The influence of random degradation on the length density function of polymer molecules

The theoretical formula for the evolution of the length density function caused by a random degradation process is derived. The formula holds for an arbitrary initial density function.     

Spacer molecules in peptide sequences: incorporation into analogues of atrial natriuretic factor

The spacer reagents FmocHN(CH₂CH₂CH₂)₃CH₂COOPfp (1b) and FmocHN(CH₂CH₂O)₃CH₂COOPfp (2d) have been prepared and used to substitute for tetra-residue sequences in the cyclic portion of Atrial Natriuretic Factor (ANF) by solid phase peptide assembly.     

Chain length effects in the second virial coefficient of linear polymer molecules

A generalized perturbation theory is presented for the second virial coefficient of linear and branched polymer systems. Results have been computed for linear chains having two to five hundred statistical segments. These are found to differ significantly from the long-chain asymptotic results of Zimm. A semi-empirical modification of the Flory--Orofino theory is also suggested.     

Synthesis of some thiopyrimidine and thiazolopyrimidines starting from 2,6-dibenzylidene-3-methylcyclohexanone and its antimicrobial activities

A series of novel N-substituted arylidene, pyrazole, thioxopyrimidine and thiazolopyrimidine derivatives 2–7 were synthesized by initial reactions of 2-methyl-cyclohexanone with aromatic aldehydes to give 2,6-dibenzylidene-3-methylcyclohexanone 2. Some of the synthesized compounds were tested as antimicrobial agents. The detailed synthesis, spectroscopic data, and antimicrobial activities of the synthesized compounds were reported.     

Interaction between chiral molecules and solvents: The importance of an interface

The recently introduced hypothesis of chiral interaction is shown here to require an additional constraint of an interface between two media in the close vicinity of the chiral molecule or element. As a result only the induced intramolecular current can exist under this constraint whereas the induced mechanical rotation is ruled out. The energy associated with this current is estimated to be of the order of (10⁻⁵–10⁻⁴)kT. Possible thermodynamical consequences of the chiral interaction for small systems are discussed.     

Silicon-containing polyphilic bent-core molecules: the importance of nanosegregation for the development of chirality and polar order in liquid crystalline phases formed by achiral molecules

Polyphilic molecules composed of a bent aromatic core, oligo(siloxane) units, and alkyl segments were synthesized, and the self-organization of these molecules was investigated. Most materials organize into polar smectic liquid crystalline phases. The switching process of these mesophases changes from antiferroelectric for the nonsilylated compounds via superparaelectric to surface-stabilized ferroelectric with increasing segregation of the silylated segments. It is proposed that the siloxane sublayers stabilize a polar synclinic ferroelectric (SmC(s)P(F)) structure, and the escape from a macroscopic polar order as well as steric effects leads to a deformation of the layers with formation of disordered microdomains, giving rise to optical isotropy. Another striking feature is the spontaneous formation of chiral domains with opposite handedness. For two compounds, a temperature-dependent inversion of the optical rotation of these domains was found, and this is associated with an increase of the tilt angle of the molecules from < 45 degrees to > 50 degrees. This observation confirms the recently proposed concept of layer optical chirality (Hough, L. E.; Clark, N. A. Phys. Rev. Lett. 2005, 95, 107802), which is a new source of optical activity in supramolecular systems. With increasing length of the alkyl chains, segregation is lost and a transition from smectic to a columnar phase is found. In the columnar phase, the switching process is antiferroelectric and takes place by rotation of the molecules around the long axes, which reverses the layer chirality; that is, the racemic ground-state structure is switched into a homogeneous chiral structure upon application of an electric field.     

Structural importance of secondary interactions in molecules: origin of unconventional conformations of phosphine-borane adducts

The series of phosphine-borane adducts, Ph2(H3C--C[triple chemical bond]C)P--B(C6F5)3 (8 c), Ph(H3C--C[triple chemical bond]C)2P--B(C6F5)3 (8 b) and (H3C--C[triple chemical bond]C)3P--B(C6F5)3 (8 a), was prepared. The X-ray crystal structure analyses revealed close to eclipsed conformations for all members of this series with average dihedral angles theta(C-P-B-C) of 8.1 degrees (8 c), 12.3 degrees (8 b) and 20.3 degrees (8 a). Quantum chemical analysis of these compounds revealed the importance of a subtle interplay between competing attractive and repulsive secondary interactions, causing the surprising eclipsed conformational preference for systems of this degree of complexity. Some cyclic phosphine-borane adducts were studied for comparison.     

NaOH-catalyzed thiolysis of alpha,beta-epoxyketones in water. A key step in the synthesis of target molecules starting from alpha,beta-unsaturated ketones

NaOH (0.02-0.3 molar equiv) is an efficient catalyst for the thiolysis reactions of alpha,beta-epoxy ketones with alkyl and aryl thiols in water. Thiolysis of 3,4-epoxyheptan-2-one (1) with thiols 2a-d has been accomplished in mild conditions (30 degrees C and pH 6 or 9) with complete C-alpha-regioselectivity and anti-stereoselectivity, and the corresponding anti-beta-carbonyl-beta-hydroxysulfides 3a-d have been prepared in excellent yields (95-98%). Compounds 3a-d, depending on their nature and pH conditions, have undergone dehydration, C-3 epimerization reaction, and retroaldol condensation. Dehydration of anti-3a-d has been chemoselectively carried out by in situ acidic treatment at 70 degrees C, giving stereoselectively the related (Z)-vinyl sulfides 4 in 89-94% overall yields. Under NaOH-catalyzed thiolysis conditions, cyclic alpha,beta-epoxyketones 6-9 have shown C-alpha attack only and spontaneously dehydrated to furnish the corresponding vinyl sulfides in high yields (90-96%). The reactions of calchone oxide (10) with thiols 2b-d have exclusively resulted in the formation of beta-carbonylsulfides 10b-d (82-93% yield), coming from the nucleophilic attack at the alpha-position and retroaldol condensation. To highlight the synthetic utility of this procedure, one-pot multisteps preparation of vinyl sulfides 7b and 7c, vinyl sulfoxides 12 and 13, and 1,5,6,7-tetrahydro-4H-1,2,3-benzotriazol-4-one (14) starting from 2-cyclohexen-1-one (11) have also been reported.     

On the use of a random parameter in the theory of depolymerization of long-chain molecules

Equations for the time course of depolymerization of long-chain polymers are derived by the method of randomization of a parameter for the case in which all bonds have equal probability of cleavage. This yields some gain in insight and avoids the necessity for solving systems of differential equations.     

Battle for the bulge: directing small molecules to DNA and RNA defects

Small molecules were tailored to specifically bind bulged DNA by complementing the geometry and nucleotide size of the bulge site. The prospect of generating small molecules that influence the secondary structure of DNA and RNA holds great promise for clinical applications.     

Radiation-induced transformations of isolated organic molecules in solid rare gas matrices

The radiation-chemical behaviour of isolated organic molecules in solid rare gas matrices was studied using a combination of ESR and IR spectroscopy. The total radiation-chemical yields of degradation of organic molecules in solid argon and xenon were estimated. The effect of electron scavengers on the radiolysis of organic molecules in solid rare gases reveals that the main primary process is positive hole transfer from matrix to guest molecule. ESR spectra of a number of radical cations (alkanes, ethers, arenes) were characterized in a low-disturbing environment. It was found that the electronic characteristics (IP, polarizability) of the matrix used had crucial effect on trapping and reactions of the isolated radical cations.     

Unusually short RNA sequences: design of a 13-mer RNA that selectively binds and recognizes theophylline

RNA plays critical roles in numerous biological processes and constitutes valuable therapeutic targets. RNA is significant not only for its roles in transmitting the genetic code but also for its enzymatic functions in ribozymes and in peptide bond formation in ribosomes. Recent studies have shown that RNAs containing as few as 22 nucleotides can be key elements in cellular functions. This suggests the possibility of using short RNAs as regulatory elements. Here, we show that ligand recognition and selectivity by RNA molecules can occur with only the presence of a binding pocket and as few as six additional scaffolding nucleotides holding the binding pocket in place. A 13-mer RNA truncation of a 33-mer aptamer for theophylline preserves the ability to bind to theophylline and to discriminate against the structurally similar compound caffeine. The truncated aptamer retains nearly all of the same structural elements in its binding site as those present in the original aptamer. This is the first demonstration of selective ligand binding by a 13-mer RNA.     

Diffusion anomaly as a function of molecular length of linear molecules: levitation effect

Previous work on monatomic spherical sorbates has shown the existence of an anomalous peak in self-diffusivity (D) when plotted as a function of size of the diffusant. Molecular dynamics studies on linear molecules of different lengths l in zeolite NaY at 140 and 200 K are reported. It is seen that there is a peak in D as a function of l, suggesting that the levitation effect exists for linear molecules, the simplest member of polyatomics. This is confirmed by the lowering of the activation energy for the molecule whose length l exhibits highest D. Related quantities of interest such as the guest-host interaction energy and preexponential factor are discussed.     

Quenching of the fluorescence of rare earth ions by high energy vibrations of solvent molecules: calculations of Franck-Condon factors from spectral data

Spectroscopic data on the intensity of vibronic satellite in aqueous solutions of Gd³⁺ are utilized to calculate relative radiationless transition rates in solutions of other rare earth ions. It is assumed that the main mechanism of fluorescence quenching of these ions is by energy transfer to single high energy OH vibrations of solvent water. The theory accounts qualitatively for the experimental results. Better quantitative agreement requires more accurate spectroscopic data on each specific ion.     

General theory of stationary random sequences with applications to the tacticity of polymers

When, in a poly-α-olefin, the probability that a given placement be isotactic depends upon the tacticity of only a finite number of immediate predecessors, the resulting diastereosequence distribution obeys the theory of Markoff chains. When this is not the case, one says that the resulting diastereosequence distribution is non-Markoffian. A special case of a Markoffian distribution is given by a simple Markof chain in which the tacticity of a given placement is assumed to be affected by only the tacticity of the immediately preceding placement. Another special case is, of course, the Bernoulli trial distribution in which the probability that a given placement be isotactic is independent of the tacticity of all other placements. A high resolution NMR spectrum can sometimes yield a quantitative determination of the concentrations of isotactic and syndiotactic placements and the concentrations of the three types of possible adjacent pairs of such placements (i.e., isotactic, syndiotactic, and heterotactic pairs). When this is the case, the spectrum can be used to determine whether or not a given diastereosequence distribution is Bernoullian. However, because the longest diastereosequences whose concentration can be measured by NMR spectroscopy involve only two placements, an NMR spectrum cannot check whether a given non-Bernoullian distribution be simple Markoffian or Markoffian in general. In fact, non-Markoffian distributions are compatible with existing NMR spectra on polymers prepared by anionic polymerizations. In this paper we work within the framework of Kac's theory of stationary statistical processes and point out some general results which are valid for both Markoffian and non-Markoffian processes. The results are applied to NMR spectroscopy and it is pointed out which calculations used to check the self-consistency of NMR data and to obtain the mean length of closed diastereosequences are valid for both Markoffian and non-Markoffian distributions.     

Effect of length and number of interlinking molecules on the strength of adhesion

The fracture energy G of an adhesive bond appears to be a product of two terms: G = G_O [1 + f(R, T)], where G_O is the intrinsic (chemical) strength of the interface and f(R, T), usually much larger than unity, reflects energy dissipated within the adherends at a crack speed R and temperature T. Values of G_O have been determined for interlinked sheets of an SBR elastomer by measuring the peel strength at low rates and high temperatures, and in the swollen state, to minimize internal losses. Both the density ΔN and molecular length L of interlinking molecules were varied. G_O was found to increase in proportion to (ΔN)L^3/2, in accord with the molecular theory of Lake and Thomas. As the peel rate was raised and the test temperature lowered, G was considerably increased by internal dissipative processes, becoming as much as 1000 × G_O near the glass transition. The loss function f(R, T) was found to depend somewhat upon the strand length L, being about twice as large at intermediate peel rates when L was increased by 40%. It also depended on the density ΔN of interlinking molecules, being about twice as large at high peel rates when the density of interlinks was reduced by a factor of six. Thus, the loss function f(R, T) is greater when the interlinking molecules are few and long, and it is lower when they are many and short. However, it is mainly governed by two parameters: peel rate R and temperature difference (T − T_g), in accord with a viscoelastic loss mechanism. © 1996 John Wiley & Sons, Inc.