A unique computer representation for molecular structures

Converting a non-unique connection table to a unique (canonical) name can be accomplished by assigning unique sequence numbers on the basis of topological (constitutional) properties. An algorithm is reported which performs this task by perceiving the topological symmetry of the molecule. A convention for assigning a single name to topologically unique structures which vary only in the position of π-electrons (resonance forms) is also presented.     

Study of poly(vinyl chloride) by inverse gas chromatography

Inverse gas chromatography (IGC) was used to investigate PVC to determine what information can be gathered using the technique. Standard IGC using a column of PVC-coated support was performed at 120°C. The polymer-solvent interaction coefficients for 25 probes were obtained and the polymer solubility parameter was determined. A 50/50 blend of PVC/poly(∈-caprolactone) was studied to determine the polymer-polymer interaction coefficient B₂₃ and the solubility parameter of the blend. A nonstandard analysis using bulk, suspension-polymerized PVC as the column packing was performed at 40°C, and the retention behavior compared with a computer simulation of Langmuir type adsorption.     

A new approach for the estimation of the axial velocity using ultrasound

The most used estimation method for calculating the blood velocity in commercial scanners is the autocorrelation approach. The calculation of the mean velocity used in this method depends on the center frequency of the interacting ultrasound pulse which downshifts as a function of depth, introducing a bias. A new velocity estimator for the mean axial velocity is presented. The estimation principle is based on the 2D Fourier transform and the Radon transform. The input data are a sequence of RF data forming a 2D data input, one column for each pulse emission. A 2D segment is selected for a specific depth. This data segment is first transformed by a 2D Fourier transform, and the result is then transformed by a Radon transform. The center of gravity for the angles of the lines intersecting the origin of the R-theta coordinate system in the Radon domain gives the mean axial velocity for the data segment. The benefit of this method is an estimate of the mean axial velocity which is independent of the center frequency of the propagating ultrasound pulse. The estimate will only depend on f(s) and f(pr f). Results of the estimation method is presented based on both simple generated RF harmonic data for different signal/noise ratios and simulated acoustic RF responses from a 3D measurement situation with an array transducer and a tube with plug flow. The new method shows improvement with a factor of 1.5-4 on the standard deviation on the estimated mean velocity for the simulated case.     

Intrinsic viscosity of polystyrene in mixed solvents

The intrinsic viscosity of a single sample of polystyrene was measured as a function of the composition of solvent in three mixed solvent pairs. The parameter Y introduced by Shultz and Flory was useful for prediction of trends, but severely overestimated the effect of solvent (1)–solvent (2) interaction on the expansion of polymer coils. The system polystyrene–cyclohexane–ethyl acetate was studied in detail for five samples of polystyrene. The analysis of the data provided strong experimental proof of a strict validity of the Mark–Houwink–Sakurada relation. The dependence of the Mark–Houwink–Sakurada exponent α on the composition of the solvent mixture was unexpectedly unsymmetrical. The unperturbed dimentions of the polystyrene chain are reduced by specific interaction of polystyrene with carbonyl groups in the solvent mixture.     

A novel formalism to characterize the degree of unsaturation of organic molecules

The existing formalism to calculate the degree of unsaturation from the molecular formula of organic molecules cannot be applied to charged and/or disconnected species. Moreover, the calculated value depends on the assumed formal valence of each of the elements. In this work, we introduce a new formalism that eliminates these problems. The suggested property, degree of unsaturation, can be calculated from the molecular formula as well as from any structural representation of a molecule corresponding to that molecular formula.     

The application of two-dimensional nuclear magnetic resonance spectroscopy in computer-assisted structure elucidation

One of the most powerful tools for the determination of an unknown organic structure is two dimensional NMR spectroscopy. In some cases, this information quickly leads to a unique structure, but in many cases there remains a larger number of structural possibilities. A linked system of computer programs has been developed which uses several types of 2-D NMR data and generates all possible topological structures consistent with these data and user-input constraints. When symmetry is present, algorithms based on group theory limit the structures to those consistent with all the different symmetries that are possible. Applications of these programs to problems described in the literature, in which 2-D NMR was used to determine the structures of natural products, are described.     

Persistence Barcodes Versus Kolmogorov Signatures: Detecting Modes of One-Dimensional Signals

We investigate the problem of estimating the number of modes (i.e., local maxima)—a well-known question in statistical inference—and we show how to do so without presmoothing the data. To this end, we modify the ideas of persistence barcodes by first relating persistence values in dimension one to distances (with respect to the supremum norm) to the sets of functions with a given number of modes, and subsequently working with norms different from the supremum norm. As a particular case, we investigate the Kolmogorov norm. We argue that this modification has certain statistical advantages. We offer confidence bands for the attendant Kolmogorov signatures, thereby allowing for the selection of relevant signatures with a statistically controllable error. As a result of independent interest, we show that taut strings minimize the number of critical points for a very general class of functions. We illustrate our results by several numerical examples.     

Time-resolved fluorescence anisotropy measurements on fluorescently tagged amphiphilic micelles

Block copolymers of polystyrene-block-poly)methacrylic acid) form multimolecular micelles in mixtures of 1,4-dioxane with a surplus of methanol at ambient temperatures. Micelles consist of compact polystyrene cores surrounded by outer shells formed by poly(methacrylic acid) and are in a reversible equilibrium with nonmicellized copolymer chains (unimers). A series of light scattering, ultracentrifugation, and fluorimetric measurements was performed on micellizing systems of end-tagged copolymers. Complex time-resolved fluorescence anisotropy decays may be explained by a distribution of fluorophores in two microenvironments, i.e., in compact polystyrene micellar cores and in unimer coils.