William A. Seitz

C60 carbon cages

Abstract Arrangements of carbon atoms in cage-like structures with no dangling bonds are considered as possible novel allotropic forms of carbon. Five different C60 cages, having certain favorable structural characteristics, are identified. Quantitative resonance-theoretic calculations are made and compared to simpe Huckel results. The favored structure is found to be the so-called Buckminsterfullerene structure.

Cellular responses to extreme water loss: The water-replacement hypothesis☆

The previously advanced hypothesis that desiccation resistance involves the replacement of water adjacent to intracellular surfaces with polyhydroxy compounds has been supported by experiments on cysts of the brine shrimp, Artemia, and in a model system of albumin--glycerol--water, using nuclear magnetic resonance spectroscopy, microwave dielectrics, and density measurements. We have also considered other problems that cells face when large fractions of their total water content are removed. Observations by other investigators have indicated that a variety of mammalian cells can lose roughly 50% of their water and survive; for a given cell type death occurs if its volume is reduced below a certain minimum level. Membrane damage has previously been suggested to be a major cause of dehydration damage. We have proposed some additional plausible mechanisms that might also be involved.

Wiener Index Extension by Counting Even/Odd Graph Distances

Chemical structures of organic compounds are characterized numerically by a variety of structural descriptors, one of the earliest and most widely used being the Wiener index W, derived from the interatomic distances in a molecular graph. Extensive use of such structural descriptors or topological indices has been made in drug design, screening of chemical databases, and similarity and diversity assessment. A new set of topological indices is introduced representing a partitioning of the Wiener index based on counts of even and odd molecular graph distances. These new indices are further generalized by weighting exponents which can be optimized during the quantitative structure-activity/-property relationship (QSAR/QSPR) modeling process. These novel topological indices are tested in QSPR models for the boiling temperature, molar heat capacity, standard Gibbs energy of formation, vaporization enthalpy, refractive index, and density of alkanes. In many cases, the even/odd distance indices proposed here give notably improved correlations.

Cationic Lipids in Gene Delivery: Principles, Vector Design and Therapeutical Applications

Gene therapy will change medicine by treating the diseases at their core levels revolutionizing the way to deliver functional proteins. The development of this technology relies in designing optimal systems for DNA transfer and expression (transfection), cationic lipids being a promising alternative. Being safer than viral vectors, they also allow the delivery of larger plasmids and can be easily GMP-manufactured and stored. The main problem associated with the use of these vectors is their transfection efficiency, which is still inferior to viral methods. In this paper we present an overview of the correlations between the chemical structure and biological activity for the principal classes of cationic lipids. Key issues in the design of this class of transfection agents are presented, as well as the future trends.

The poly-polyphenanthrene family of multi-phase π-network polymers in a valence-bond picture☆

Abstract A family of long polymer strips are argued to exhibit distinct phases each with a different long-range ordering in electronic structure. Degenerate ground-state phases necessarily occur (due to symmetry) for odd width strips, and may also occur for even width strips. A new type of solitonic excitation is indicated; it may be viewed as consisting of a group of bound solitons that cannot mutually annihilate (i.e. there are no antisolitons in the group). Valence-bond and band-theoretic molecular-orbital views are compared.

Dimer coverings and Kekulé structures on honeycomb lattice strips

The problem of covering every site of a subsection of the honeycomb lattice with disjoint edges is considered. It is pointed out that a type of long-range order associated to such coverings can occur, so that different phases can arise as a consequence of the subsections boundaries. These features are quantitatively investigated via a new analytic solution for a class of strips of arbitrary widths, arbitrary lengths, and arbitrary long-range-order values. Relations to work on the dimer covering problem of statistical mechanics and especially to the resonance theory of benzenoid hydrocarbons are noted.

Excluded volume effects for branched polymers: Monte Carlo results

As a simple model of randomly branched polymers, we consider the different ways that N‐bond trees may be self‐avoidingly embedded on a lattice. A general Monte Carlo approach is developed and applied. The mean square radii of gyration of the conformations with a given N are found to vary with N as Nν, where ν?1.23 and ν?0.92 in two and three dimensions. The critical importance of volume exclusion for branched structures is discussed.

Quantitative Structure-Retention Relationships for Gas Chromatographic Retention Indices of Alkylbenzenes with Molecular Graph Descriptors

Abstract Quantitative structure-retention relationships (QSRR) represent statistical models that quantify the connection between the molecular structure and the chromatographic retention indices of organic compounds, allowing the prediction of retention indices of novel, not yet synthesized compounds, solely from their structural descriptors. Using multiple linear regression, QSRR models for the gas chromatographic Kovàts retention indices of 129 alkylbenzenes are generated using molecular graph descriptors. The correlational ability of structural descriptors computed from 10 molecular matrices is investigated, showing that the novel reciprocal matrices give numerical indices with improved correlational ability. A QSRR equation with 5 graph descriptors gives the best calibration and prediction results, demonstrating the usefulness of the molecular graph descriptors in modeling chromatographic retention parameters. The sequential orthogonalization of descriptors suggests simpler QSRR models by eliminating redundant structural information.

Wavefunction comparisons for the valence-bond model for conjugated π-networks

Approximate ground-state wavefunctions for valence-bond (or Heisenberg) models are obtained both within Néel-state-based and within Kekulé-state-based resonance-theoretic approaches. Comparisons are made between these and other general approaches, with particular emphasis on organic π-network systems. Attention is drawn to the manner in which the quality of the different approximation schemes changes with variations in structural characteristics of the system. It is suggested that resonance-theoretic ideas are most appropriate for (aromatic benzenoid) systems with low coordination number, whereas Néel-state based ideas are most appropriate for (3-dimensional) structures with higher coordination number (and little “frustration”).

Extended ?-networks with multiple spin-pairing phases: resonance-theory calculations on poly-polyphenanthrenes

The poly-polyphenanthrene family of extended π-network strips with members ranging from polyacetylene to graphite is considered in terms of the locally correlated valence-bond or Heisenberg Hamiltonian. Resonance theory wavefunctions which provide a variational upper bound to the ground state energy are developed in a graph-theoretic formalism extendable to more general localized wavefunction cluster expansions. The graph-theoretic formalism facilitates the use of general transfer matrix techniques, which are especially powerful in application to quasi-one-dimensional systems such as are illustratively treated here. It is argued that these strips exhibit states of different long-range spin-pairing orderings. Novel properties associated with these different resulting phases are briefly indicated, including the possibilities of solitonic excitations and the reactivity at the ends of the strips. The qualitative arguments are supported by numerical calculations for strips up to width 8.