Heterogeneous Structure, Heterogeneous Dynamics, and Complex Behavior in Two-Dimensional Liquids.

Analysis of the metrical and topological features of the local structure in a freezing two-dimensional Lennard-Jones system found that in a narrow strip [Formula: see text] of thermodynamic states close to the melting line, the liquid becomes a complex liquid characterized by a super-Arrhenius increase of relaxation times, stretched-exponential decay of correlations in time, and a power-law distribution of waiting times for changes in the local order. In [Formula: see text], the structure of the liquid and its dynamics are spatially heterogeneous; the sizes of ordered clusters are power-law distributed. Those features are governed by local structure evolution between solid-like and liquid-like (disordered) patterns. The liquid inside the strip [Formula: see text] gives a unique opportunity to study how heterogeneous structure, dynamics and complexity are intertwined with each other on a microscopic level.

A stochastic surrogate Hamiltonian approach of coherent and incoherent exciton transport in the Fenna-Matthews-Olson complex.

The capture and transduction of energy in biological systems is clearly necessary for life, and nature has evolved remarkable macromolecular entities to serve these purposes. The Fenna-Matthews-Olson (FMO) complex serves as an intermediate to transfer the energy from the chlorosome to the special pairs of different photo systems. Recent observations have both suggested the importance of coherent exciton transport within the FMO and motivated an elegant and appropriate theoretical construct for interpreting these observations. Here we employ a different approach to exciton transport in a relaxing environment, one based on the stochastic surrogate Hamiltonian method. With it, we calculate the quantum trajectories through the FMO complex both for the model involving seven bacteriochlorophylls that has been used before, and for one involving an eighth bacteriochlorophyll, which has been observed in some new and very important structural work. We find that in both systems, efficient energy transfer to the ultimate receptor occurs, but that because of the placement of, and energy relaxation among, the different bacteriochlorophyll subunits in the FMO complex, the importance of coherent oscillation that was discussed extensively for the seven site system is far less striking for the eight site system, effectively because of the weak mixing between the initial site and the remainder of the system. We suggest that the relevant spectral densities can be determinative for the energy transport route and may provide a new way to enhance energy transfer in artificial devices.

A time-dependent approach to electronic transmission in model molecular junctions.

We present a simple method to compute the transmission coefficient of a quantum system embedded between two conducting electrodes. Starting from the solution of the time-dependent Schrodinger equation, we demonstrate the relationship between the temporal evolution of the state vector, |ψ(t)>, initially localized on one electrode and the electronic transmission coefficient, T(E). We particularly emphasize the role of the oscillation frequency and the decay rate of |ψ(t)> in the line shape of T(E). This method is applied to the well-known problems of the single impurity, two-site systems and the benzene ring, where it agrees with well-accepted time-independent methods and gives new physical insight to the resonance and interference patterns widely observed in molecular junctions.

Atomic-scale roughness effect on capillary force in atomic force microscopy.

We study the capillary force in atomic force microscopy by using Monte Carlo simulations. Adopting a lattice gas model for water, we simulated water menisci that form between a rough silicon-nitride tip and a mica surface. Unlike its macroscopic counterpart, the water meniscus at the nanoscale gives rise to a capillary force that responds sensitively to the tip roughness. With only a slight change in tip shape, the pull-off force significantly changes its qualitative variation with humidity.

Pathways for photoinduced charge separation in DNA hairpins.

By means of correlated quantum-chemical calculations, we explore the chain-length dependence of the electronic coupling for photoinduced charge separation in DNA hairpins associated to conjugated linkers. Pathways for charge transfer from the linker chromophore to a guanine site located at a well-defined distance along the DNA strand are identified. Importantly, these involve not only the frontier molecular orbitals of the interacting donor, bridge, and acceptor units, but also deeper lying orbitals possessing both the appropriate energy and the symmetry to overlap significantly. The relative efficiency of these channels is found to be sensitive to the chemical structure of the linker, leading to falloff parameters for the charge-transfer rates ranging from approximately 0.4 to approximately 1.2 A(-1).

Anomalous surface diffusion in nanoscale direct deposition processes.

We report the first observation of anomalous diffusion in nanometer scale direct deposition processes utilizing dip-pen nanolithography (DPN). DPN permits quite general nanostructure patterns to be drawn on flat surfaces. Here we demonstrate experimentally, and discuss theoretically, the situation in which the molecular ink in DPN binds weakly to the surface. We observe, for the weak-binding case of 1-dodecylamine on mica, that anomalous diffusion occurs, leading to nearly fractal deposition patterns.

Understanding charge transport in molecular electronics.

For molecular electronics to become a viable technology the factors that control charge transport across a metal-molecule-metal junction need to be elucidated. We use an experimentally simple crossed-wire tunnel junction to interrogate how factors such as metal-molecule coupling, molecular structure, and the choice of metal electrode influence the current-voltage characteristics of a molecular junction.

Photoinduced electron-transfer processes along molecular wires based on phenylenevinylene oligomers: a quantum-chemical insight.

Quantum-chemical techniques are applied to model the mechanisms of photoinduced charge transfer from a pi-electron donating group (tetracene, D) to a pi-electron-acceptor moiety (pyromellitimide, A) separated by a bridge of increasing size (p-phenylenevinylene oligomers, B). Correlated Hartree-Fock semiempirical approaches are exploited to calculate the four main parameters controlling the transfer rate (k(RP)) in the framework of Marcus-Jortner-Levich's formalism: (i) the electronic coupling between the initial and final states; (ii) and (iii) the internal and external reorganization energy terms; and (iv) the variation of the free Gibbs energy. The charge transfer is shown to proceed in these compounds through two competing mechanisms, coherent (superexchange) versus incoherent (bridge-mediated) pathways. While superexchange is the dominant mechanism for short bridges, incoherent transfer through hopping along the phenylene vinylene segment takes over in longer chains (for ca. three phenylenevinylene repeat units). The influence of the chemical structure of the pi-conjugated phenylenevinylene bridge on the electronic properties and the rate of charge transfer is also investigated.

Model for a random laser.

The laser action in random media is studied numerically for a planar system of resonant scatterers pumped by an external laser. The eigenmodes of the finite system (quasistates) are "lossy" in the absence of gain because of the leakage of light outside the medium and can be characterized by their decay rates. Lasing occurs when the gain compensates the decay rate of the quasistate with the longest lifetime. The dependence of the lasing threshold on the number of scatterers (size of the system) is found to be I varies with 1/square root of (N), which agrees with recent experiments. We demonstrate that this dependence is strongly related to the fluctuations of quasistate decay rates and discuss the nature of these fluctuations.

Conformational gating of long distance electron transfer through wire-like bridges in donor-bridge-acceptor molecules.

A series of five donor-bridge-acceptor (DBA) molecules in which the donor is tetracene, the acceptor is pyromellitimide, and the bridge molecules are oligo-p-phenylenevinylenes (OPV) of increasing length has been shown to undergo electron transfer (ET) by means of two mechanisms. When the bridge is short, strongly distance dependent superexchange dynamics dominates, whereas when the bridge is longer, bridge-assisted hopping dynamics prevails. The latter mechanism results in relatively soft distance dependence for ET in which the OPV oligomers act effectively as molecular wires. We now report studies on the critical influence that bridge dynamics have on electron transfer through these oligomers. The temperature dependence of the charge separation (CS) rates in all five molecules does not appear to obey the predictions of standard ET theories based upon the Condon approximation. All five molecules show behavior consistent with CS being "gated" by torsional motion between the tetracene donor and the first bridge phenyl ring. This is based on the near equivalence of the CS activation energies measured for all five molecules with the frequency of a known vibrational mode in 5-phenyltetracene. In the molecule containing a trans-stilbene bridge, a competition occurs between the tetracene-phenyl torsional motion and one that occurs between the vinyl group and the phenyls linked to it. This results in complex temperature-dependent CS that exhibits both activated and negatively activated regimes. The charge recombination (CR) reactions within the molecules which have the two shortest bridges, namely phenyl and trans-stilbene, show a weaker dependence on these molecular motions. The three molecules with the longest bridges all display complex temperature dependencies in both their rates of CS and CR, most likely because of the complex torsional motions, which arise from the multiple phenyl-vinyl linkages. The data show that long-distance electron transfer and therefore wire-like behavior within conjugated bridge molecules depend critically on these low-frequency torsional motions. Molecular device designs that utilize such bridges will need to address these issues.

Molecular dynamics simulations of the rehydration of folded and unfolded cytochrome C ions in the vapor phase.

Molecular dynamics (MD) simulations have been performed to study the rehydration of compact and unfolded cytochrome c ions in the vapor phase. Experimental studies have shown that the compact conformations adsorb many more water molecules than unfolded ones when exposed to water vapor. MD simulations performed with up to 150 water molecules reproduce the key experimental observations, including a partial refolding caused by hydration. According to the calculations it is more energetically favorable to hydrate the compact conformation in the initial stages of hydration, because it is easier for a water molecule to interact simultaneously with several polar groups (due to their proximity). The protonated side chains are not favored hydration sites in the simulations because they have "self-solvation" shells which must be disrupted for the water to penetrate. For both conformations, the adsorbed water molecules are mainly located in surface crevices.

Charge hopping in DNA.

The efficiency of charge migration through stacked Watson-Crick base pairs is analyzed for coherent hole motion interrupted by localization on guanine (G) bases. Our analysis rests on recent experiments, which demonstrate the competition of hole hopping transitions between nearest neighbor G bases and a chemical reaction of the cation G(+) with water. In addition, it has been assumed that the presence of units with several adjacent stacked G bases on the same strand leads to the additional vibronic relaxation process (G(+)G...G) --> (GG...G)(+). The latter may also compete with the hole transfer from (G(+)G...G) to a single G site, depending on the relative positions of energy levels for G(+) and (G(+)G...G). A hopping model is proposed to take the competition of these three rate steps into account. It is shown that the model includes two important limits. One corresponds to the situation where the charge relaxation inside a multiple guanine unit is faster than hopping. In this case hopping is terminated by several adjacent G bases located on the same strand, as has been observed for the GGG triple. In the opposite, slow relaxation limit the GG...G unit allows a hole to migrate further in accord with experiments on strand cleavage exploiting GG pairs. We demonstrate that for base pair sequences with only the GGG triple, the fast relaxation limit of our model yields practically the same sequence- and distance dependencies as measurements, without invoking adjustable parameters. For sequences with a certain number of repeating adenine:thymine pairs between neighboring G bases, our analysis predicts that the hole transfer efficiency varies in inverse proportion to the sequence length for short sequences, with change to slow exponential decay for longer sequences. Calculations performed within the slow relaxation limit enable us to specify parameters that provide a reasonable fit of our numerical results to the hole migration efficiency deduced from experiments with sequences containing GG pairs. The relation of the results obtained to other theoretical and experimental studies of charge transfer in DNA is discussed. We propose experiments to gain a deeper insight into complicated kinetics of charge-transfer hopping in DNA.

Conformationally tuned large two-photon absorption cross sections in simple molecular chromophores.

We investigate here the relationship between molecular architecture and two-photon absorption (TPA) processes in a class of alkyl-substituted 4-quinopyran chromophores. We find that TPA cross sections diverge as the one-photon gap energy nears one-half of the two-photon gap. The molecular strategy proposed here to tune these two-excitation gaps for maximizing TPA cross sections is to twist the molecule about the bond connecting the chromophore donor and acceptor phenylene fragments. Extremely large TPA cross sections, determined by the absorption bandwidth, can then be realized (imaginary part of the third-order polarizability approximately 2.6 x 10(5) x 10(-36) esu) for fundamental photon energies near 1.0 eV, when the torsional angle approaches 104 degrees. The required torsional angle is achieved by introduction of sterically encumbered 2,2',2' ',2' " tertiary alkyl substituents.

Presentation of carcinoma in a patient with a previous operation for Hirschsprung's disease.

The usual complications after a definitive pull-through procedure for Hirschsprung's disease include stricture formation, enterocolitis, bowel obstruction, and, occasionally, wound infection. The authors report a case of mucinous adenocarcinoma arising 32 years later at a stricture site from a previous pull-through procedure.

Ketorolac reduces postoperative narcotic requirements.

BACKGROUND/PURPOSE: Adverse effects from narcotics complicate pain management in children. Ketorolac, a potent nonsteroidal antiinflammatory agent can be used as an adjuvant analgesic, yet concerns of bleeding and nephrotoxicity have limited routine use. The authors hypothesized that postoperative use of ketorolac in healthy pediatric surgical patients would limit narcotic requirements without increasing morbidity. METHODS: A case-control clinical trial was conducted of 29 pediatric surgical cases prospectively administered ketorolac (0.5 mg/kg intravenously every 6 hours) supplemented with morphine. Controls receiving morphine only were matched for age (+/- 6 months) and surgical procedure. Incidence of respiratory depression, urinary retention, emesis, nephrotoxicity, and bleeding were recorded. RESULTS: Patients receiving ketorolac plus morphine had significantly less morphine requirements in the first 48 postoperative hours (Ketorolac plus Morphine: 0.36+/-0.16 mg/kg/d, Morphine only: 1.08+/-0.16 mg/kg/d [P<.05, analysis by paired t test]). This decrease was noted despite mode of analgesia (patient controlled or nurse administered). Adverse effects of morphine including respiratory depression, emesis, and urinary retention were not affected by ketorolac. Patients administered ketorolac had no significant increase in bleeding or nephrotoxicity. CONCLUSION: Ketorolac exhibits significant opiate-sparing effects in the immediate postoperative period without introducing additional morbidity to pediatric surgical procedures.

[Primary focal segmental glomerulosclerosis: clinical and morphological prognostic factors]. / Pervichnyi fokal'no-segmentarnyi glomeruloskleroz: klinicheskie i morfologicheskie faktory prognoza.

AIM: To retrospectively analyze clinical course and results of immunodepressive therapy of patients with primary focal-segmental glomerulosclerosis (FSGS), to reveal prognostic factors of the disease progression and patients' sensitivity to immunosuppressive therapy. MATERIAL AND METHODS: Morphological diagnosis was specified, morphological indices of activity and sclerosis were estimated, renal survival was analysed, mono- and multivariate analysis of prognostic factors was made by the evidence obtained in the study of 135 biopsy specimens from CRF patients meeting the criteria of FSGS. RESULTS: At the moment of the disease onset only age of the patients was related to FSGS: 5- and 10-year survival was 100% if the disease started under 16 years of age, if older--the survival was 80 and 65%, respectively. Nephrotic syndrome, hematuria, high creatinine, racemose alterations in the glomeruli worsened the disease prognosis. When cytostatics and corticosteroids were used in combination they produced better results and were associated with better prognosis than each of them in monotherapy. Patients with marked hematuria and low proteinuria were less sensitive to therapy than those with weak hematuria and high proteinemia. Patients with FSGS having high IA and SI required more aggressive therapy for response. CONCLUSION: Renal biopsy with quantitation of IA and IS increases the prognosis accuracy and is important for choice of the treatment policy in patients with primary FSGS.

[Histomorphological tubulointerstitial changes in progression of chronic glomerulonephritis]. / Gistomorfologicheskie tubulointerstitsial'nye izmeneniia pri progressirovanii khronicheskogo glomerulonefrita

The role of histological changes associated with primary types of glomerulonephritides in progression of chronic glomerulonephritis (CGN) is analysed. It is shown that CGN progression acceleration occurs much more frequently if chronic renal failure arises within 7 years since the disease onset, in the presence of concomitant tubulointerstitial changes (TIC)in unfavorable morphological types--mesangiocapillary glomerulonephritis and focal-segmental hyalinosis/ sclerosis. It was found that more frequent occurrence of CGN accelerated progression in unfavorable morphological types and TIC depends on the presence of unfavorable clinical types--active nephritic and nephrotic-hypertensive types (classification of M. Ya. Ratner et al.). In TIC there were primarily unfavorable clinical types which contribute to accelerated progression of CGN. This relationship is explained by involvement of factors belonging to the above clinical types in the mechanism of the morphological changes. In these clinical types CGN accelerated progression takes place irrespectively of the presence of TIC and unfavorable morphological types.

[Prediction of accelerated progression of chronic glomerulonephritis basing on clinical and histomorphological data]. / Prognozirovanie uskorennogo progressirovaniia khronicheskogo glomerulonefrita na osnovanii klinicheskikh i gistomorfologicheskikh dannykh.

AIM: To find out predictive value of three factors in progression of chronic glomerulonephritis (CGN): unfavorable clinical course, unfavorable morphological type and tubulointerstitial changes. MATERIALS AND METHODS: 150 CGN patients entered the trial. Frequency of onset of chronic renal failure (CRF) within 7 years after the diagnosis was chosen as a criterium of accelerated progression of CGN (AP CGN). Chi-square criterium was used for testing relationships between AP CGN and the parameters under study. RESULTS: The findings support previously published data on statistically more frequent occurrence of AP CGN in unfavorable clinical types (active nephritic and nephrotically-hypertensive), in unfavorable morphological types (mesangiocapillary CGN and focal-segmental hyalinosis/sclerosis and tubulointerstitial lesions). In unfavorable clinical types there was a significantly more frequent occurrence of AP CGN irrespective of unfavorable morphological changes. In contrast, both in unfavorable and favorable clinical types, frequency of AP CGN in unfavorable morphological types of CGN and tubulointerstitial changes was the same. CONCLUSION: Clinical type of CGN is a valuable prognostic criterium for AP CGN.