Counterion effect on sulfonatocalix[n]arene recognition

Sulfonatocalixarenes, like other ionic receptors, possess counterions that can affect the molecular recognition process. In the present review it is shown that the competitive effect of the alkaline cations frequently used as counterions determines not only the magnitude of the external guest association constant, but also the stoichiometry of the complexes. Experimental evidences are shown about the interaction of the counterions with sulfonatocalixarene, allowing to quantify its association equilibrium constants. The counterions recognition will be a competitive process that must be taken into account when investigating the interaction of calixarenes with an external guests. When the external guest is a neutral molecule it will be possible to form ternary complexes where the counterion shows a competitive and cooperative effect. By increasing the size of the receptor, sulfonatocalix[6] and sulfonatocalix[8]arene, the complexity of the system is increased due to the formation of counterion complexes with stoichiometries 1:1 and 1:2. In the presence of an external guest, the formation of heteroternary complexes with 1:1:1 stoichiometries including a counterion and an organic cation will be possible.

The C Terminus of the Ribosomal-Associated Protein LrtA is an Intrinsically Disordered Oligomer

The 191-residue-long LrtA protein of Synechocystis sp. PCC 6803 is involved in post-stress survival and in stabilizing 70S ribosomal particles. It belongs to the hibernating promoting factor (HPF) family, intervening in protein synthesis. The protein consists of two domains: The N-terminal region (N-LrtA, residues 1101), which is common to all the members of the HPF, and seems to be well-folded; and the C-terminal region (C-LrtA, residues 102-191), which is hypothesized to be disordered. In this work, we studied the conformational preferences of isolated C-LrtA in solution. The protein was disordered, as shown by computational modelling, 1D-1H NMR, steady-state far- UV circular dichroism (CD) and chemical and thermal denaturations followed by fluorescence and far-UV CD. Moreover, at physiological conditions, as indicated by several biochemical and hydrodynamic techniques, isolated C-LrtA intervened in a self-association equilibrium, involving several oligomerization reactions. Thus, C-LrtA was an oligomeric disordered protein.