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Lead is widely distributed in the environment; it is known to mankind for thousands of years and its toxicity is nowadays (again) well recognized, though on the molecular level only partly understood. One of the reasons for this shortcoming is that the coordination chemistry of the biologically important lead(II) is complicated due to the various coordination numbers it can adopt (CN = 4 to 10) as well as by the 6s2 electron lone pair which, with CN = 4, can shield one side of the Pb2+ coordination sphere. The chapter focuses on the properties of Pb2+ complexes formed with nucleotides and their constituents and derivatives. Covered are (among others) the complexes formed with hydroxy groups and sugar residues, the interactions with the various nucleobases occurring in nucleic acids, as well as complexes of phosphates. It is expeced that such interactions, next to those like with lipids and proteins, are responsible for the toxic properties of lead. To emphasize the special properties of Pb2+ complexes, these are compared as far as possible with the corresponding properties of the Ca2+, Fe2+, Cu2+, Zn2+, and Cd2+ species. It needs to be mentioned that the hard-soft rule fails with Pb2+. This metal ion forms complexes with ligands offering O donors of a stability comparable to that of Cu2+. In contrast, with aromatic N ligands, like imidazole or N7 sites of purines, complex stability is comparable to that of the corresponding Fe2+ complexes. The properties of Pb2+ towards S donor sites are difficult to generalize: On the one hand Pb2+ forms very stable complexes with nucleoside 5'-O-thiomonophosphates by coordinating to nearly 100% at S in the thiophosphate group; however, on the other hand, once a sulfur atom replaces one of the terminal oxygen atoms in the phosphodiester linkage, macrochelate formation of the phosphate-bound Pb2+ occurs with the O and not the S site. Quite generally, the phosphodiester linkage is a relatively weak binding site, but the affinity increases further to the mono- and then to the di- and triphosphate. The same holds for the corresponding nucleotides, though the Pb2+ affinity had to be estimated via that of the Cu2+ complexes for some of these ligands. Complex stability of the pyrimidine-nucleotides (due to their anti conformation) is solely determined by the coordinating tendency of the phosphate group(s); this also holds for the Pb2+ complex of adenosine 5'-monophosphate. For the other purinenucleotides macrochelate formation takes place by the interaction of the phosphate-coordinated Pb2+ with the N7/(C6)O site of, e.g., the guanine residue. The extents of the formation degrees of these chelates are summarized. Unfortunately, information about mixed ligand (ternary) or other higher order comlexes is missing, but still it is hoped that the present overview will help to understand the interaction of Pb2+ with nucleotides and nucleic acids, and especially that it will facilitate further research in this fascinating area.

Cadmium(II), commonly classified as a relatively soft metal ion, prefers indeed aromatic-nitrogen sites (e.g., N7 of purines) over oxygen sites (like sugar-hydroxyl groups). However, matters are not that simple, though it is true that the affinity of Cd(2+) towards ribose-hydroxyl groups is very small; yet, a correct orientation brought about by a suitable primary binding site and a reduced solvent polarity, as it is expected to occur in a folded nucleic acid, may facilitate metal ion-hydroxyl group binding very effectively. Cd(2+) prefers the guanine(N7) over the adenine(N7), mainly because of the steric hindrance of the (C6)NH(2) group in the adenine residue. This Cd(2+)-(N7) interaction in a guanine moiety leads to a significant acidification of the (N1)H meaning that the deprotonation reaction occurs now in the physiological pH range. N3 of the cytosine residue, together with the neighboring (C2)O, is also a remarkable Cd(2+) binding site, though replacement of (C2)O by (C2)S enhances the affinity towards Cd(2+) dramatically, giving in addition rise to the deprotonation of the (C4)NH(2) group. The phosphodiester bridge is only a weak binding site but the affinity increases further from the mono- to the di- and the triphosphate. The same also holds for the corresponding nucleotides. Complex stability of the pyrimidine-nucleotides is solely determined by the coordination tendency of the phosphate group(s), whereas in the case of purine-nucleotides macrochelate formation takes place by the interaction of the phosphate-coordinated Cd(2+) with N7. The extents of the formation degrees of these chelates are summarized and the effect of a non-bridging sulfur atom in a thiophosphate group (versus a normal phosphate group) is considered. Mixed ligand complexes containing a nucleotide and a further mono- or bidentate ligand are covered and it is concluded that in these species N7 is released from the coordination sphere of Cd(2+). In the case that the other ligand contains an aromatic residue (e.g., 2,2'-bipyridine or the indole ring of tryptophanate) intramolecular stack formation takes place. With buffers like Tris or Bistris mixed ligand complexes are formed. Cd(2+) coordination to dinucleotides and to dinucleoside monophosphates provides some insights regarding the interaction between Cd(2+) and nucleic acids. Cd(2+) binding to oligonucleotides follows the principles of coordination to its units. The available crystal studies reveal that N7 of purines is the prominent binding site followed by phosphate oxygens and other heteroatoms in nucleic acids. Due to its high thiophilicity, Cd(2+) is regularly used in so-called thiorescue experiments, which lead to the identification of a direct involvement of divalent metal ions in ribozyme catalysis.

The four acidity constants of threefold protonated xanthosine 5'-monophosphate, H(3)(XMP)(+), reveal that in the physiological pH range around 7.5 (X - H x MP)(3-) strongly dominates and not XMP(2-) as commonly given in textbooks and often applied in research papers. Therefore, this nucleotide, which participates in many metabolic processes, should be addressed as xanthosinate 5'-monophosphate as is stated in this critical review. Micro acidity constant schemes allow quantification of intrinsic site basicities. In 9-methylxanthine nucleobase deprotonation occurs to more than 99% at (N3)H, whereas for xanthosine it is estimated that about 30% are (N1)H deprotonated and for (X - H x MP)(3-) it is suggested that (N1)H deprotonation is further favored, especially in macrochelates where the phosphate-coordinated M(2+) interacts with N7. The formation degree of these macrochelates in the (X - H x MP x M)(-) species of Co(2+), Ni(2+), Cu(2+), Zn(2+) or Cd(2+) amounts to 90% or more. In the monoprotonated (M x X - H x MP x H)(+/-) complexes, M(2+) is located at the N7/[(C6)O] unit as the primary binding site and it forms macrochelates with the P(O)(2)(OH)(-) group to about 65% for nearly all metal ions considered (i.e., including Ba(2+), Sr(2+), Ca(2+), Mg(2+)); this indicates outer-sphere binding to P(O)(2)(OH)(-). Finally, a new method quantifying the chelate effect is applied to the M(X - H x MP)(-) species, stabilities and structures of mixed-ligand complexes are considered, and the stability constants for several M(X - H x DP)(2-) and M(X - H x TP)(3-) complexes are estimated (112 references).

The acidity constants of the (N7)H(+) sites of inosylyl(3'-->5')inosine (IpI(-)) were estimated and those of its (N1)H sites were measured by potentiometric pH titrations in aqueous solution (25 degrees C; I = 0.1 M, NaNO3). The same method was used for the determination of the stability constants of the 1:1 complexes formed between Mg(2+), Co(2+), Ni(2+), Zn(2+), or Cd(2+) (= M(2+)) and (IpI - H)(2-) and, in the case of Mg(2+), also of (IpI - 2H)(3-). The stability constants of the M(IpI)(+) complexes were estimated. The acidity constants of H(inosine)(+) and the stability constants of the M(Ino)(2+) and M(Ino - H)(+) complexes were taken from the literature. The comparison of these and related data allows the conclusion that, in the M(IpI - H) species, chelates are formed; most likely they are preferably of an N7/N7 type. For the metal ions Co(2+), Ni(2+), Zn(2+), or Cd(2+), the formation degrees of the chelates are on the order of 60-80%; no chelates could be detected for the Mg(IpI - H) complexes. It is noteworthy that the (N1)H deprotonation, which leads to the M(IpI - H) species, occurs in all M(IpI)(+) complexes in the physiological pH range of about 7.5 or even below.

Previously published stability constants of several metal ion (M2+) complexes formed with thiouridines and their 5'-monophosphates, together with recently obtained log K(M(U))(M) versus pK(U)(H) plots for M2+ complexes of uridinate derivatives (U-) allowed now a quantitative evaluation of the effect that the exchange of a (C)O by a (C)S group has on the stability of the corresponding complexes. For example, the stability of the Ni2+, Cu2+ and Cd2+ complexes of 2-thiouridinate is increased by about 1.6, 2.3, and 1.3 log units, respectively, by the indicated exchange of groups. Similar results were obtained for other thiouridinates, including 4-thiouridinate. The structure of these complexes and the types of chelates formed (involving (N3)- and (C)S) are discussed. A recently advanced method for the quantification of the chelate effect allows now also an evaluation of several complexes of thiouridinate 5'-monophosphates. In most instances the thiouracilate coordination dominates the systems, allowing only the formation of small amounts of phosphate-bound isomers. Among the complexes studied only the one formed by Cu2+ with 2-thiouridinate 5'-monophosphate leads to significant amounts of the macrochelated isomer, which means that in this case Cu2+ is able to force the nucleotide from the anti to the syn conformation, allowing thus metal ion binding to both potential sites and this results in the formation of about 58% of the macrochelated isomer. The remaining 42% are species in which Cu2+ is overwhelmingly coordinated to the thiouracilate residue; Cu2+ binding to the phosphate group occurs in this case only in trace amounts.

The acidity constants of the two-fold protonated acyclic 9-[2-(phosphonomethoxy)ethyl]-8-azaadenine, H2(9,8aPMEA)(+)(-), and its 8-isomer, 8-[2-(phosphonomethoxy)ethyl]-8-azaadenine, H2(8,8aPMEA)(+)(-), both abbreviated as H2(PA)(+)(-), as well as the stability constants of their M(H;PA)+ and M(PA) complexes with the metal ions M2+=Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ or Cd2+, have been determined by potentiometric pH titrations in aqueous solution at I=0.1 M (NaNO3) and 25 degrees C. Application of previously determined straight-line plots of log K(M)M(R-PO3) versus pK(H)H(R-PO3)for simple phosph(on)ate ligands, R-PO3(2-), where R represents a residue without an affinity for metal ions, proves that for all M(PA) complexes a larger stability is observed than is expected for a sole phosphonate coordination of the metal ion. This increased stability is attributed to the formation of five-membered chelates involving the ether oxygen present in the aliphatic residue (-CH2-O-CH2-PO3(2-)) of the ligands. The formation degrees of these chelates were calculated; they vary between about 13% for Ca(8,8aPMEA) and 71% for Cu(8,8aPMEA). The adenine residue has no influence on complex stability except in the Cu(9,8aPMEA) and Zn(9,8aPMEA) systems, where an additional stability increase attributable to the adenine residue is observed and equilibria between four different isomers exist. This means (1) an open isomer with a sole phosphonate coordination, M(PA)op, where PA(2-)=9,8aPMEA2-, (2) an isomer with a five-membered chelate involving the ether oxygen, M(PA)cl/O, (3) an isomer which contains five- and seven-membered chelates formed by coordination of the phosphonate group, the ether oxygen and the N3 site of the adenine residue, M(PA)cl/O/N3, and finally (4) a macrochelated isomer involving N7, M(PA)cl/N7. For Cu(9,8aPMEA) the formation degrees are 15, 30, 48 and 7% for Cu(PA)op, Cu(PA)cl/O, Cu(PA)cl/O/N3 and Cu(PA)cl/N7, respectively; this proves that the macrochelate involving N7 is a minority species. The situation for the Cu(PMEA) system, where PMEA2- represents the parent compound, i.e. the dianion of 9-[2-(phosphonomethoxy)ethyl]adenine, is quite similar. The relationship between the antiviral activity of acyclic nucleoside phosphonates and the structures of the various complexes is discussed and an explanation is offered why 9,8aPMEA is biologically active but 8,8aPMEA is not.

Acyclic nucleoside phosphonates (ANPs), i.e., analogues of (2'-deoxy)nucleoside 5'-monophosphates, have been studied during the past 15 years for their potential as antiviral drugs. One of these compounds, 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA; Adefovir) was recently approved in the form of its bis(pivaloyloxymethyl)ester (Adefovir dipivoxil) for use in hepatitis B therapy, a disease evoked by a DNA virus. Diphosphorylated PMEA(2-), i.e., PMEApp(4-), is initially recognized by nucleic acid polymerases as an excellent substrate, but after insertion in the growing nucleic acid chain, this is terminated due to the lack of a 3'-hydroxy group. Based on the metal ion-binding properties of PMEApp(4-) it can be explained why the ether oxygen in the aliphatic chain, R-CH(2)-O-CH(2)-PO(3)pp(4-), is compulsory for a useful biological activity. Consequently, this critical review presents an overview on the coordination chemistry of various ANPs and correlates this to their biological properties.

The occurrence of high concentrations of ATP in certain cell organelles prompted us to study the self-association of ATP via the concentration dependence of the 1H-NMR chemical shifts for H2, H8 and H1' in D2O at pD 8.4 (25 degrees C) in the range 0.0025-0.4 M in the presence and absence of poly(alpha, L-lysine), where [Lys units] was 0.4 M. The experiment in the presence of poly(Lys) was repeated at pD 12.1. At pD 8.4, the poly(amino acid) is protonated, i.e. poly(H.Lys)n+, whereas at pD 12.1 only approximately 10% of the epsilon-amino groups are positively charged. The results in all three systems are consistent with the isodesmic model of indefinite non-cooperative stacking. The stacking tendency follows the series: ATP4- (K = 1.3 M-1; pD 8.4) < ATP4-/poly(H.Lys)n+ (K = 11.5 M-1; pD 8.4) > ATP4-/90% poly(Lys)/10% poly(H.Lys)n+ (K = 3.1 M-1; pD 12.1). It is evident that poly(H.Lys)n+ assists the association of ATP by a factor of approximately 10, and it is suggested that, via its positively charged epsilon-ammonium groups, poly(H.Lys)n+ acts as a matrix by aligning ATP4- ions via ionic interactions with the negatively charged phosphate residues. The intragranular concentrations of various constituents of several storage or secretory cell organelles, as reported in the literature, are tabulated. The chromaffin granules of the adrenal medulla and the dense granules of blood platelets contain particularly high concentrations of nucleotides ([ATP] is approximately 0.14 M in the chromaffin granules and 0.5 M in the dense granules of rabbit blood platelets) and amines, such as epinephrine or 5-hydroxytryptamine. These granules, and probably also the storage vesicles of certain neurons (which seem to have a similar composition), appear, if the total concentrations of the various solutes are considered, to be osmotically unstable, which means that the intragranular solutes must be associated. This aggregation is discussed, especially with regard to the nucleotides.

First-order rate constants (50 degrees C; I = 0.1 M, NaClO4) for the dephosphorylation of UTP and TTP (1 mM) in the pH range 2-10 are compared with those of ATP and CTP; they all show the same properties indicating that the nucleic base has no influence on the rate. In the presence of Cu2+ or Zn2+ (NTP:M2+ = 1:1) this changes drastically: ATP-M2+ much greater than UTP-M2+ approximately equal to TTP-M2+ approximately equal to CTP-M2+ greater than NTP, the Cu2+ systems being always more reactive than the Zn2+ systems, and these more than the Ni2+ systems. An interaction between the nucleic base and metal ion is important for the Cu2+-ATP and Zn2+-ATP systems, but not for the pyrimidine-nucleotide systems (these behave like methyltriphosphate). Accordingly, prevention of the Cu2+-purine interaction by the addition of one equivalent of 2,2'-bipyridyl, leading to Cu(Bpy) (NTP)2-, strongly reduces the activity and all four ternary Cu2+ systems now show the same dephosphorylation rate. Addition of a second equivalent of Cu2+ to the Cu2+-UTP 1:1 system enhances the dephosphorylation rate significantly and Job's method provides evidence that a 2:1 complex is the most reactive intermediate. The relation between the initial rate, vo = d[PO3-4]/dt, and the concentration of Cu2+-UTP in 1:1 and 2:1 systems was determined. The results suggest that the reactive complex with pyrimidine nucleotides is a monomeric, dinuclear species of the type M2(NTP) (OH)- (its formation is inhibited by ligands like tryptophanate), while with M2+-ATP the reactive complex is a dimer. The connection between the indicated dephosphorylations in vitro, i.e. trans-phosphorylations to H2O, and related reactions in vivo are discussed.

The self-association of nucleosides decreases within the series adenosine greater than guanosine greater than inosine greater than cytidine approximately uridine. The same trend is observed for the corresponding nucleotides, though less pronounced, as the charge effect governs series like adenosine much greater than AMP2- greater than ADP3- approximately greater than ATP4-. Protonation of adenosine considerably reduces its self-stacking tendency: this is different with ATP4-, where a maximum is reached for H2(ATP)2- caused by additional ionic interactions in the [H2(ATP)]2(4-) dimer. Metal ion coordination may promote self-association, e.g., of ATP4- via (mainly) charge neutralization (Mg2+) and the formation of intermolecular bridges in dimeric stacks (Zn2+, Cd2+). These results allow definition of conditions with negligible self-association and thus the determination of the stability and structure of monomeric nucleotide complexes in aqueous solution, e.g., quantification of macrochelate formation in M(ATP)2- complexes. Some biological implications of the results are indicated.

By means of UV difference spectra, the acidity constants KHLH of the N-1-proton in the nucleotides inonine, guanosine, uridine, and thymidine 5`-triphosphate were measured. The same deprotonation (KcuHLH) in the respective Cu2+ complexes was also determined. The measurement were carried out with and without (Table 1) addition of NaClO4 (0.1 M). It was found that the influence of the ionic strength is great, possibly due to the formation of Na+ complexes. The difference, ΔpKA = pKHLH-pK–H ≈ 2.0, for the nucleotide systems is of the same order as the respective value 2.2 of the Cu2+ guanosine system. This indicates a strong interaction between Cu2+ and the nucleic base in the Cu2+-nucleotide complexes. From experimental indices and the analogy to the Cu2+-ATP complex, it is concluded that macrochelates are formed. The acidity constants determined by means of UV difference spectra were confirmed by potentiometric titrations (Table 2). The Cu2+-nucleoside triphosphates are known to form stable complexes with 2,2`-bipyridyl. In such ternary complexes, the nucleic base can no longer coordinate to the Cu2+. The relatively small ΔpKBipy/A values (Table 2) are in agreement with this. The hydrolytic tendencies of Cu2+ bound to the different nucleotides were also estimated (Table 2). The pKCuL(H2O)xH values show that the hydrolytic tendency of the Cu2+ complexes decreases in the order CTP > ATP > TTP > UTP > ITP ≈ GTP. The influence of the coordination number of the different ligands and also other problems concerning nucleotide complexes were discussed. [ABSTRACT FROM AUTHOR]

By means of UV difference spectra, the acidity constants KHLH of the N-1-proton in the nucleotides inonine, guanosine, uridine, and thymidine 5`-triphosphate were measured. The same deprotonation (KcuHLH) in the respective Cu2+ complexes was also determined. The measurement were carried out with and without (Table 1) addition of NaClO4 (0.1 M). It was found that the influence of the ionic strength is great, possibly due to the formation of Na+ complexes. The difference, ΔpKA = pKHLH-pK–H ≈ 2.0, for the nucleotide systems is of the same order as the respective value 2.2 of the Cu2+ guanosine system. This indicates a strong interaction between Cu2+ and the nucleic base in the Cu2+-nucleotide complexes. From experimental indices and the analogy to the Cu2+-ATP complex, it is concluded that macrochelates are formed. The acidity constants determined by means of UV difference spectra were confirmed by potentiometric titrations (Table 2). The Cu2+-nucleoside triphosphates are known to form stable complexes with 2,2`-bipyridyl. In such ternary complexes, the nucleic base can no longer coordinate to the Cu2+. The relatively small ΔpKBipy/A values (Table 2) are in agreement with this. The hydrolytic tendencies of Cu2+ bound to the different nucleotides were also estimated (Table 2). The pKCuL(H2O)xH values show that the hydrolytic tendency of the Cu2+ complexes decreases in the order CTP > ATP > TTP > UTP > ITP ≈ GTP. The influence of the coordination number of the different ligands and also other problems concerning nucleotide complexes were discussed. [ABSTRACT FROM AUTHOR]