Accurate thermodynamic properties of gas phase hydrogen bonded complexes

The increase in temperature causes a decrease in equilibrium constant, which makes accurate determination of the enthalpy of complex formation possible.

Role of Secondary Structure in Discrimination between Constitutive and Inducible Activators

ABSTRACT We have examined structural differences between the proto-oncogene c-Myb and the cyclic AMP-responsive factor CREB that underlie their constitutive or signal-dependent activation properties. Both proteins stimulate gene expression via activating regions that articulate with a shallow hydrophobic groove in the KIX domain of the coactivator CREB-binding protein (CBP). Three hydrophobic residues in c-Myb that are conserved in CREB function importantly in cellular gene activation and in complex formation with KIX. These hydrophobic residues are assembled on one face of an amphipathic helix in both proteins, and mutations that disrupt c-Myb or CREB helicity in this region block interaction of either factor with KIX. Binding of the helical c-Myb domain to KIX is accompanied by a substantial increase in entropy that compensates for the comparatively low enthalpy of complex formation. By contrast, binding of CREB to KIX entails a large entropy cost due to a random coil-to-helix transition in CREB that accompanies complex formation. These results indicate that the constitutive and inducible activation properties of c-Myb and CREB reflect secondary structural characteristics of their corresponding activating regions that influence the thermodynamics of formation of a complex with CBP.

Coordinating properties of pyrrolic and pyridinic nitrogen with metals in aqueous solution. A calorimetric study

Enthalpy of complex formation of pyrrole-2-carboxylate ligand with calcium(II), cobalt(II), nickel(II) and copper(II) cations has been determined by direct calorimetry. By means of the equilibrium constants, Gibbs function and entropy were also obtained. The measurements were carried out in aqueous medium at 25�C and an ionic strength of 1 (NaNO3). The thermodynamic quantities for pyrrole-2-carboxylate, compared with those for pyridine-2-carboxylate obtained previously, lead to the following conclusions. (A) The enthalpy factors which make the metal-pyridinic ligand complexes rather stable are absent in the case of pyrrolic ligand. The absence is probably due to the electronic structure of the pyrrole aromatic ring. (B) The enthalpy data seem to indicate, for the metal complexes of pyrazole and imidazole, a metal-pyridinic nitrogen bond rather than a metal-pyrrolic nitrogen bond.