Hemoglobin is a tetrameric oxygen transport protein in animal bodies. However, there is a paucity of information regarding differences between alpha and beta subunits of hemoglobin in terms of oxygen affinity. The sequential model of Koshland, Nemthy and Filmer (KNF model) has attributed similar affinities to both alpha and beta subunits. The main purpose of the present study is to construct a new sequential model for hemoglobin oxygenation based on higher oxygen affinities for alpha subunits. To this end, coordinate files of 19 oxy and 41 deoxy hemoglobin structures were used as starting structures. These files were processed using Microsoft Excel and SPSS software in order to calculate Euclidean distances between each pair of proximal and distal histidine Fe2+ as well as other pairs of atoms of interest. The calculated distances were then compared for either set of hemoglobin conformations, i.e. oxy and deoxy conformations. Our results showed that a2 subunit show higher structural changes that could be related to oxygen affinity. This subunit could be introduced as initiator of hemoglobin oxygenation and COOPERATIVITY. Subunit a2 in our sequential model induces relaxed conformation in a1, b2 and b1 respectively. The order of oxygen affinity in our model is as follow: a2>a1>b1>b2.

A COOPERATIVITY is observed between CH/p and hydrogen bond interactions in HF···Py^X1benX2 complexes. The COOPERATIVITY is more influenced by the inductive effects in comparison with the resonance effects. Increase in the electron withdrawing nature of substituents is accompanied by decrease in the COOPERATIVITY energy (Ecoop). The vital effect of the reactivity centre on the Ecoop is determined through the good relationship between spara+ (the substituted constant estimated for the para placed reactivity center) and Ecoop values.

The rate of an enzymatic reaction may be changed by a moderator. Usually, the effect is to reduce the rate, and this is called inhibition. Sometimes the rate of enzyme reaction is raised, and this is called activation. Not only enzyme activation is subject of a less detailed presentation, but also enzyme inhibition and activation are very often discussed independently in enzymology. I attempt to introduce a general model of enzyme inhibition and activation to allow one to interpret inhibition and activation from a mechanistic or physical perspective using the significance of COOPERATIVITY as a new approach. The magnitude of interaction between substrate and inhibitor binding sites is given by the a parameter and the magnitude of increasing catalytic reaction constant is given by the b parameter, which both parameter values characterize the type of inhibition and activation. The moderation of mushroom tyrosinse is described by application of the model as a typical.

In this study, the role of interaction of p-electrons on the strength of simultaneous s-hole interactions (pnicogen, chalcogen and halogen bonds) is investigated using the quantum chemical calculations. X-ben||TAZ∙∙∙Y1, Y2, Y3 complexes (X=CN, F, Cl, Br, CH3, OH and NH2, TAZ=s-triazine and Y1, Y2 and Y3 denote PH2F, HSF and ClF molecules) is introduced as a model. The results show that interaction of p-electrons of X-ben and TAZ rings in X-ben||TAZ∙∙∙Y1, Y2, Y3 complexes are effective in enhancing the strength of simultaneous σ-hole interactions than that in the TAZ∙∙∙Y1, Y2, Y3 complexes. We show that the effect of the substituents on the studied complexes strongly depends on the nature of the substituents on the X-ben ring. The electron-donor and electron-acceptor substituents increase and decrease the stability of complexes, respectively. The electronic properties of the complexes have been analyzed using molecular electrostatic potential (MEP), and the parameters were derived from the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) methodologies.

In this paper, ab initio calculations were performed on the ternary complex formed by HB (CO)2, XCN (X=Cl, Br) and YF (Y=Li, H, Cl). In these complexes boron acts as a non-classical electron donor to form an unconventional halogen bond. The cooperative effect between the B···X halogen bond and lithium/hydrogen/halogen bond was investigated. The calculated results show that the B···X and N···Y interactions in the termolecular complexes are stronger compared with those in their corresponding bimolecular complexes. The COOPERATIVITY energies of these termolecular complexes span a range from -1.06 to -2.75 kJ mol-1 and -1.63 to -4.34 kJ mol-1 for X=Cl and Br, respectively, indicating the presence of the COOPERATIVITY effect. The nature of interactions is analyzed in terms of parameters derived from molecular electrostatic potential (MEP), natural bond orbital (NBO) and atoms in molecules (AIM) analyses. The amount of charge transfer in the termolecular complexes is stronger compared with those in their corresponding bimolecular complexes. The obtained results from AIM analyses demonstrate that B···X and N···Y bonds in the termolecular complexes are amplified compared to the bimolecular complexes.