Science & Mathematics

Enthalpy, entropy, and free energy values for the addition of a series of substituted pyridine ligands to a number of porphyriniron(II) dimethyl esters in benzene solution were measured spectrophotometrically. Variations in free energy of reaction with the changes in ligand and metalloporphyrin were not great but changes in enthalpy and entropy of reaction were large. The results obtained best fit a model in which the ligand is bound by both σ bonding from the pyridine nitrogen to the metal atom and π bonding by back-donation from the iron atom to the pyridine. While π bonding becomes more important in the order 4-methyl < 4-vinyl < pyridine < 4-carboxybutyl ester < 4-cyanopyridine, σ bonding is most important for 4-methylpyridine and decreases in importance in the same order, π bonding was not as important for 2,4-diacetyldeuteroporphyriniron(II) dimethyl ester as it was for the meso-, deutero-, and protoporphyrin complexes. Entropies of reaction were large and variable and were well correlated with the enthalpy changes observed. This and the compensatory nature of the entropy change in relation to the enthalpy change gave an isoequilibrium temperature of 340 °K. The close proximity of this to the temperature at which the reactions were studied led to the small changes observed in the free energy of reaction.

Free energy, enthalpy, and entropy changes for the addition of substituted pyridines to various nickel and zinc porphyrins have been determined in chloroform and benzene solutions. The stoichiometry of the reactions indicates formation of mooo pyridinates with some nickel porphyrins and bis pyridinates with others, the latter requiring very high ligand concentrations. In contratt to the situation in iron porphyrin pyridinates, the ir-acceptor ability of the ligands has little effect on the enthalpy of reaction of the nickel and zinc complexes. The dependence of the enthalpy of reaction on ligand basicity is strong for nickel porphyrin pyridinates in chloroform and for the corresponding zinc complexes in benzene but is less noticeable when these solvents are reversed. Specific solute-solvent interactions are suggested to explain this behavior. It is suggested that in nickel porphyrins hydrogen bonding between chloroform and the solvent-accessible side of the five-coordinate nickel porphyrins is very sensitive to changes in the metal-ligand bond strength on the opposite side of the porphyrin plane. In agreement with the expectation that zinc compounds will moee readily accommodate the forced square-pyramidal structuee than nickel, the changes attributed to hydrogen-bond effects in chloroform weee found to be smaller in the zinc than the nickel compounds studied. In this respect the zinc porphyrins moee closely resemble the iron(II) porphyrins which have been studied previously than do the nickel porphyrins and interactions between the aromatic porphyrin plane and benzene are thought to be moee important. to illustrate this complexity. We would agree with this referee's comment but would point out that the supposition that hydrogen bonding changes are reflected by the differences in heats of mixing seems to u.s a reasonable interpretation for the purposes of this discussion.

The echidna, Tachyglossus aculeatus, has two haemoglobins and polymorphic variants of both have been described. Haemoglobins Hb-IA, Hb-IB and Hb-IIA have been separated and their oxygen binding parameters studied. Hb-IIA has a markedly higher oxygen affinity and lower cooperativity than the polymorphs ofhaemoglbin I. In phosphate buffer, pH 7·1 and 25°C, Hb-IIA has an oxygen half-saturation pressure (Pso) of 1·3 kPa and a Hill coefficient of 2·4, whereas the polymorphic forms of Hb-I have P so values of about 1·6 kPa and Hill coefficients of about 2·9. Differences in diphosphoglyceric acid interactions account for some of these differences. When the polymorphic forms of haemoglobin I were compared at pH 7 ·1, Hb-IA had a AHo value of -27·9 kJjmol and a Hill coefficient of 2·9, whereas Hb-IB had a AHo value of -31·7 kJjmol and a Hill coefficient of 3·0.

Analytical formulas are derived for integrals of the orbit‐orbit operator in the Breit‐Pauli Hamiltonian. The present method differs from an earlier one in the introduction of a new auxiliary function Gn(a) which is an integral of Shavitt's Fm(t) function. A method for its evaluation is discussed and some numerical examples are given. Copyright © 1975 John Wiley & Sons, Inc.

There has been much discussion in recent years of the bonding in metalloporphyrins generally and of the bonding of oxygen in oxyhaemoglobin and oxymyoglobin in particular. In this article we wish to outline some conclusions reached from recent physico-chemical studies, supported by molecular orbital calculations.

Previous Hückel calculations applied to metal trisacetylacetonates are shown to be in error and are repeated and extended in the light of recent ionisation potential data, indicating that the approach is quantitatively inadequate for the present system.

The study of free energy, enthalpy, and entropy changes accompanying the addition of substituted pyridines to iron(II) porphyrins has been extended to follow the effect of solvent change on these thermodynamic functions. Free energy changes are an unreliable index of strength of binding, but very large solvent effects are observed for enthalpy and entropy changes. Enthalpy effects were most favorable and entropy effects least favorable in carbon tetrachloride with decreasing magnitudes in chloroform and benzene. Statistical treatment of the results (62 data pairs) suggests that the contributions to these thermodynamic functions from different solvents, ligands, and metalloporphyrins operate independently and are additive. A linear model is proposed which simulates these effects. The thermodynamic data may be rationalized in terms of a relaxation in the binding of solvent molecules by heme when ligands are attached. The types of heme-solvent interaction occurring in the three solvents are different and solva.tion energies appear to decrease in the order benzene < chloroform < carbon tetrachloride. The associated large entropy changes follow the same order.

The effect of proximity of cues was studied with 2 pairs of visual stimuli in transfer from spatial to non-spatial discrimination learning tasks, with 225 Ss. The prediction that proximity promotes positive transfer was confirmed with both successive and simultaneous viewing of stimuli, with temporal and spatial proximity, and with reduced viewing time. The hypothesis that the spatial proximity effect is mediated by background stimuli was confirmed in the further finding that variation in background gives the same result as spatial non-proximity. The temporal proximity effect was ascribed to temporal S-R contiguity as a variable. © 1966 American Psychological Association.

Basis set expansion in molecules containing second-row atoms by inclusion of s and p-type gaussian functions centred on the bond axes is shown to be efficient and is compared with the addition of d-orbitals to the basis set, in the test molecule S02. Calculations on S02 and other molecules at MBS and double-zeta level have been carried out with wave-function polarization introduced by bond functions at various locations in the molecule. Some simple rules for basis set enhancement by this method are presented. © 1976 Taylor & Francis Group, LLC.