Abstract
The thermodynamics of adsorption of four acidic proline-rich salivary proteins from their aqueous solutions onto hydroxyapatite (HA) and fluorapatite (FA) have been analyzed. Enthalpies and entropies of adsorption were derived from isotherms at 37 degrees C and 4 degrees C and led to the unexpected conclusion that adsorption of the macromolecules studied is an endothermic process. Consequently, adsorption is driven by an increase in entropy. Most of this increase originates in the adsorbate molecules, probably through disruption of internal ion pairs, hydrogen bonds, and displacement of water molecules. Integral free energies of adsorption (delta G) and the partial contributions of adsorbent and adsorbate were both calculated as a function of the adsorption coverages. For equal coverages (of each adsorbate), delta G is lower with FA than with HA. Most of the contribution to delta G originates from the adsorbate, although the adsorbent contribution becomes significant as adsorption saturation is approached. The differences in the various thermodynamic quantities are qualitatively related to the known structural features of the four proteins. The results are important in understanding the role played by the proline-rich proteins in the oral environment.
Highlights
Onto hydroxyapatite (HA) and fluorapatite (FA) have Kousvelari et al, 1980)that the proline-rich proteins (PRP) areconstituents of the been analyzed
In thermodynamic quantities are qualitatively related to addition, the 30-residueNH2-terminalfragment of PRPl was studied
Hay (1973) identified a group of anionic proline-rich proteins (PRP)' among the macromolecules selectively adsorbed by varied from 8 to 55 mg; the initial adsorbate concentration in the equilibrating systems was from 13to 100pg/ml, and the total solution volumes varied from 5 to 15 ml
Summary
Adsolption Experiments-Accurately weighed samples of the apatites were placed in tightly stoppered plastic tubes containing a given volume of 0.05 M NaCl solution. The suspensions were dispersed, by magnetic stirrers, for 20 h when a measured volume of the adsorbate solution (in 0.05 M NaCl) was added. The stoppered tubes were rotated end over end for 4 h a t 37 f 0.5 "C; this length of time was in pellicle formation in vivo or the mechanisms by which it verified to be sufficient to reach equilibrium. Hay (1973) identified a group of anionic proline-rich proteins (PRP)' among the macromolecules selectively adsorbed by varied from 8 to 55 mg; the initial adsorbate concentration in the equilibrating systems was from 13to 100pg/ml, and the total solution volumes varied from 5 to 15 ml. ' The abbreviations used are: PRP, proline-rich proteins; HA, hydroxyapatite, CaSOH(POJ3; FA, fluorapatite, Ca6F(PO4).
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