Showing papers on "Chemical binding published in 1971"

Paradoxical Role of the Kinetic‐Energy Operator in the Formation of the Covalent Bond

Abstract: The origin of chemical binding is analyzed with the help of variational reasoning for the ground state of the hydrogen molecule–ion The bond‐parallel component of the kinetic‐energy integral is shown to be the critical term The effect of electron sharing on this term is such that the variation process yields, at all internuclear distances, a lower energy for the molecule than for the separated atoms This appears to be paradoxical in as much as, at the equilibrium distance, the potential part of the binding energy is negative and the kinetic part is positive The paradox is resolved by showing that the actual values of the potential and kinetic binding energy are the result of a scale variation which is similar to that found in the isolated atom and contracts the wavefunction towards the two nuclei The variational origin of this redistribution of the kinetic‐ and potential‐energy integrals, which establishes the virial relationship, is analyzed It is concluded that the variational facts are inadequately described by the view that the chemical bond is formed “because the potential energy can be lowered through accumulation of charge in the bond” Rather, the accumulation of charge in the bond is essential for chemical binding because it profoundly influences the behavior of the kinetic‐energy integral under scale variations

Molecular SCF calculations for the ground state of some three-membered ring molecules: Cis and trans diaziridine, oxaziridine and the corresponding imminium ions

Abstract: LCAO SCF MO calculations with minimal basis sets of Slater-type orbitals are performed for some three-membered heterocycles. A reinterpretation of the resulting wavefunctions in terms of localized (exclusive) orbitals is employed to show the bending of the ring bonds and to measure the degree of transferability of some groups among chemically different yet related molecules. The electrostatic potentials produced in the neighbouring space by the nuclear and electronic charge distributions are evaluated and employed to indicate the molecular sites most likely subject to electrophilic attack. Some features of the electrostatic potential may be easily related to the entities employed in the usual intuitive description of chemical binding (lone pairs, etc.).

Structure of Daunomycin; X-ray Analysis of N-Br-Acetyl-Daunomycin Solvate

Abstract: THE antibiotic daunomycin was discovered and studied by Di Marco and co-workers1–5 who found it to have cytotoxic and antimitotic activity. Extensive chemical work by Arca-mone and co-workers6–9 has established the total absolute configuration as in Fig. 1, which gives the formula of the N-Br-acetyl derivative. Daunomycin interferes with nucleic acid metabolism in both mammalian10 and bacterial11 cells and the formation of a complex between daunomycin and nucleic acids has been studied12–16. The nature of the chemical binding between antibiotics which affect ribonucleic acid synthesis and DNA has been discussed16 and it has been suggested that the amino as well as either quinone17,18 or hydroxyl groups of the chromophore are responsible for hydrogen bonding to the DNA helix. An understanding of this effect is important as the binding of daunomycin to DNA is most likely responsible for the biological activity of this antibiotic. To determine the detailed stereochemical features of daunomycin, the relative orientation of the sugar ring to the aglycone moiety and the nature of the hydrogen bonding in the solid state, we began X-ray crystallographic studies.

Mechanism of the antipeptic action of anionic carbohydrate and its clinical application for the treatment of peptic ulcer.

Abstract: The antipeptic effects of various anionic carbohydrate preparations were found to vary depending on the source and molecular weight of carbohydrate, the types and the grades of esterification, and minerals attached to anionic groups. Dextran sulfate inhibits all proteases separated from gastric mucosa. The antipeptic effects are assumed to be due to the chemical binding of anionic carbohydrate not only with pepsin, but also with substrate proteins basing on the results obtained in both in vitro and electrophoretic experiments. Immunoelectrophoretic analysis revealed that the peptic ulcer-covering ‘white coating’ contains plasma proteins. These findings are thought to indicate that anionic carbohydrate can be used for the treatment of peptic ulcer. The significance of chemical binding of anionic carbohydrate to ‘white coating’ is stressed.

The mechanism of binding of polycyclic aromatic hydrocarbons to nucleic acids: A theoretical investigation

Abstract: A kinetic model employing calculated atom and bond localization energies to approximate relative activation energies of reaction is used to analyse previously obtained experimental results for in vitro and in vivo chemical binding of polycyclic aromatic hydrocarbons to nucleic acids. It is found that in vitro linkage of hydrocarbons to DNA induced by a microsomal hydroxylating system is compatible with mechanisms involving either attack at the most reactive hydrocarbon center or attack at the most reactive hydrocarbon bond. Independent evidence leads us to favor the former mechanism. Further, the limited experimental data for in vivo linkage of hydrocarbons to DNA is found to be consistent with a model involving attack at the most reactive bond of the molecule: the “K region”. This model is supported by a close parallelism found between extent of hydrocarbons bound to DNA in vivo and the experimentally determined relative reactivities of their K regions.