Random and sequential copolypeptides containing O-phospho-L-threonine and L-aspartic acid; roles in CaCO3 biomineralization

The present study describes the synthesis of novel polypeptides containing O-phospho-L-threonine [Thr(PO(3)H(2))] and L-aspartic acid. Random copolypeptides copoly[Thr(PO(3)H(2))(X)Asp(Y)] (X:Y = 25:75, 50:50, 75:25), were conventionally prepared by copolymerization of Thr(PO(3)Ph(2)) N-carboxyanhydride (NCA) and Asp(OBzl) NCA followed by deprotection of the phenyl and benzyl groups by catalytic hydrogenolysis over PtO(2). Polycondensation of the protected peptide p-nitrophenyl esters [Thr(PO(3)Ph(2))](Z)-Asp(OBzl)-ONp and subsequent deprotection yielded the sequential polypeptides poly[Thr(PO(3)H(2))(Z)-Asp] (Z = 1-4). By using the synthetic polypeptides, their effects on the growth of CaCO(3) crystals were examined. In the poly[Thr(PO(3)H(2))(Z)-Asp]/CaCO(3) systems, brushlike calcite and spherical vaterite were formed, with the former being found at [Ca(2+)]/[Res] ratios of > or =180, > or =140, > or =120, and > or =100 for Z = 1, 2, 3, and 4, respectively. These results indicate that an increase of Thr(PO(3)H(2)) residues in the repetitive unit induces the characteristic brushlike calcite, a fact indicating that Thr(PO(3)H(2)) residues can modify the CaCO(3) crystal morphology.     

Fibrous and helical calcite crystals induced by synthetic polypeptides containing o-phospho-L-serine and o-phospho-L-threonine

The modification of CaCO(3) crystal growth by synthetic L-Ser(PO(3)H(2)) and L-Thr(PO(3)H(2)) containing polypeptides is described. The amino acids Gly, L-Glu, L-Asp, L-Ser, L-Ala, and L-Lys induced rhombohedral calcite with a rough surface. Dipeptides, Xaa-L-Ser(PO(3)H(2)) (Xaa = Gly, L-Glu, L-Asp, L-Ser, L-Ala and L-Lys) induced vaterite crystals in the lower [Ca(2+)]. On the other hand, L-Ser(PO(3)H(2))-containing polypeptides formed spherical vaterite and fibrous calcite. The characteristic helical calcite was found in the presence of copoly[L-Ser(PO(3)H(2))(75)L-Asp(25)] or poly[L-Ser(PO(3)H(2))(3)-L-Asp]. Fibrous calcite, spherical vaterite, and helical calcite crystals were subjected to XRD and EDX analysis. XRD revealed the specific faces of these crystals. EDX spectra and surface analysis visualized the localization of the polypeptides and CaCO(3) components. Together with TEM and SAED data, we propose hypothetical growth mechanisms for the fibrous and helical calcite crystals.     

医用CVIC/C复合材料表面仿生沉积生物活性钙磷涂层

为了在医用化学气相渗透工艺(CVI)C/C复合材料表面制备生物活性钙磷涂层,用阴极声电化学工艺处理CVIC/C复合材料,再将试样浸泡于过饱和钙磷溶液中,使其诱导钙磷晶体生长.XPS研究表明,经声电工艺处理后,CVIC/C复合材料表面发生了改性;SEM,XRD和FTIR研究表明,在过饱和钙磷溶液中,未经声电化学处理的CVIC/C复合材料不具有诱导钙磷晶体生长的功能,而改性的CVIC/C表面能够诱导钙磷晶体生长,形成片状五水磷酸八钙(OCP)涂层.同时讨论了OCP沉积的形成机理.     

Surface properties of various powdered hydroxyapatites

Electrophoretic mobilities of various synthetic and semisynthetic hydroxyapatites (Ca10(PO4)6(OH)2, HAP) suspended in aqueous solutions have been measured as a function of pH and calcium concentration. The studied powders differ in particle size, crystallinity degree and surface contamination (carbonate). When equilibrated in mineral acids or bases, a large plateau of negative mobility is observed in the pH range 5-8, with increasing negative values at higher pH. Only in the case of the sample composed of nanoparticles, positive mobility obtains at pH < 8.9. When Ca2+ is added, positive mobility values are observed for all samples, and a bell-shaped profile results as a function of pH. Two possible models are explored to describe the results: the Nernstian approach, which assumes solubility equilibrium and surface potentials determined by the three potential-determining ions (Ca2+, PO3-4, and OH-), and the surface complexation approach, based on the idea of negligible phase transfer of structural phosphate. The Nernstian model is inadequate, whereas a very simple surface complexation model based on the equations Ca5(PO4)+3 = Ca4(PO4)-3 + Ca2+,Ca4(PO4)-3 + H+ = Ca4(PO4)2(PO4H),Ca5(PO4)+3 + OH- = Ca5(PO4)3(OH),coupled with a very simple electrical double layer, model suffices to reproduce the bell-shaped profile of the mobility as a function of pH in the presence of added calcium salts. The results also show that the sample composed of nanoparticles exchanges ions more easily with the solution, without reaching the solubility equilibrium in the explored timespans. In the presence of soluble phosphate salts, it is postulated that the same surface ensembles define the surface charge, with participation of phosphate as described by the equation Ca5(PO4)+3 + PO3-4 = Ca4(PO4)-3.HAP is just one member of a family of calcium phosphates with different (Ca)/(P) ratios. Electrophoretic mobilities of another member, tricalcium diphosphate, Ca3(PO4)2, were also measured and shown to be described by the same basic model. Comparison with previous literature data shows that the negative plateau in the mobility is a general feature of many HAP samples at low Ca2+, again in agreement with the surface complexation model. FTIR data demonstrates that surface phosphate indeed undergoes protonation, as postulated in the model.     

Solid-State P and H NMR Investigations of Amorphous and Crystalline Calcium Phosphates Grown Biomimetically From a Mesoporous Bioactive Glass

By exploiting (1)H and (31)P magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy, we explore the proton and orthophosphate environments in biomimetic amorphous calcium phosphate (ACP) and hydroxy-apatite (HA), as grown in vitro at the surface of a 10CaO-85SiO(2)-5P(2)O(5) mesoporous bioactive glass (MBG) in either a simulated body fluid or buffered water. Transmission electron microscopy confirmed the presence of a calcium phosphate layer comprising nanocrystalline HA. Two-dimensional (1)H-(31)P heteronuclear correlation NMR established predominantly (1)H(2)O?(31)PO(4) (3-) and O(1)H?(31)PO(4) (3-) contacts in the amorphous and crystalline component, respectively, of the MBG surface-layer; these two pairs exhibit distinctly different (1)H→(31)P cross-polarization dynamics, revealing a twice as large squared effective (1)H-(31)P dipolar coupling constant in ACP compared with HA. These respective observations are mirrored in synthetic (well-crystalline) HA, and the amorphous calcium orthophosphate (CaP) clusters that are present in the pristine MBG pore walls: besides highlighting very similar local (1)H and (31)P environments in synthetic and biomimetic HA, our findings evidence closely related NMR characteristics, and thereby similar local structures, of the CaP clusters in the pristine MBG relative to biomimetic ACP.     

胆盐与磷酸钙的相互作用

胆汁的pH条件下(pH=6～8)，应该生成无定形磷酸钙(ACP)，而在胆结石中磷酸钙通常以羟基磷灰石的形式出现.利用谱学方法研究了ACP与胆盐的作用.结果表明，胆盐以胶团的形式与ACP作用，在溶液中形成复合胶团，使其溶解度增加.不同类型胆盐与ACP的作用能力不同：脱氧胆酸钠(NaDC) > 牛磺胆酸钠(NaTC) > 胆酸钠(NaC).胆盐与ACP中结合钙的亲和能力大于结合钙的亲和能力，使ACP在胆汁的环境下容易转化为羟基磷灰石.     

A comparative study of the metastable equilibrium solubility behavior of high-crystallinity and low-crystallinity carbonated apatites using pH and solution strontium as independent variables

Using solution strontium and pH as independent variables, the metastable equilibrium solubility (MES) behavior of two carbonated apatite (CAP) samples has been examined, a high-crystallinity CAP (properties expected to be similar to dental enamel) and a low-crystallinity CAP (properties expected to be similar to bone mineral). CAP samples were prepared by precipitation/digestion: (CAP A: high-crystallinity, 1.3 wt% CO3, synthesized at 85 degrees C; CAP B: low-crystallinity, 6.4 wt% CO3, synthesized at 50 degrees C). Baseline MES distributions were determined in a series of 0.1 M acetate buffers containing only calcium and phosphate (no strontium) over a broad range of solution conditions. To assess the influence of strontium, MES profiles were determined in a similar fashion with 20, 40, 60, and 80% of the solution calcium being replaced on an equal molar basis by solution strontium. To determine the correct function governing CAP dissolution, ion activity products (IAPs) were calculated from the compositions of buffer solutions based on the hydroxyapatite template (Ca(10-n)Sr(n)(PO4)6(OH)2 (n = 0-10)) and the calcium/hydroxide deficient hydroxyapatite template (Ca(9-n)Sr(n)(HPO4)(PO4)5OH (n = 0-9)). Findings: (a) for CAP A, at high solution strontium/calcium ratios, the MES profiles were essentially superimposable when the solution IAPs were calculated using the stoichiometry of Ca6Sr4(PO4)6(OH)2 and for CAP B by a stoichiometry of Ca7Sr2(HPO4)(PO4)5OH; (b) for CAP A, at low strontium/calcium ratios, the stoichiometry yielding MES data superpositioning was found to be that of hydroxyapatite and for CAP B, that of calcium/hydroxide deficient hydroxyapatite. When other stoichiometries were assumed, good superpositioning of the data was not possible.     

Analysis and confirmation of synthetic anorexics in adulterated traditional Chinese medicines by high-performance capillary electrophoresis.

Six synthetic anorexics, clobenzorex, diethylpropion, fenfluramine, methamphetamine, phenylpropanolamine and phentermine, which can be found as adulterants in traditional Chinese medicines were assayed simultaneously by high-performance capillary electrophoresis. The electrolyte was a buffer solution containing 120 mM phosphate buffer (NaH2PO4/H3PO4, pH 2.0) and 15% acetonitrile. Applied voltage was 16 kV and temperature was 30 degrees C. Fluoren-2,7-diammonium chloride was used as an internal standard and detector set at 200 nm. The recoveries of the synthetic anorexic adulterants in traditional Chinese medicinal formula using C8-SCX mixed solid-phase extraction were studied. Several traditional Chinese medicinal powders obtained from clinics were also studied by the above HPCE method and confirmed by GC-MS. Clobenzorex, diethylpropion and fenfluramine were found and determine in these samples.     

酸性溶液中NaH2PO2在铂电极上氧化的原位红外光谱研究

用SNIFTIRS和循环伏安法研究酸性溶液中次亚磷酸钠在多晶铂电极上的电氧化机理.分析了0.5mol/LH2SO4+0.1mol/LNaH2PO2溶液中原位红外反射谱图与电极电位的关系,发现在发生反应的电位下Pt电极上的吸附物种有氢原子和H2PO2,最终的氧化产物是H3PO4而不是H2PO3-,据此提出了酸性介质中次亚磷酸根离子在Pt上氧化的新机理.     

Vapor-phase synthesis of mesoporous SiO2-P2O5 thin films

Mesoporous SiO2-P2O5 films were synthesized from the vapor phase onto a silicon substrate. First, a precursor solution of cetyltrimethylammonium bromide (C16TAB), H3PO4, ethanol, and water was deposited on a silicon substrate by a spin-coating method. Then, the C16TAB-H3PO4 composite film was treated with tetraethoxysilane (TEOS) vapor at 90-180 degrees C for 2.5 h. The H3PO4-C16TAB composite formed a hexagonal structure on the silicon substrate before vapor treatment. The TEOS molecules penetrated into the film without a phase transition. The periodic mesostructure of the SiO2-P2O5 films was retained after calcination. The calcined films showed a high proton conductivity of about 0.55 S/cm at room temperature. The molar ratio of P/Si in the SiO2-P2O5 film was as high as 0.43, a level that was not attained by a premixing sol-gel method. The high phosphate group content and the ordered periodic mesostructure contributed to the high proton conductivity.     

Derivatization of phosphopeptides with mercapto- and amino-functionalized conjugate groups by phosphate elimination and subsequent Michael addition

Kinetics of the beta-elimination of the phosphate group from H-Tyr-Ser(PO3H2)-Phe-OH and H-Tyr-Thr(PO3H2)-Phe-OH and subsequent addition of thiols and amines to the dehydroalaninyl and beta-methyldehydroalaninyl residues formed, were followed by RP HPLC under alkaline conditions in the absence and presence of Ba2+ ions. By this reaction sequence, the phosphoserinyl peptide was conjugated with mono-N-(2-mercaptoethyl)amide of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (4), a mercapto-functionalized pentapeptide, H-His-Gly-Gly-His-Gly-NH(CH2)4SH, and an amino-functionalized fluorescent dye, 5-dimethylaminonaphthalene-1-[N-(5-aminopentyl)]sulfonamide (dansyl cadaverine). The beta-methyldehydroalanine residue was, in turn, observed to be a poor Michael acceptor.     

Biological and medical significance of calcium phosphates

The inorganic part of hard tissues (bones and teeth) of mammals consists of calcium phosphate, mainly of apatitic structure. Similarly, most undesired calcifications (i.e. those appearing as a result of various diseases) of mammals also contain calcium phosphate. For example, atherosclerosis results in blood-vessel blockage caused by a solid composite of cholesterol with calcium phosphate. Dental caries result in a replacement of less soluble and hard apatite by more soluble and softer calcium hydrogenphosphates. Osteoporosis is a demineralization of bone. Therefore, from a chemical point of view, processes of normal (bone and teeth formation and growth) and pathological (atherosclerosis and dental calculus) calcifications are just an in vivo crystallization of calcium phosphate. Similarly, dental caries and osteoporosis can be considered to be in vivo dissolution of calcium phosphates. On the other hand, because of the chemical similarity with biological calcified tissues, all calcium phosphates are remarkably biocompatible. This property is widely used in medicine for biomaterials that are either entirely made of or coated with calcium phosphate. For example, self-setting bone cements made of calcium phosphates are helpful in bone repair and titanium substitutes covered with a surface layer of calcium phosphates are used for hip-joint endoprostheses and tooth substitutes, to facilitate the growth of bone and thereby raise the mechanical stability. Calcium phosphates have a great biological and medical significance and in this review we give an overview of the current knowledge in this subject.     

Constant composition dissolution of mixed phases. II. Selective dissolution of calcium phosphates

Characterization of the dissolution kinetics of individual synthetic and biological calcium phosphates is of considerable importance since these phases often coexist in biological minerals. The constant composition method has been used to study the dissolution kinetics of a series of synthetic calcium phosphates, brushite (DCPD), beta-tricalcium phosphate (TCP), octacalcium phosphate (OCP), hydroxyapatite (HAP), and carbonated apatite (CAP) in the presence and absence of citric acid, as a function of pH and thermodynamic driving force. While citric acid markedly accelerates the dissolution of TCP, HAP dissolution is significantly inhibited. Moreover, this additive has almost no influence on the dissolution of DCPD, OCP, and CAP. Dual constant composition dissolution studies of mixed calcium phosphates in the presence of citric acid have also been made. Another factor, pH, also plays an important role in the dissolution of these calcium phosphates. In suspensions of calcium phosphate mixtures, specific phases can be selectively dissolved by changing experimental parameters such as pH and the presence of rate modifiers. This result has important applications for the dissolution control of dental hard tissues such as dentin, enamel, and calculus.     

Removal of aqueous ammonium with magnesium phosphates obtained from the ammonium-elimination of magnesium ammonium phosphate

In order to recycle magnesium ammonium phosphate (MgNH4PO4.6H2O: MAP) obtained from MAP process, which is one of the attractive processes for removal of aqueous ammonium and phosphate from wastewater, ammonium elimination from MAP to magnesium phosphates and ammonium incorporation into the magnesium phosphates have been investigated in the present study. It is confirmed that magnesium hydrogen phosphate (MgHPO4) is favorably obtained from the ammonium elimination from MAP at temperatures greater than 353 K, although magnesium phosphate (Mg3(PO4)2) and magnesium pyrophosphate (Mg2P2O7) have been suggested as possible candidates. Based on the dissolution-precipitation mechanism for the removal of aqueous ammonium with magnesium phosphates, three magnesium phosphates were employed for the removal of aqueous ammonium. The order of the removal rate of the aqueous ammonium was MgHPO4>Mg3(PO4)2>Mg2P2O7, as expected from the solubility of those magnesium phosphates. The removability of the solid obtained from ammonium elimination of MAP is also confirmed. The present results show that MAP can be employed as an advanced material for the removal/recovery of ammonium, although it is generally accepted that an excess of MAP obtained from the wastewater treatment can be only used as a slow-acting fertilizer.     

Synthesis and characterization of new calcium phenylphosphonates and 4-carboxyphenylphosphonates

Three new calcium phenylphosphonates, CaC(6)H(5)PO(3).2H(2)O, Ca(3)(C(6)H(5)PO(3)H)(2)(C(6)H(5)PO(3))(2).4H(2)O, and CaC(6)H(5)PO(3).H(2)O, and two calcium 4-carboxyphenylphosphonates, Ca(HOOCC(6)H(4)PO(3)H)(2) and Ca(3)(OOCC(6)H(4)PO(3))(2).6H(2)O, were prepared. It was found that CaC(6)H(5)PO(3).2H(2)O transformed into previously known Ca(C(6)H(5)PO(3)H)(2) via Ca(3)(C(6)H(5)PO(3)H)(2)(C(6)H(5)PO(3))(2).4H(2)O in the presence of phenylphosphonic acid, and vice versa, Ca(C(6)H(5)PO(3)H)(2) turned into CaC(6)H(5)PO(3).2H(2)O in a weak basic medium. A similar relationship was found between Ca(HOOCC(6)H(4)PO(3)H)(2) and Ca(3)(OOCC(6)H(4)PO(3))(2).6H(2)O; i.e., Ca(3)(OOCC(6)H(4)PO(3))(2).6H(2)O transformed into Ca(HOOCC(6)H(4)PO(3)H)(2) in the presence of 4-carboxyphenylphosphonic acid. On the contrary, Ca(3)(OOCC(6)H(4)PO(3))(2).6H(2)O is formed from Ca(HOOCC(6)H(4)PO(3)H)(2) in the presence of ammonium as a weak base. The structure of Ca(HOOCC(6)H(4)PO(3)H)(2) was solved from X-ray powder diffraction data by an ab initio method using a FOX program. The compound is monoclinic, space group C2/c (No. 15), a = 49.218(3) A, b = 7.7609(4) A, c = 5.4452(3) A, beta = 128.119(3) degrees , and Z = 4. Its structure is one-dimensional with [Ca(2)(HOOCC(6)H(4)PO(3)H)(4)](infinity) ribbons forming basic building blocks. The ribbons are held together by hydrogen bonds between carboxylic groups.     

Vibrational spectroscopy of synthetic stercorite H(NH4)Na(PO4)·4H2O--a comparison with the natural cave mineral

In order to mimic the chemical reactions in cave systems, the analogue of the mineral stercorite H(NH(4))Na(PO(4))·4H(2)O has been synthesised. X-ray diffraction of the stercorite analogue matches the stercorite reference pattern. A comparison is made with the vibrational spectra of synthetic stercorite analogue and the natural Cave mineral. The mineral in nature is formed by the reaction of bat guano chemicals on calcite substrates. A single Raman band at 920 cm(-1) (Cave) and 922 cm(-1) (synthesised) defines the presence of hydrogen phosphate in the mineral. In the synthetic stercorite analogue, additional bands are observed and are attributed to the dihydrogen and phosphate anions. The vibrational spectra of synthetic stercorite only partly match that of the natural stercorite. It is suggested that natural stercorite is more pure than that of synthesised stercorite. Antisymmetric stretching bands are observed in the infrared spectrum at 1052, 1097, 1135 and 1173 cm(-1). Raman spectroscopy shows the stercorite mineral is based upon the hydrogen phosphate anion and not the phosphate anion. Raman and infrared bands are found and assigned to PO(4)(3-), H(2)O, OH and NH stretching vibrations. Raman spectroscopy shows the synthetic analogue is similar to the natural mineral. A mechanism for the formation of stercorite is provided.     

Surface modification of calcium hydroxyapatite with pyrophosphoric acid

The surface of synthetic colloidal calcium hydroxyapatite (Ca(10)(PO(4))(6)(OH)(2); CaHap) was treated with pyrophosphoric acid (H(4)P(2)O(7); PP) in acetone and the materials were characterized by XRD, thermal analysis, N(2) adsorption, TEM, and FTIR. No remarkable change in XRD patterns or in particle morphology by the modification was observed. The additional amount of PO(4) of CaHap was increased with an increase in PP concentration. The Ca/P molar ratio of CaHap was decreased from 1.62 to 1.35 by the modification. IR results indicated that the PP reacts with surface P-OH groups of CaHap to form additional surface P-OH groups as follows: surface P-OH+H(4)P(2)O(7)--> surface P-O-PO(OH)(2) + H(3)PO(4). When the modified CaHap with Ca/P molar ratio less than 1.50 was treated at 850 degrees C in air, the materials transformed into beta-Ca(3)(PO(4))(2).     

Sandbed Consolidation with Mineral Precipitation

A new method has been developed to prevent sand reentrainment during oil production from unconsolidated or poorly consolidated reservoir formations. Consolidation of the zformation around the well is achieved through in situ precipitation of a sparingly soluble salt, namely, calcium phosphate. Control of the depth of salt formation is achieved by alternating injection, mixing, and reaction of two aqueous solutions of calcium chloride and potassium phosphate. Calcium phosphate crystals precipitate and grow on the grain surfaces, forming sufficiently uniform coatings. The formation of relatively uniform coatings on the grains causes an acceptably small decrease of the permeability, which is a feature of primary importance for oil production. The grains are gradually "cemented" with bridges of calcium phosphate crystallites and form a consolidated and still porous structure. As a result, the rate of hydrocarbon production for the problematic reservoir can be increased considerably without undesirable reentrainment of sand. The proposed method for consolidation has been successfully tested in sandbeds. Several series of experiments have been carried out under diverse conditions to establish the optimum parameter values for the implementation of this method. A set of optimum conditions at 25 degrees C were determined and these conditions gave satisfactory consolidation with permeability loss of ca. 60% of the initial value. The conditions of precipitation were chosen so that the precipitated phase was octacalcium phosphate [Ca(4)H(PO(4))(3).2.5H(2)O], along with its byproduct hydroxyapatite [Ca(5)(PO(4))(3)OH]. Experiments were also carried out at 70 degrees C and have shown that it is feasible to consolidate loose sandpacks at oil reservoir conditions. Copyright 2000 Academic Press.     

Non-interpenetrating transition metal diorganophosphate 2-dimensional rectangular grids from their 1-dimensional wires: structural transformations under mild conditions

The manganese, cobalt, and cadmium complexes [M(dtbp)2]n (M = Mn (1) and Co 2) and [Cd(dtbp)2(H2O)]n (3) (dtbp = di-tert-butyl phosphate), which exist as one-dimensional molecular wires, transform to non-interpenetrating rectangular grids [M(dtbp)2(bpy)2.2H2O]n (M = Mn (4), Co (5), and Cd (6)) by the addition of 4,4-bipyridine (bpy) at room temperature. Products 4-6 have also been prepared by a room-temperature reaction or by solvothermal synthesis in methanol through a direct reaction between the metal acetate, di-tert-butyl phosphate, and 4,4'-bipyridine (bpy) in a 1:2:2 molar ratio. Single-crystal X-ray structure determination of 4-6 shows that these compounds are composed of octahedral transition metal ions woven into a two-dimensional grid structure with the help of bpy spacer ligands. The axial coordination sites at the metal are occupied by bulky unidentate dtbp ligands, which prevent any interpenetration of the individual grids. The change of reaction conditions from solvothermal to hydrothermal, for the attempted synthesis of a magnesium grid structure, however leads to the isolation of an organic phosphate [(H2bpy)(H2PO4)2] (7) and an inorganic phosphate [Mg(HPO4)(OH2)3] (8). Compound 7 can also be prepared quantitatively from a direct reaction between bpy and H3PO4. The new organic phosphate 7 is a unique example of a phosphate material with alternating layers of [H2bpy]2+ cations and [H2PO4]- anions that are held together by hydrogen bonds. Solid-state thermal decomposition of 4-6 produced the respective metaphosphate materials [M(PO3)2] (M = Mn (9), Co (10), and Cd (11)). All new metal-organic phosphates have been characterized by elemental analysis, thermal analysis (TGA, DTA, DSC), and IR and NMR spectroscopy. The metaphosphate ceramic materials were characterized by IR spectral and powder X-ray diffraction studies.     

Density functional calculations of ATP systems. 2. ATP hydrolysis at the active site of actin

The hydrolysis of adenosine 5'-triphosphate (ATP) at the active site of actin has been studied using density functional calculations. The active site is modeled by the triphosphate tail of ATP, an Mg cation, surrounding water molecules, and the nearby protein residues. Four reaction paths have been followed by constraining coordinates that allow phosphate stretching, nucleophilic attack of the catalytic water, and OH(-) formation via water deprotonation. The lowest-energy barrier (21.0 kcal/mol) is obtained for a dissociative reaction where the terminal phosphate breaks on approaching the catalytic water, followed by proton release via a proton wire mechanism. A higher barrier (39.6 kcal/mol) results for an associative reaction path where OH(-) is formed first, with a pentacoordinated phosphorus atom (P-O distances 2.1 A). Stretching the terminal bridging P-O bond results in bond rupture at 2.8 A with an energy barrier of 28.8 kcal/mol. The residues Gln137 and His161 are not important in the reactions, but insight into their roles in vivo has been obtained. The favored coordination of the end products H(2)PO(4)(-) and ADP(3-) includes a hydrogen bond and an O-Mg-O bridge between the phosphates as well as a hydrogen bond between H(2)PO(4)(-) and the Ser14 side chain. The total energy is 2.1 kcal/mol lower than in the initial reactants. Classical simulations of ATP- and ADP.P(i)-actin show few hydrolysis-induced differences in the protein structure, indicating that phosphate migration is necessary for a change in conformation.