Epitaxial 2D nucleation of the stable polymorphic form of the steroid 7alphaMNa on the metastable form: implications for Ostwald's rule of stages.

This paper presents in situ observations of the epitaxial nucleation and growth of the stable polymorph of a steroid, 7alphaMna, on a specific face of the metastable form at low supersaturation, using optical microscopy and in situ Raman spectroscopy. The presence of the metastable polymorph is essential for the nucleation and growth of the stable one. The order of the metastable zones of the stable and metastable polymorphs is reversed for the epitaxial growth process as compared to the case of 3D nucleation. The rate of transformation of the metastable polymorph to the stable one can be controlled by the supersaturation.

Fluoroaluminates do not affect the guanine-nucleotide binding centre of the peptide chain elongation factor EF-Tu.

EF-Tu is often referred to as a model for guanine-nucleotide-binding regulatory proteins (G-proteins), since X-ray diffraction analysis of its GTP-binding domain shows a detailed location of the 'consensus' amino acid sequences involved in nucleotide binding. Fluoroaluminates are thought to mimick the gamma-phosphate in the GTPase centre on account of their activating effect on a variety of GTP binding proteins. In the case of EF-Tu, we could find no such effects on the basis of at least three independent functional assays. We did notice, however, complicating interactions between free nucleotides, fluoroaluminates and other ligands. By consequence, if indeed AlF4- behaves as a gamma-phosphate analogue in G-proteins, then EF-Tu must have a different GDP/GTP binding site, despite of the conserved consensus sequences.