FT–IR and FT–Raman spectroscopic investigations of adsorption of 2-aminopyridine on natural and ion-exchanged bentonites

Anharmonic vibrational spectra and mode-mode couplings analysis of 2-aminopyridine

Vibrational spectra of 2-aminopyridine (2AP) have been analyzed using the vibrational self-consistence field theory (VSCF), correlated corrected vibrational self-consistence field theory (CC-VSCF) and vibrational perturbation theory (VPT2) at B3LYP/6-311G(d,p) framework. The mode-mode couplings affect the vibrational frequencies and intensities. The coupling integrals between pairs of normal modes have been obtained on the basis of quartic force field (2MR-QFF) approximation. The overtone and combination bands are also assigned in the FTIR spectrum with the help of anharmonic calculation at VPT2 method. A statistical analysis of deviations shows that estimated anharmonic frequencies are closer to the experiment over harmonic approximation. Furthermore, the anharmonic correction has also been carried out for the dimeric structure of 2AP. The fundamental vibration bands have been assigned on the basis of potential energy distribution (PED) and visual look over the animated modes. Other important molecular properties such as frontier molecular orbitals and molecular electrostatics potential mapping have also been analyzed.

Surface complexation modeling of the sorption of 2-, 3-, and 4-aminopyridine by montmorillonite

The sorption of 2-, 3-, and 4-aminopyridine on K-saturated Wyoming (SWy-K) and Texas (STx-K) and Ca-enriched Texas (STx-Ca) montmorillonite was measured at 25 degrees C with 10 mM KNO(3) or 3.3 mM Ca(NO(3))(2) as the background electrolyte. The aminopyridines adsorbed to montmorillonite at low pH, but not at high pH. Extended constant capacitance surface complexation models (ECCMs) and attenuated total reflectance-FTIR data indicate that aminopyridines sorb to the silica-like faces by cation exchange, forming outer-sphere complexes between aminopyridinium ions and permanent negatively charged surface sites (X(-)). X-ray diffraction data and sorption kinetics suggest that sorption occurs not only at external X(-) sites but also at those in the interlayer spaces. Differences in the sorption behaviors of 2-, 3-, and 4-aminopyridine result from differences in their pK(a)s. The extent of sorption of aminopyridines by the montmorillonite samples (SWy-K>STx-K>STx-Ca) results from the higher cation-exchange capacity of SWy-K, and from the fact that Ca(2+) is much more effective than K(+) in competing with protonated aminopyridines for the X(-) sites.

Adenine and RNA in mineral samples. Surface-enhanced Raman spectroscopy (SERS) for picomole detections

Studies on the interactions of biological macromolecules with mineral surfaces are crucial for the detecting biomarkers. But before this can be done for real samples like rocks or sediments, rational methods based on mineral models plus known amounts of nucleic acids must be developed. The methods must be very sensitive, as the amount of bound macromolecule may be very small. Surface-enhanced Raman spectroscopy (SERS) is perfect for detecting picomolar amounts of nucleic acid materials. In this study, the models used were adenine and GAAA hairpin for nucleic acids materials and a clay (montmorillonite) plus colloidal silver (used for SERS detection) for mineral supports. We have shown that OH(-) anions compete with adenine and the adenyl residues in the GAAA loop for adsorption onto nano-sized silver particles in basic medium. The GAAA adenyl moieties are less well adsorbed onto either clay or silver than is adenine. Also, the transfer of either adenine or the RNA hairpin from the clay to the silver aggregates is pH-dependent. Contact between adenine and the montmorillonite also seems to disperse adenine aggregates. The clay could also increase the flexibility of the RNA hairpin so that it is released from the clay at pH 10, and the affinity of its adenyl moieties for the metallic substrate is enhanced.