International Journal of Quantum ChemistryVolume 28, Issue S19 p. 707-718 Article Overview of Hückel- and resonance-theoretic approaches to π-network polymers D. J. Klein, D. J. Klein Department of Marine Sciences, Texas A&M University at Galveston, Galveston, Texas 77553, USASearch for more papers by this authorT. G. Schmalz, T. G. Schmalz Department of Marine Sciences, Texas A&M University at Galveston, Galveston, Texas 77553, USASearch for more papers by this authorW. A. Seitz, W. A. Seitz Department of Marine Sciences, Texas A&M University at Galveston, Galveston, Texas 77553, USASearch for more papers by this authorG. E. Hite, G. E. Hite Department of Marine Sciences, Texas A&M University at Galveston, Galveston, Texas 77553, USASearch for more papers by this author D. J. Klein, D. J. Klein Department of Marine Sciences, Texas A&M University at Galveston, Galveston, Texas 77553, USASearch for more papers by this authorT. G. Schmalz, T. G. Schmalz Department of Marine Sciences, Texas A&M University at Galveston, Galveston, Texas 77553, USASearch for more papers by this authorW. A. Seitz, W. A. Seitz Department of Marine Sciences, Texas A&M University at Galveston, Galveston, Texas 77553, USASearch for more papers by this authorG. E. Hite, G. E. Hite Department of Marine Sciences, Texas A&M University at Galveston, Galveston, Texas 77553, USASearch for more papers by this author First published: 18/23 March 1985 https://doi.org/10.1002/qua.560280861Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Abstract Various possible behaviors for a diversity of π-network strips are considered, including long-range-ordering, Peierls-type distortions, and solitonic excitations. Such novel features are shown to arise both in Hückel and resonance-theoretic models. Qualitative correspondences between these complementary models are given in terms of formal propositions and a conjecture. Bibliography 1a H. A. Pohl, J. Polym. Sci. C 17, 13 (1967) 10.1002/polc.5070170104 Google Scholarb A. Rembaum, J. Polym. Sci. C 29, 157 (1970). 10.1002/polc.5070290117 Google Scholarc A. Graovac, I. Gutman, M. Randić, and N. Trinajstić, Colloid and Polym. Sci. 255, 480 (1977). 10.1007/BF01536464 CASWeb of Science®Google Scholard A. A. Ovchinnokov and I. A. Misurkin, Russ. Chem. Rev. (Engl. Trans.) 46, 967 (1977). 10.1070/RC1977v046n10ABEH002185 Google Scholare C. B. Duke and H. W. 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A variational derivation of Heisenberg or valence-bond models from a Hubbard (or Pariser-Parr-Pople) Hamiltonian is described. Calculations within a low-order approximation are carried out for linear-chain and graphite lattices. The results seem to be quite accurate with little computational effort when compared with other approaches.

Many tri- and tetra-alkyl- substituted pyrylium and pyridinium salts having n-alkyl groups with more than three carbon atoms have melting points below 100 degrees C (and in some cases even below room temperature), and thus qualify as ionic liquids. Uses of ionic liquids are mentioned, and bibliographic data are provided.

Cationic lipids provide a promising alternative to the use of viruses for delivering genes therapeutically. Among the several classes of lipidic vectors, those bearing a heterocyclic cationic head have shown important advantages, such as low cytotoxicity and improved efficiency across different cell lines. We recently reported a simple and efficient strategy for obtaining pyridinium cationic lipids, starting from pyrylium salts and primary amines. The present study is aimed to compare the cellular toxicity and transfection efficiency generated by the pyridinium polar head versus the tetramethylammonium one on several tumor cell lines and also in experimental animals, delivered via intratumor injections. Thus, the lead compound 1-(2,3-dioleoyloxypropyl)-2,4,6-trimethylpyridinium lipid (2Oc), coformulated with different helper lipids in various molar ratios, was tested against its ammonium congener DOTAP-a standard transfection reagent. The results revealed that when formulated with cholesterol at 1:1 molar ratio, the pyridinium lipid 2Oc was able to transfect several cancer cell lines with similar or better efficiency than its tetraalkylammonium congener DOTAP, while producing lower cytotoxicity. The NCI-H23 lung cancer cell line was found to be the most susceptible to be transfected. Therefore, we designed an in vivo assay based on this type of carcinoma in nude mice, which were injected intratumoral with 2Oc- and DOTAP-based lipoplexes. The red fluorescent protein reporter revealed that the pyridinium cationic lipid was superior to its tetraalkylammonium congener, transfecting the tissue on a higher area and with higher efficiency. These encouraging findings, together with the simple and efficient synthetic strategy, lay the foundation for further development of pyridinium lipids for gene therapy with improved transfection efficiency in vivo and even further reduced cytotoxicity.

The success of gene therapy as a revolutionary method to treat diseases at their core level relies on the design of transfection systems that can overcome the different delivery barriers present in living organisms. Cationic lipids and related cationic gemini surfactants provide promising alternatives to the use of viruses for delivering genes therapeutically due to important advantages such as low immunogenicity and cytotoxicity, but their efficiency must be increased [M. A. Ilies, W. A. Seitz and A. T. Balaban, Curr. Pharm. Design, 2002, 8, 2441; A.J. Kirby et al. Angew. Chem. Int. Ed. 2003, 42, 1448].

Cationic lipids are a promising alternative to viral vectors for gene therapy, allowing the delivery of larger plasmids without immunogenicity, despite their lower transfection efficiency. Among them, heterocyclic systems with imidazolium or pyridinium polar head groups have definite advantages such as the excellent transfection profiles and low cytotoxicity. Our approach for synthesizing heterocyclic cationic lipids differs from those previously described because we synthesize a pyridinium ring from simple starting materials. First a pyrylium salt is formed via diacylation of alkenes. The pyrylium salt is then converted by primary amines into pyridinium salts. Appropriate choice of the primary amine allows the attachment of two hydrophobic chains yielding compounds 21A and 25A (with various chain lengths derived from palmitic, stearic and oleic acids). The same strategy allowed the preparation of lipophilic derivatives 21B, 25B useful as strongly fluorescent markers for the study of the properties of biological membranes. Preliminary tests with some of the compounds 21A and 25A, on several cell lines, showed comparable transfection efficiencies and lower cytotoxicity than those obtained with standard commercial transfection agents. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).

Using experimental data for the melting temperature T-m for four Li halides (F to I), it is demonstrated that T(m)Z(1/3) is close to constant, congruent to (2530 +/- 250) K, where Z is the atomic number of the halogen. This formula is not appropriate for what sometimes is considered the simplest halide, LiH, and therefore alternatives are investigated which involve a characteristic electrostatic energy e(2)/epsilons with s the internuclear separation and epsilon the static dielectric constant.

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.

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.

Two types of directed self-avoiding walks (SAW's), namely, three-choice directed SAW and outwardly directed SAW, have been studied on infinite percolation clusters on the square lattice in two dimensions. The walks on the percolation clusters are generated via a Monte Carlo technique. The longitudinal extension R(N) and the transverse fluctuation W(N) have been measured as a function of the number of steps N. Slight swelling is observed in the longitudinal direction on the random lattices. A crossover from shrinking to swelling of the transverse fluctuations is found at a certain length N(c) of the walks. The exponents related to the transverse fluctuations are seen to be unchanged in the random media even as the percolation threshold is reached. The scaling function form of the extensions are verified.

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 Kovats 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.

Diffusion on 2D site percolation clusters at p = 0.7, 0.8, and 0.9 above pc on the square lattice in the presence of two crossed bias fields, a local bias B and a global bias E, has been investigated. The global bias E is applied in a fixed global direction whereas the local bias B imposes a rotational constraint on the motion of the diffusing particle. The rms displacement Rt ~ tk in the presence of both biases is studied. Depending on the strength of E and B, the behavior of the random walker changes from diffusion to drift to no-drift or trapping. There is always diffusion for finite B with no global bias. A crossover from drift to no-drift at a critical global bias Ec is observed in the presence of local bias B for all disordered lattices. At the crossover, value of the rms exponent changes from k = 1 to k < 1, the drift velocity vt changes from constant in time t to decreasing power law nature, and the "relaxation" time τ has a maximum rate of change with respect to the global bias E. The value of critical bias Ec depends on the disorder p as well as on the strength of local bias B. Phase diagrams for diffusion, drift, and no-drift are obtained as a function of bias fields E and B for these systems.

The possibility is considered of a special class of fullerenes which are not spheres but disks, similar in shape to healthy red-blood cells (or erythrocytes). Such a fullerene would consist of two large, nearly parallel, hexagonal ways of benzenoid rings, each array connected by 12 pentagons (and typically some supplementary benzenoid rings). Different families of such fullerenes are presented, sometimes via more than one construction. They can be considered to approximate graphite without dangling bonds, especially if the central parts of the hexagonal arrays consisting only of hexagonal rings flatten to yield intramolecular Van der Waals attractive interactions between the two parallel faces of the erythrocyte-like pillow-shaped fullerene.

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