Ca2+ complexes

The absorption maximum for the Ca2+-complexed form of KBC-002 is observed at 550 nm (Figure 24a). When Ca2+ measurements were performed at a pH of 9.0 using the optode membrane, a dynamic response range between 10 pM and 10 mM was observed for the sensors as illustrated by the calibration function shown in Figure 24b. 

An investigation of the kinetics of formation of the Li+ and Ca2+ complexes of cryptand 2.1.1 using stopped-flow calorimetry suggests that complexation occurs initially at one face of the cryptand such that the metal is only partially enclosed (to yield an exclusive complex). Then follows rearrangement of this species to yield the more stable product, containing the metal ion inside the cryptand (the inclusive product) (Liesegang, 1981). X-ray diffraction studies have indeed demonstrated that exclusive complexes are able to be isolated for systems in which the metal is too large to readily occupy the cryptand cavity (Lincoln et al., 1986). 

The kinetics of formation and dissociation of the Ca2+, Sr2+ and Ba2+ complexes of the mono- and di-benzo-substituted forms of 2.2.2, namely (214) and (285), have been studied in water (Bemtgen et al., 1984). The introduction of the benzene rings causes a progressive drop in the formation rates the dissociation rate for the Ca2+ complex remains almost constant while those for the Sr2+ and Ba2+ complexes increase. All complexes undergo first-order, proton-catalyzed dissociation with 0bs — kd + /ch[H+]. The relative degree of acid catalysis increases in the order Ba2+ < Sr2+ < Ca2+ for a given ligand. The ability of the cryptate to achieve a conformation which is accessible to proton attack appears to be inversely proportional to the size of the complexed metal cation in these cases. 

Stability Constants, Logi0Ai, foe Ca2+ Complexes with Inorganic Ligands ... 

Lo gK values for Ca2+-protein complexes do not seem to vary greatly according to source. Thus the Ca2+ complex of pigeon lysozyme is ten times more stable than that of equine lysozyme,... 

Stability constants, measured in methanol solution, for alkaline earth complexes of a number of ionophores are given in Table XVI (280,289,571-577).8 The values for the complexes of valinomycin and enniatin B lie between the values for the crown ethers 15C5 and 18C6 (cf. Section II.C.5 above), for the middle four entries the values are slightly higher. Stabilities of enniatin B complexes show a modest maximum for Ca2+, and of valinomycin complexes show stabilities increasing up to Ba2+ (281). LogAi values for the Ca2+ complexes of acetate, benzoate, and salicylate are between 4.5 and 4.7 in methanol (578) - the... 

However, the stability of the Ca2+ complex of the antibiotic squalestatin-1 is significantly greater than that of its citrate component (Bal, W. Drake, A. F. Jezowska-Bojczuk, M. Kozlowski, H. Pettit, L. D. Sadler, P. J. J. Chem. Soc. Chem. Commun. 1994, 555-556). 

Ionophores such as A-23187 and X-14885A are flexible, so despite the need for conformational change, established for A-23187 by a 1H and 13C NMR solution study (556), their complex formation reactions can take place quite quickly as they can change their conformations rapidly as required for sequential bonding to the cation, and thus proceed in a series of energetically not-too-demanding steps. Formation rate constants for the Ca2+ complexes of A-23187 and X-14885A are 6 x 105 and... 

Stability constants for Ca2+ complexes of several inorganic phosphate ligands have been given in Table V (Section II.C.l above), for Ca2+ complexes of glucose-1-phosphate and glycerol-1-phosphate (log]0 A — 2.5, 1.4 respectively) in Sections V.B.2. For... 

Table XIX contains stability constants for complexes of Ca2+ and of several other M2+ ions with a selection of phosphonate and nucleotide ligands (681,687-695). There is considerably more published information, especially on ATP (and, to a lesser extent, ADP and AMP) complexes at various pHs, ionic strengths, and temperatures (229,696,697), and on phosphonates (688) and bisphosphonates (688,698). The metal-ion binding properties of cytidine have been considered in detail in relation to stability constant determinations for its Ca2+ complex and complexes of seven other M2+ cations (232), and for ternary M21 -cytidine-amino acid and -oxalate complexes (699). Stability constant data for Ca2+ complexes of the nucleosides cytidine and uridine, the nucleoside bases adenine, cytosine, uracil, and thymine, and the 5 -monophosphates of adenosine, cytidine, thymidine, and uridine, have been listed along with values for analogous complexes of a wide range of other metal ions (700). Unfortunately comparisons are sometimes precluded by significant differences in experimental conditions.

MePO2- or PME2- (Table XIX), but the open closed equilibrium lies very much on the side of the chelated form of the complex (87% for the Ca2+ complex - compare 15% for [Ca(atp)]2 and just 7% for [Ca(amp)] (695)). The availability of stability constants both for methylphosphonate and for benzimidazole (a purine model) complexes means that the chelate effect for complexes of (1H-benzimidazol-2-yl-methyl)phosphonate can be discussed without the usual complications, such as the differences between ethane-1,2-diamine and two ammonia or two methylamine ligands and disparities between units (704). 

In solution, the potential for binding is six-coordinate for edta and four-coordinate for nta. The total coordination, however, is not well known and water molecules are also involved. [Ca(edta)]2- is often represented as an octahedral complex but it would be anticipated that Ca2+ could have a coordination number in excess of six. The denticity of edta has been found to vary from one to six, and the coordination number about the metal from four to nine.349 A review of the crystal structures of Ca2+ complexes of amino acids, peptides and related model systems surveys the coordination patterns found for Ca2+-carboxyl and Ca2+-carbonyl interactions.351... 

We have studied crystals of both the uninhibited nuclease and the nuclease-pdTp-Ca2+ complex and treated them as independent structures. The crystals of uninhibited nuclease, hereafter referred to as Type I, have unit cell parameters a = b = 47.75 A, c = 63.5 A the ternary complex (inhibited nuclease) crystals, hereafter called Type II, have unit cell parameters a=b = 48.3A, c = 63.3A the space group of both is P4i (31). We shall show views of the structure of the nuclease at three levels of resolution these structures were determined as follows ... 

Similar studies were carried out on other enniatin analogues12671, enniatin itself12681, and valinomycin12691. The Ca2+ complexes of a series of C3 symmetric trispep-tides were also modeled, and it was shown that some of these have the ability to form chiral Ca2+ complexes12701. 

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