Calcium phosphate solubility -- In the blind spot

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Calcium phosphate solubility -- In the blind spot
  Colloids and Surfaces B: Biointerfaces 82 (2011) 263–264 Contents lists available at ScienceDirect ColloidsandSurfacesB:Biointerfaces  journal homepage: Letter to the Editor Calcium phosphate solubility—In the blind spot Dear Editor, We refer to the paper “Hydroxyapatite surface solubility andeffectoncelladhesion,”recentlypublishedin ColloidsandSurfacesB (2010;78:177–184).Unfortunately,wehavesomedoubtsaboutitsvalue:notonlyisthesolubilityworkunreliable,butmoreseriously,while appreciating the kind words, our previous work has beenwrongly cited as making certain statements (refs. 41–44), perhapsdue to a misunderstanding.Firstly, while the solubilities of the calcium phosphates cer-tainly need to be re-evaluated, the problem is that the methodordinarily used is itself faulty. Attempts to calculate the solubil-ity of hydroxyapatite (HA) can be traced back some 80 years, butlittle agreement on or precision for the solubility product ( K  sp ) hasbeen obtained so far. Seidell [1] reported that it was known that the solubility of calcium phosphates depends on the amount of excess solid present. Then, Levinskas and Neuman [2] f ound that the dissolution of HA was incongruent, and also that the solubil-ity varied with the amount of solid present. They concluded thatHA has no solubility product constant. In fact, the problem lies inthe method itself, given the chemistry of the calcium phosphatesin general where, as we have reported [3], superficial phase trans- formations are strongly suspected in conventional solubility workusing a large excess of solid. The solution Ca/P ratio after equilib-riumhasbeenlittleconsidered,buthasoftendeviatedsubstantiallyfrom that of the solid (see Fig. 2 in [3]), violating the congruence requirement. DCPD has been reported to form below pH 7.2 dur-ing the dissolution of HA [4], but this only indicates that the solid surface may already have been contaminated with another phase;the‘solubility’therebyobtainedisthereforenotatruesolubilityforHA. Thus, any such calculated  K  sp  cannot express equilibrium withHA;thepredictionofanyphenomena invitro and invivo usingsim-plified values for  K  sp  is meaningless, as we have discussed in detailelsewhere [3].Secondly, although it was claimed that carbonate does not con-tribute significantly to the ionic strength of the solution, we havefoundthatitmarkedlyaffectsthetruemasssolubilityofHAduetosolutioncomplexation[5];inaddition,theadsorptionofcarbonate wasfoundatphysiologicalpH[6].Again,thereported K  sp  basedonsimplification is not reliable. As we have recently shown using theassumption-free solid titration method, the (congruent) solubilityisotherm of HA lies, surprisingly, substantially lower than hithertobelieved: at pH  ∼ 5, the solubility is only about one-hundredth of that previous reported [7]. In fact, the calculation of   K  sp  dependsnot only on the validity of a measured mass solubility, but also onthe assumed solution speciation, as the distribution of an analyteacrossallspeciescontrolstheactivityofeachoftherepresentativeentitiesinsertedinthesolubilityproduct.Thatistosay,allionicandneutralformsderivedfromthecomponentsofthesolidduringthedissolution process need to be taken into account. Even in a “sim-ple” solution, there may be more than a dozen equilibria involved.But for the calcium phosphates the case is rather more compli-cated due to the many known, assumed, and postulated species;attention has been drawn to this [8]. Any missing or neglected solutionspeciesandcomplexesmustresultinthefalserepresenta-tionofthesolutionequilibria.A K  sp  valuefromagrosslysimplifiedcalculation is therefore meaningless without explicit, detailed jus-tification. Using solid titration, we have re-mapped the solubilitydiagram for several calcium phosphates [3,7,9,10].Thirdly,ourworkiswronglycited.HAisthethermodynamicallystable phase above (at most) pH 4, and the formation of DCPD is akinetic issue. Thus, the statement “the formation of CaHPO 4 · 2H 2 Ois stable at pH over 4” is quite incorrect but in any case not due tous, being contrary to our results – it is clearly metastable at bestunderthoseconditions.Wehaveshownthatitispossibletoobtaina solubility isotherm for DCPD, but this arises solely because of theapparent difficulty (impossibility?) of nucleation of HA on DCPD[10].Butthenow(wrongly)assumedlowersolubilityofDCPDwithrespect to HA at low pH is not real: it is simply a case of nucleationbeing easier, again the kinetic issue. We had hoped that we hadmade this clear.The authors claimed that better biological performance wasattributable to the formation of a new phase,  i.e.  other than HA,such as CaHPO 4  or Ca(H 2 PO 4 ) 2  (which latter is particularly solu-ble [11,12]), by soaking HA in NaCl solution for 15 days. However, there was no chemical analysis of these treated solids, neitherconstitution (XRD) nor chemical composition (FTIR). The forma-tion of a second phase is, of course, possible (indeed, likely, undercertain conditions), but it is difficult to understand how a moresoluble phase can under such circumstances spontaneously formon the most stable phase: strong claims require strong evidence.This is particularly inconsistent with thermodynamic principles(reverse Ostwald succession does not occur like this – precipita-tion from a supersaturated solution is a quite different matter). Infact, since we are still missing a full statement of relevant equilib-ria, all such work is hypothetical, but still deeply flawed. Biologicalperformance can be affected by many factors without invokingphase transitions,  e.g.  crystallinity, substituted ions, particle sizeand roughness. “Apatites”, or apatitic minerals, have a remarkablestructural versatility, with variations in both lattice and morphol-ogy(crystalhabit),andsufferingreadysolid–solutionsubstitutionsbymanyforeignions.Thus,“apatite”mustbetreatedasastructuretype rather than a specific compound. Indeed, any modificationof its structure or a detail of composition may result in a shift inbiological performance, even though no phase transition occurs.Itisalsodebatablewhetherabetterresponseofpre-osteoblastsis indeed attributable to the presence of monetite or brushite. Nosurface characterization was made. In fact, it is most likely thatapatite-like material was formed on the surface, for example acalcium-deficient HA, since DMEM is supersaturated with respect 0927-7765/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2010.08.005  264  Letter to the Editor / Colloids and Surfaces B: Biointerfaces 82 (2011) 263–264 to HA. Again, there was no surface characterization where at leastsurface topography (SEM) and chemical composition (XPS) couldprovide useful information.Solubility is fundamentally important in general chemistry,geology,andthepharmaceuticalindustry,butitalsoplaysakeyroleinbiologicalprocesseswhichinvolvetheformationandresorptionof hard tissues, as well as in pathological calcifications. However,theconventionalapproachhasbeenconfirmedtobeinappropriatefor such complicated systems, primarily because of incongruentdissolution. Phase transitions were not truly considered in the cal-culationsnow.Butitisworthnotingthatreportedvaluesforthe K  sp for calcium phosphates, derived from faulty work, are still widelyused to predict many phenomena  in vitro  and  in vivo , and even if apparentlyself-consistent,theyhavebeenshownbyustobequiteunreliable. Thus, we deem it appropriate to say that it is both nec-essary and urgent for a proper appreciation and reconsideration of calcium phosphate studies, regretting that it has been necessarynow to use the paper in question to make the reminder. References [1] A. Seidell, Solubility of Inorganic and Metal Organic Compounds, 3rd ed., VanNostrand, New York, 1940.[2] G.L. Levinskas, W.F. Neuman, J. Phys. Chem. 59 (1955) 164–168.[3] H.B. Pan, B.W. Darvell, Cryst. Growth Des. 9 (2009) 639–645.[4] P.P. Mahapatra, H. Mishra, N.S. Chickerur, Thermochim. Acta 52 (1982)333–336.[5] H.B. Pan, B.W. Darvell, Cryst. Growth Des. 10 (2010) 845–850.[6] H.B. Pan, Z.Y. Li, W.M. Lam, J.C. Wong, B.W. Darvell, K.D.K. Luk, W.W. Lu, ActaBiomater. 5 (2009) 1678–1685.[7] H.B. Pan, B.W. Darvell, Arch. Oral Biol. 52 (2007) 618–624.[8] B.W. Darvell, Z.F. Chen, J. Sun, Yat-sen Univ. (Med. Sci.) 24 (2004) 1–2.[9] H.B. Pan, B.W. Darvell, Arch. Oral Biol. 54 (2009) 671–677.[10] H.B. Pan, B.W. Darvell, Caries Res. 43 (2009) 254–260.[11] V.L. Snoeyink, D. Jenkins, Aquatic Chemistry: Chemical Equilibria and Rates inNatural Waters, Wiley, New York, 1980.[12] L.C. Chow, W.E. Brown, J. Dent. Res. 52 (1973) 1158. Haobo Pan ∗ Department of Orthopaedics & Traumatology,The University of Hong Kong, R9-12, Lab Block, 21 Sassoon Rd., Hong Kong  Brian W. Darvell 1 Dental Materials Science, Department of BioclinicalSciences, Faculty of Dentistry, Health Sciences Centre,PO Box 24923, Safat 13110, Kuwait  ∗ Corresponding author. Tel.: +852 28199613;fax: +852 28185210. E-mail addresses: (H. Pan), (B.W. Darvell) 1 Tel.: +965 2498 6698; fax: +965 2498 6698.4 July 2010 Available online 12 August 2010
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