The synthesis of the new alcohol-pendant macrocycle 4-(2-hydroxyethyl)-1,4,7,16,19,22-hexaaza-10,13,25,28-tetraoxacyclotriacontane (L2) is reported. This ligand contains two different triamine moieties, one of them bearing an ethanolic sidearm. L2 binds two Zn(II) ions in aqueous solution. The stability constants of the L2 complexes have been determined at 298.1 and 308.1 K by means of potentiometric measurements. Besides a [Zn2 L2]4+ species, a deprotonated [Zn2(L2-H)]3+ complex and a hydroxo [Zn2(L2-H)(OH)]2+ complex are formed in aqueous solution. Zn(II)-assisted deprotonation of the alcoholic group takes place at neutral pH, giving the [Zn2(L2-H)]3+ complex. In [Zn2(L2-H)]3+, the deprotonated R−O- function bridges the two metals, as shown by the crystal structure of [Zn2(L2-H)Br2]BPh4·MeOH. The hydroxo species [Zn2(L2-H)(OH)]2+ is formed at slightly alkaline pH's. This complex contains both a Zn(II)-bound alkoxide and a Zn(II)−OH nucleophilic function. Therefore, it may provide a simple model system for alkaline phosphatases, where both a deprotonated serine and a Zn−OH function are involved in phosphate ester hydrolysis. Indeed, this complex promotes the hydrolysis of the carboxy ester p-nitrophenyl acetate (NA) as well as the cleavage of phosphate ester bis(p-nitrophenyl) phosphate (BNP). The kinetics of promoted hydrolysis of NA and BNP were studied by means of UV and 1H and 31P NMR measurements. In NA hydrolysis, the R−O-−Zn(II) function acts as nucleophile in the first step of the hydrolytic mechanism, to give an acetyl derivative, which is subsequently hydrolyzed to acetate by a Zn−OH group. Similarly, in BNP cleavage, the nucleophilic attack of alkoxide on phosphorus gives a pendant-alcohol phosphorylated intermediate, which undergoes subsequent intramolecular nucleophilic attack of a Zn(II)-bound hydroxide to yield a phosphomonoester product.

The ligand [30]aneN(6)O(4) (L1) binds two Zn(II) in aqueous solution. The stability constants of the L1 complexes have been determined at 308.1 K by means of potentiometric measurements. Dinuclear monohydroxo [Zn(2)L1OH](3+) and dihydroxo [Zn(2)L1(OH)(2)](2+) complexes are formed in aqueous solution from neutral to alkaline pH. The kinetics of promoted hydrolysis of p-nitrophenyl acetate (NA) was studied. Both hydroxo species promote p-nitrophenyl acetate (NA) hydrolysis at 298.1 K with second-order kinetics. The activity of these species in NA hydrolysis is similar to that found for the mononuclear L2-Zn-OH(+) complex (L2 = [15]aneN(3)O(2)), indicating that the hydrolytic process takes place via a simple bimolecular mechanism. The hydrolysis rate of bis(p-nitrophenyl) phosphate (BNP) was measured in aqueous solution at 308.1 K in the presence of the L1and L2 zinc complexes. The hydrolysis rate of BNP is increased almost 10-fold by the dinuclear [Zn(2)L1(OH)(2)](2+) complex with respect to the mononuclear L2-Zn-OH(+) one. This result indicates a cooperative role of the two metals in the hydrolytic mechanism. A bridging coordination of the phosphate ester to the two Zn(II) ions can be suggested. The crystal structure of [Zn(2)L1(&mgr;-PP)(2)(MeOH)(2)](ClO(4))(2) (PP(-) = diphenyl phosphate) (space group P&onemacr;, a = 10.681(5) Å, b = 12.042(1) Å, c = 13.191(3) Å, alpha = 74.63(2) degrees, beta = 71.74(3) degrees, gamma = 68.41(2) degrees, V = 1476.4(8) Å(3), Z = 1, R = 0.0472, R(w)(2) = 0.1166) strongly supports this hypothesis, since in the [Zn(2)L1(&mgr;-PP)(2)(MeOH)(2)](2+) cation the diphosphate anions bridge the two metals. The dinuclear Zn(II) complexes of L1 provide a simple model system for hydrolytic dizinc enzymes.

The interactions of phosphate and pyrophosphate anions with polyammonium cations deriving from 14 polyamines (five polyazacycloalkanes, four polyazacyclophanes, and five acyclic polyamines) in aqueous solution have been studied by means of potentiometric, microcalorimetric, and NMR measurements in solution. Very stable 1:1 receptor-to-anion complexes are formed. The stability trends of such complexes are not strictly determined by electrostatic forces, hydrogen bond interactions being of considerable importance in the complex formation, the thermodynamic data being consistent with different hydrogen bonding modes. ΔH°−TΔS° compensatory relationships hold for such complexation reactions. The crystal structures of (H4 L1)(H2P2O7)2·2H2O and (H4 L2)(H2P2O7)2·6H2O (L1 = 1,4,7,10,13,16-hexaazacyclooctadecane, L2 = 1,10-dimethyl-1,4,7,10,13,16-hexaazacyclooctadecane) have been solved by X-ray analysis, evidencing different substrate/anion binding characteristics.

The CO2 uptake by single 2-amino-2-methyl-1-propanol (AMP) and its blends with 2-(ethylamino)­ethanol (EMEA) or N-methyl-2,2′-iminodiethanol (MDEA) has been investigated in both aqueous and nonaqueous solutions, and compared with aqueous 2-aminoethanol (MEA), the most used sorbent in carbon capture and storage (CCS) processes. The loading capacity, the rate of absorption, and the heat of CO2 absorption have been experimentally determined for all the amine solutions. 13C NMR analysis allowed the identification of the carbonated species formed in solution and evaluation of their relative amounts. The most promising sorbents have been further tested in continuous cycles of absorption and desorption carried out in packed columns, in order to verify their CO2 (15% in N2) capture efficiency. Thanks to their good CO2 loading, high rate of reaction with CO2, and low heat of absorption, the AMP–EMEA blend solutions, both in water and in organic diluents, are good candidates for CO2 capture as an alternative to the conventional aqueous MEA solution.

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Thermodynamic and structural aspects of manganese(II) complexes with polyaminopolycarboxylic ligands based upon 1,4,7,10tetraazacyclododecane (cyclen). Crystal structure of dimeric [MnL] 2 ؒ2CH 3 OH containing the new ligand 1,4,7,10-tetraazacyclododecane-1,4-diacetate

The hexapeptide Ser-Gly-Ala-Gly-Lys-Thr has been synthesized and characterized. It was designed as a minimal soluble peptide that would be likely to have the phosphate-binding properties observed in the P-loops of proteins that bind the β-phosphate of GTP or ATP. The β-phosphate in such proteins is bound by a combination of the side chain ε-amino group of the lysine residue plus the concavity formed by successive main chain peptide NH groups called a nest, which is favored by the glycines. The hexapeptide is shown to bind HPO(4) (2-) strongly at neutral pH. The affinities of the various ionized species of phosphate and hexapeptide are analyzed, showing that they increase with pH. It is likely the main chain NH groups of the hexapeptide bind phosphate in much the same way as the corresponding P-loop atoms bind the phosphate ligand in proteins. Most proteinaceous P-loops are situated at the N-termini of α-helices, and this observation has frequently been considered a key aspect of these binding sites. Such a hexapeptide in isolation seems unlikely to form an α-helix, an expectation in accord with the CD spectra examined; this suggests that being at the N-terminus of an α-helix is not essential for phosphate binding. An unexpected finding about the hexapeptide-HPO(4) (2-) complex is that the side chain ε-amino group of the lysine occurs in its deprotonated form, which appears to bind HPO(4) (2-) via an N···H-O-P hydrogen bond.

The synthesis and characterization of a new bis([9]aneN3) ligand (H2L) containing two [9]aneN3 macrocyclic moieties separated by a 2,2'-methylene-bis-cresol (cresol = 4-methyl-phenol) unit is reported. A potentiometric and (1)H NMR study in aqueous solution reveals that H2L is in a zwitterionic form, and protonation of the cresolate oxygens occurs only with the formation of the highly charged (H5L)(3+) and (H6L)(4+) species at acidic pH values. The coordination properties of H2L toward Cu(II), Zn(II), Cd(II), and Pb(II) were studied by means of potentiometric and UV spectrophotometric measurements. The ligand gives both mono- and binuclear complexes in aqueous solution. At acidic pH values the ligand forms stable binuclear [M2H2L](4+) complexes, where each metal is coordinated by two amine groups of [9]aneN3 and the deprotonated oxygen of the adjacent cresol unit; the remaining amine group is protonated. Deprotonation of the [M2H2L](4+) species at alkaline pH values affords [M2L](2+) complexes, where all amine groups of the [9]aneN3 moieties are involved in metal coordination. Binding of mono-, di- and triphosphate, and adenosine triphosphate (ATP) was studied by means of potentiometric, (1)H and (31)P NMR measurements and by molecular dynamics simulations. The receptor forms stable 1:1 adducts with di-, triphosphate, and ATP, while the interaction with monophosphate is too low to be detected. In the complexes both the [9]aneN3 moieties act cooperatively in the substrate binding process. The stability of the adducts increases in the order diphosphate < triphosphate < ATP. This trend is explained in terms of increasing number of charge-charge interactions between the phosphate chains and the protonated [9]aneN3 subunits and, in the case of ATP, of stacking interactions between the adenine and cresol units.