Orientation distribution functions based upon both (P1), (P3) order parameters and upon the four (P1) up to (P4) values: application to an electrically poled nonlinear optical azopolymer film.

The most probable orientational distribution functions of rod-like polar molecules contained in a noncentrosymmetric uniaxial system are established using the first-rank and third-rank Legendre polynomials, (P1(cos theta)) and (P3(cos theta)) order parameters, and the maximum entropy method. Emphasis is put on the different domains of existence in the ((P1), (P3)) plane for the various shapes of the distributions: it is thus shown that, for any positive (P1(cos theta)) value and for decreasing (P3(cos theta)) values, the distribution function may exhibit either a distorted oblate form with an intense maximum at 0 degrees, or a three-leaved rose curve with maxima at 60 degrees, 180 degrees, and 300 degrees, and finally another markedly oblate shape with a strong maximum at 180 degrees. As an illustrative example, we have considered the azobenzene molecular orientations in an electrically poled p(DR1M) homopolymer thin film after a thermal process and several relaxation periods. We have made use not only of the (P1) and (P3) parameters determined from polarized second-harmonic generation (SHG) measurements, but also of the (P2) values extracted from UV-visible spectra and of the (P4) values adjusted according to the information entropy theory. In such a thin film with very large nonlinear properties (d33 coefficients were varying from 437.0 to 117.0 pm/V at 1064 nm) it is evidenced that a strong polar order is maintained even after a long relaxation period of 42 days. So, the distribution functions demonstrate that the poling treatment was quite efficient and they emphasize the importance in the determination of both couples of odd and even order parameters in such uniaxially oriented optical elements.

Hydroxyapatite Growth Inhibition Effect of Pellicle Statherin Peptides.

In our recent studies, we have shown that in vivo-acquired enamel pellicle is a sophisticated biological structure containing a significant portion of naturally occurring salivary peptides. From a functional aspect, the identification of peptides in the acquired enamel pellicle is of interest because many salivary proteins exhibit functional domains that maintain the activities of the native protein. Among the in vivo-acquired enamel pellicle peptides that have been newly identified, 5 peptides are derived from statherin. Here, we assessed the ability of these statherin pellicle peptides to inhibit hydroxyapatite crystal growth. In addition, atomistic molecular dynamics (MD) simulations were performed to better understand the underlying physical mechanisms of hydroxyapatite growth inhibition. A microplate colorimetric assay was used to quantify hydroxyapatite growth. Statherin protein, 5 statherin-derived peptides, and a peptide lacking phosphate at residues 2 and 3 were analyzed. Statherin peptide phosphorylated on residues 2 and 3 indicated a significant inhibitory effect when compared with the 5 other peptides (P < 0.05). MD simulations showed a strong affinity and fast adsorption to hydroxyapatite for phosphopeptides, whereas unphosphorylated peptides interacted weakly with the hydroxyapatite. Our data suggest that the presence of a covalently linked phosphate group (at residues 2 and 3) in statherin peptides modulates the effect of hydroxyapatite growth inhibition. This study provides a mechanism to account for the composition and function of acquired enamel pellicle statherin peptides that will contribute as a base for the development of biologically stable and functional synthetic peptides for therapeutic use against dental caries and/or periodontal disease.