complexes.     

AFM studies of DNA structures on mica in the presence of alkaline earth metal ions

As counterions of DNA on mica, Mg(2+), Ca(2+), Sr(2+) and Ba(2+) were used for clarifying whether DNA molecules equilibrate or are trapped on mica surface. End to end distance and contour lengths were determined from statistical analysis of AFM data. It was revealed that DNA molecules can equilibrate on mica when Mg(2+), Ca(2+) and Sr(2+) are counterions. When Ba(2+) is present, significantly crossovered DNA molecules indicate that it is most difficult for DNA to equilibrate on mica and the trapping degree is different under different preparation conditions. In the presence of ethanol, using AFM we have also observed the dependence of B-A conformational transition on counterion identities. The four alkaline earth metal ions cause the B-A transition in different degrees, in which Sr(2+) induces the greatest structural transition.     

Calcium in the toad (Bufo marinus) cornea: binding by the stroma

1. The Ca concentration in the toad (Bufo marinus) cornea was 2.6 mmol/kg wet wt compared at 1.0 mmol/l in the bathing aqueous humor and 2.8 mmol/kg wet wt in the separated corneal stromal layer. Cell Ca content was calculated to be about 1.8 mmol/kg wet wt. 2. About 80% of the total Ca appears to be sequestered or bound to tissue components most of which (68% of the total) is associated with the stroma (2.2 mmol/kg wet wt stroma). 3. About 85-90% of the Ca in the stroma is readily exchangeable with external 45Ca. 4. The loss of accumulated 45Ca from the stroma was measured in vitro. This efflux of the isotope was enhanced by multivalent ions and was greatest when Ca2+ or La3+ was present in the external media. Other alkaline earth metal ions were not as effective. The relative effectiveness of this displacement of 45Ca was Ca = La greater than Sr greater than Ba greater than Mg. 5. The results suggest that the Ca2+ is bound by the amphibian stroma at sites that have a preference or specificity for this divalent ion as compared to the other alkaline earth metals. 6. The possible functional role of this bound Ca is discussed.     

Raman spectroscopy of DNA-metal complexes. II. The thermal denaturation of DNA in the presence of Sr2+, Ba2+, Mg2+, Ca2+, Mn2+, Co2+, Ni2+, and Cd2+.

Differential scanning calorimetry, laser Raman spectroscopy, optical densitometry, and pH potentiometry have been used to investigate DNA melting profiles in the presence of the chloride salts of Ba2+, Sr2+, Mg2+, Ca2+, Mn2+, Co2+, Ni2+, and Cd2+. Metal-DNA interactions have been observed for the molar ratio [M2+]/[PO2-] = 0.6 in aqueous solutions containing 5% by weight of 160 bp mononucleosomal calf thymus DNA. All of the alkaline earth metals, plus Mn2+, elevate the melting temperature of DNA (Tm > 75.5 degrees C), whereas the transition metals Co2+, Ni2+, and Cd2+ lower Tm. Calorimetric (delta Hcal) and van't Hoff (delta HVH) enthalpies of melting range from 6.2-8.7 kcal/mol bp and 75.6-188.6 kcal/mol cooperative unit, respectively, and entropies from 17.5 to 24.7 cal/K mol bp. The average number of base pairs in a cooperative melting unit (<nmelt>) varied from 11.3 to 28.1. No dichotomy was observed between alkaline earth and transition DNA-metal complexes for any of the thermodynamic parameters other than their effects on Tm. These results complement Raman difference spectra, which reveal decreases in backbone order, base unstacking, distortion of glycosyl torsion angles, and rupture of hydrogen bonds, which occur after thermal denaturation. Raman difference spectroscopy shows that transition metals interact with the N7 atom of guanine in duplex DNA. A broader range of interaction sites with single-stranded DNA includes ionic phosphates, the N1 and N7 atoms of purines, and the N3 atom of pyrimidines. For alkaline earth metals, very little interaction was observed with duplex DNA, whereas spectra of single-stranded complexes are very similar to those of melted DNA without metal. However, difference spectra reveal some metal-specific perturbations at 1092 cm-1 (nPO2-), 1258 cm-1 (dC, dA), and 1668 cm-1 (nC==O, dNH2 dT, dG, dC). Increased spectral intensity could also be observed near 1335 cm-1 (dA, dG) for CaDNA. Optical densitometry, employed to detect DNA aggregation, reveals increased turbidity during the melting transition for all divalent DNA-metal complexes, except SrDNA and BaDNA. Turbidity was not observed for DNA in the absence of metal. A correlation was made between DNA melting, aggregation, and the ratio of Raman intensities I1335/I1374. At room temperature, DNA-metal interactions result in a pH drop of 1.2-2.2 units for alkaline earths and more than 2.5 units for transition metals. Sr2+, Ba2+, and Mg2+ cause protonated sites on the DNA to become thermally labile. These results lead to a model that describes DNA aggregation and denaturation during heating in the presence of divalent metal cations; 1) The cations initially interact with the DNA at phosphate and/or base sites, resulting in proton displacement. 2) A combination of metal-base interactions and heating disrupts the base pairing within the DNA duplex. This allows divalent metals and protons to bind to additional sites on the DNA bases during the aggregation/melting process. 3) Strands whose bases have swung open upon disruption are linked to neighboring strands by metal ion bridges. 4) Near the midpoint of the melting transition, thermal energy breaks up the aggregate. We have no evidence to indicate whether metal ion cross-bridges or direct base-base interactions rupture first. 5) Finally, all cross-links break, resulting in single-stranded DNA complexed with metal ions.     

Investigation of restriction enzyme cofactor requirements: a relationship between metal ion properties and sequence specificity

Restriction enzymes are important model systems for understanding the mechanistic contributions of metal ions to nuclease activity. These systems are unique in that they combine distinct functions which have been shown to depend on metal ions: high-affinity DNA binding, sequence-specific recognition of DNA, and Mg(II)-dependent phosphodiester cleavage. While Ca(II) and Mn(II) are commonly used to promote DNA binding and cleavage, respectively, the metal ion properties that are critical to the support of these functions are not clear. To address this question, we assessed the abilities of a series of metal ions to promote DNA binding, sequence specificity, and cleavage in the representative PvuII endonuclease. Among the metal ions tested [Ca(II), Sr(II), Ba(II), Eu(III), Tb(III), Cd(II), Mn(II), Co(II), and Zn(II)], only Mn(II) and Co(II) were similar enough to Mg(II) to support detectable cleavage activity. Interestingly, cofactor requirements for the support of DNA binding are much more permissive; the survey of DNA binding cofactors indicated that Cd(II) and the heavier and larger alkaline earth metal ions Sr(II) and Ba(II) were effective cofactors, stimulating DNA binding affinity 20-200-fold. Impressively, the trivalent lanthanides Tb(III) and Eu(III) promoted DNA binding as efficiently as Ca(II), corresponding to an increase in affinity over 1000-fold higher than that observed under metal-free conditions. The trend for DNA binding affinity supported by these ions suggests that ionic radius and charge are not critical to the promotion of DNA binding. To examine the role of metal ions in sequence discrimination, we determined specificity factors [K(a)(specific)/K(a)(nonspecific)] in the presence of Cd(II), Ba(II), and Tb(III). Most interestingly, all of these ions compromised sequence specificity to some degree compared to Ca(II), by either increased affinity for a noncognate sequence, decreased affinity for the cognate sequence, or both. These results suggest that while amino acid-base contacts are important for specificity, the properties of metal ion cofactors at the catalytic site are also critical for sequence discrimination. This insight is invaluable to our efforts to understand and subsequently design sequence-specific nucleases.     

The crystal structure of lithium L-ascorbate dihydrate.

The crystal structure of lithium L-ascorbate dihydrate is triclinic, Pl; with a = 5.964(9), b = 5.299(9), c = 7.760(15) A; alpha = 100.82(9), beta = 109.78(9), gamma = 92.02(9) degrees. The plant fragment of the ascorbate anion is a part of the five-membered ring [C-1,C-2,C-3(O-3),C-4], and O-4 deviates by 0.053(2) A from this plane. Deprotonated O-3 is an acceptor of three hydrogen bonds, but does not interact with Li+. The coordination number of the Li+ is 5 and it is bonded to two water molecules and three hydroxyl oxygen atoms of two ascorbate anions: O-2 and the gauche O-5, 6 of the side chain.     

Interaction of D-glucose with alkaline-earth metal ions. Synthesis, spectroscopic, and structural characterization of Mg(II)- and Ca(II)-D-glucose adducts and the effect of metal-ion binding on anomeric configuration of the sugar

The interaction of D-glucose with the hydrated alkaline-earth metal halides has been studied in solution, and adducts of the type Mg(D-glucose)X2.4 H2O, Ca(D-glucose)X2.4 H2O, and Ca(D-glucose)2X2.4 H2O, where X = Cl- and Br-, have been isolated, and characterized by means of F.t.-i.r. and 1H-n.m.r. spectroscopy, X-ray powder diffraction, and molar conductivity measurements. Spectroscopic and other evidence suggested that the Mg(II) ion in the Mg(D-glucose)X2.4 H2O adducts six-coordinate, binding to a D-glucose molecule (possibly via O-1 and O-2 atoms) and to four H2O molecules, whereas, in the corresponding 1:1 Ca-D-glucose adduct, the Ca(II) ion is possibly seven-coordinate, binding to a sugar moiety (through the O-1, O-2, and other sugar donor atoms) and to four H2O molecules. In 1:2 Ca(D-glucose)2X2.4 H2O, the calcium ion may be eight-coordinate, binding to two D-glucose molecules (possibly via the O-1 and O-2 atoms of each sugar moiety) and to four H2O molecules. The strong, sugar H-bonding network is rearranged upon D-glucose adduct-formation, and the alpha-anomeric configuration is favored by these metal cation coordinations.     

The effects of monovalent cations Li+, Na+, K+, NH4+, Rb+ and Cs+ on the solid and solution structures of the nucleic acid components. Metal ion binding and sugar conformation.

The interactions of the monovalent ions Li+, Na+, K+, NH4+, Rb+ and Cs+ with adenosine-5'-monophosphoric acid (H2-AMP), guanosine-5'-monophosphoric acid (H2-GMP) and deoxyguanosine-5'-monophosphoric acid (H2-dGMP) were investigated in aqueous solution at physiological pH. The crystalline salts M2-nucleotide.nH2O, where M = Li+, Na+, K+ NH4+, Rb+ and Cs+, nucleotide = AMP, GMP and dGMP anions and n = 2-4 were isolated and characterized by Fourier Transform infrared (FTIR) and 1H-NMR spectroscopy. Spectroscopic evidence showed that these ions are in the form of M(H2O)n+ with no direct metal-nucleotide interaction, in aqueous solution. In the solid state, Li+ ions bind to the base N-7 site and the phosphate group (inner-sphere), while the NH4+ cations are in the vicinity of the N-7 position and the phosphate group, through hydrogen bonding systems. The Na-nucleotides and K-nucleotides are structurally similar. The Na+ ions bind to the phosphate group of the AMP through metal hydration shell (outer-sphere), whereas in the Na2-GMP, the hydrated metal ions bind to the base N-7 or the ribose hydroxyl groups (inner-sphere). The Na2-dGMP contains hydrated metal-carbonyl and metal-phosphate bindings (inner-sphere). The Rb+ and Cs+ ions are directly bonded to the phosphate groups and indirectly to the base moieties (via H2O). The ribose moiety shows C2'-endo/anti conformation for the free AMP acid and its alkali metal ion salts. In the free GMP acid, the ribose ring exhibits C3'-endo/anti conformer, while a C2'-endo/anti sugar pucker was found in the Na2-GMP and K2-GMP salts and a C3'-endo/anti conformation for the Li+, NH4+, Rb+ and Cs+ salts. The deoxyribose has C3'-endo/anti conformation in the free dGMP acid and O4'-endo/anti in the Na2-dGMP, K2-dGMP and a C3'-endo/anti for the Li+, NH4+, Rb+ and Cs+ salts. An equilibrium mixture of the C2'-endo/anti and C3'-endo/anti sugar puckers was found for these metal-nucleotide salts in aqueous solution.     

Carbohydrate adducts with zinc-group-metal ions. Interactions of β-d-fructose with Zn(II), Cd(II), and Hg(II) cations,and the effects of metal-ion coordination on the sugar isomer binding

Interaction of β-d-fructose with hydrated salts of zinc-group-metal has been studied in aqueous solution and solid adducts of the type M(d-fructose)X₂·nH₂O, where M = Zn(II), Cd(II), and Hg(II) ions, X = Cl⁻ or Br⁻, and n = 0–2, have been isolated, and characterized by means of F.t.-i.r. spectroscopy, X-ray powder diffraction, and molar conductivity measurements. The marked spectral similarities observed with the Mg(d-fructose)X₂·4 H₂O (X = Cl⁻ or Br⁻) compounds indicated that the Zn(II) and Cd(II) ions are six-coordinated, binding to two d-fructose molecules through O-2, O-3 of the first d-fructose, and O-4, O-5 of the second, as well as to two H₂O. The Hg(II) ion binds to two sugar moieties in the same fashion as do the Zn(II) and Cd(II) ions, resulting in four-coordination geometry around the mercury ion. The crystalline sugar is in the β-d-fructopyranose form, and the coordination of the of the Ca(II) ion takes place through the β-d-fructopyranose isomer, whereas the binding of the Mg(II), Zn(II), Cd(II), Hg(II), and UO²⁺₂ cations could be via the β-d-fructopyranose and the β-d-fructofuranose structures.     

Tolerance ofChlorella vulgaris for metallic and non-metallic ions

The well-known, extreme sensitivity of algae towards Cu⁺⁺ ions prompted a systematic investigation of the tolerance ofChlorella vulgaris for both metallic (49) and non-metallic (7) ions. With thirty metals forming weak bases, pH effects were to some extent super-imposed on the toxic effects of the metal ions themselves. With the elements U, Zr, V and Sb, oxy-compounds had to be used. The elements Mo, W and Bi were tested as components of anions.From the metals that form strong bases, the salts of Na, K, Rb, Ca, Sr and Mg were tolerated in high concentrations; the maximum values of these ranged from 0.11 – 0.98 g at/liter. Notwithstanding some unavoidable simplifications of the experimental technique, it could be concluded from the results that in four intermetal groups, arranged according to I.U.P.A.C., toxicity has a definite tendency to increase with increasing atomic number. This held for the series: Na, K, Rb, Cs; Mg, Ca, Sr, Ba; Zn, Cd, Hg; Al, In, Tl. In like manner, the rare earths were found to be more toxic than the alkaline earth metals.Co, Ni and Cu completely inhibit growth at very low concentrations ranging from 4.2×10^–6–2×10^–5 g at/liter; in view of the relatively low atomic numbers of these metals, the toxicity must be regarded as specific (algotoxicity).Among the non-metals, Sb and As proved highly toxic. Fluoride ions were considerably more toxic than chloride and bromide ions.     

IR and Raman study on the interactions of the 5′-GMP and 5′-CMP phosphate groups with Mg(II), Ca(II), Sr(II), Ba(II), Cr(III), Co(II), Cu(II), Zn(II), Cd(II), Al(III) and Ga(III)

The chief motive behind this research is the interest provoked by the presence of metal ions as necessary stabilizers of the negative charges of phosphate groups in nucleic acids. The effect that the presence of different metal ions produces on the band principally assigned to the nu(s) PO(3)(2-) mode has been studied using FT-IR and FT-Raman spectroscopy. The results obtained reveal the diagnostic capacity of these techniques in determining the type of metal ion interaction with respect to the mononucleotides that form DNA and RNA, providing a tool for improving the knowledge of the stabilizing or destabilizing effects of these ions on such macromolecules. The metal complexes of the ribonucleotides 5'-CMP and 5'-GMP with Mg(II), Ca(II), Sr(II), Ba(II), Cr(III), Co(II), Cu(II), Zn(II), Cd(II), Al(III) and Ga(III) were obtained in this study. After studying and analyzing the IR and Raman spectra of all these complexes and comparing them with the spectra of the corresponding disodium salts, it was verified that, independently of the type of nucleotide involved, the presence of the metal in the vicinity of the phosphate group produces an alteration in the aforementioned nu(s) PO(3)(2-) band. This effect is related to the type of interaction that the phosphate group has with the metal. Three components are observed: (1) one near 983-975 cm(-1) (detectable in IR and Raman), associated with phosphate groups in an electrostatic type of interaction with the metal ion, separated by two or more water molecules; (2) another near 989-985 cm(-1) (only in IR), associated with phosphate groups in indirect interaction through the water molecules of the coordination sphere of the metal ions; and (3) the IR and Raman bands near 1014-1001 cm(-1), which represent phosphate groups directly bonded to the metal ion. These results are supported by the behavior of 5'-CMP in aqueous solution in the presence of Mg(II) ions.     

The permeability of endplate channels to monovalent and divalent metal cations

The relative permeability of endplate channels to monovalent and divalent metal ions was determined from reversal potentials. Thallium is the most permeant ion with a permeability ratio relative to Na+ of 2.5. The selectivity among alkali metals is weak with a sequence, Cs+ greater than Rb+ greater than K+ greater than Na+ greater than Li+, and permeability ratios of 1.4, 1.3, 1.1, 1.0, and 0.9. The selectivity among divalent ions is also weak, with a sequence for alkaline earths of Mg++ greater than Ca++ greater than Ba++ greater than Sr++. The transition metal ions Mn++, Co++, Ni++, Zn++, and Cd++ are also permeant. Permeability ratios for divalent ions decreased as the concentration of divalent ion was increased in a manner consistent with the negative surface potential theory of Lewis (1979 J. Physiol. (Lond.). 286: 417--445). With 20 mM XCl2 and 85.5 mM glucosamine.HCl in the external solution, the apparent permeability ratios for the alkaline earth cations (X++) are in the range 0.18--0.25. Alkali metal ions see the endplate channel as a water-filled, neutral pore without high-field-strength sites inside. Their permeability sequence is the same as their aqueous mobility sequence. Divalent ions, however, have a permeability sequence almost opposite from their mobility sequence and must experience some interaction with groups in the channel. In addition, the concentrations of monovalent and divalent ions are increased near the channel mouth by a weak negative surface potential.     

Divalent Ions and the Surface Potential of Charged Phospholipid Membranes

Phospholipid bilayer membranes were bathed in a decimolar solution of monovalent ions, and the conductance produced by neutral carriers of these monovalent cations and anions was used to assess the electric potential at the surface of the membrane. When the bilayers were formed from a neutral lipid, phosphatidylethanolamine, the addition of alkaline earth cations produced no detectable surface potential, indicating that little or no binding occurs to the polar head group with these ions. When the bilayers were formed from a negatively charged lipid, phosphatidylserine, the addition of Sr and Ba decreased the magnitude of the surface potential as predicted by the theory of the diffuse double layer. In particular, the potential decreased 27 mv for a 10-fold increase in concentration in the millimolar-decimolar range. A 10-fold increase in the Ca or Mg concentration also produced a 27 mv decrease in potential in this region, which was again due to screening, but it was necessary to invoke some specific binding to account for the observation that these cations were effective at a lower concentration than Ba or Sr. It is suggested that the ability of the alkaline earth cations to shift the conductance-voltage curves of a nerve along the voltage axis by 20–26 mv for a 10-fold increase in concentration may be due to essentially a screening rather than a binding phenomenon.