Effect of sequence distribution on the isothermal crystallization kinetics and successive self-nucleation and annealing (SSA) behavior of poly(ε-caprolactone-co-ε-caprolactam) copolymers

Two types of miscible poly(ε-caprolactone-co-ε-caprolactam) copolymers were studied. In both cases catalyzed hydrolytic ring-opening polymerization was employed. For the first type, the comonomers were added simultaneously to obtain random copolymers. For the second type, the comonomers were added sequentially to obtain block copolymers. Successive self-nucleation and annealing (SSA) and isothermal crystallization studies were performed to both types of copolymers. The SSA results reflect the differences in molecular microstructure: block versus random copolymers. In a wide composition range only the polycaprolactam sequences were capable of crystallization in the random copolymers. Avrami indexes of approximately 3-4 were obtained corresponding to the spherulitic crystallization of these units within the copolymers. The block copolymer samples experienced a relatively small reduction of crystallization kinetics with composition, and this was attributed to the dilution effect caused by the miscible non-crystalline polycaprolactone units. On the other hand, for the random copolymers, the rate of crystallization strongly increased with polycaprolactam content while the energy barrier for secondary nucleation decreased exponentially. The comparison between miscible block and random copolymers provides a unique opportunity to distinguish the dilution effect of the polycaprolactone units (a moderate effect) on the isothermal crystallization and melting of the polyamide phase from the molecular microstructural effect in the random copolymers case (a dramatically strong effect), where the polycaprolactam sequences are interrupted statistically by polycaprolactone sequences.     

The coexistence of γ(γ′) and θ alumina observed by STM and LEED on top of oxide layer grown on CoAl(1 0 0)

We report the coexistence of γ(γ′) and θ alumina grown on the CoAl(1 0 0) surface. Both phases were observed after annealing of alumina at 1150 K by means of scanning tunnelling microscopy (STM). The crystal structures of γ(γ′) and θ alumina were indexed based on low energy electron diffraction (LEED) and STM experimental data.     

PHOTO-CIDNP DETECTION OF PYRIMIDINE DIMER RADICAL CATIONS IN ANTHRAQUINONESULFONATE- SENSITIZED SPLITTING

Anthraquinone-2-sulfonate (AQS) photosensitizes pyrimidine dimer splitting. Electron abstraction from the dimer is thought to induce dimer splitting, but direct evidence for the existence and intermediacy of dimer radical cations has been lacking. By employing photochemically induced dynamic nuclear polarization, we have found emission signals in the NMR spectra of dimers upon photolysis of dimers in the presence of anthraquinone-2-sulfonate. The two dimers employed were cis, syn-thymine dimer in which the N(1)-positions were linked by a three-carbon bridge and the N(3), N(3')-dimethyl derivative of that compound. The anthraquinone-2-sulfonate sensitized photochemically induced dynamic nuclear polarization spectrum of the methylated derivative exhibited an emission signal from the dimer-C(6) hydrogens. This result implied the existence of a dimer radical cation (mD+.) formed by electron abstraction by excited anthraquinone-2-sulfonate and nuclear spin sorting within a solvent caged radical ion pair [mD+. AQS-.]. Product pyrimidine photochemically induced dynamic nuclear polarization signals were also seen [enhanced absorption by C(6)-hydrogens and emission by C(5)-methyl groups]. Nuclear spin polarization in the product resulted from spin sorting in one or more of its precursors, including mD+. The results support the conclusion that dimer radical cations not only exist but are intermediates in the photosensitized splitting of pyrimidine dimers by anthraquinonesulfonate.     

PHOTO-CIDNP STUDY OF PYRIMIDINE DIMER SPLITTING II: REACTIONS INVOLVING PYRIMIDINE RADICAL ANION INTERMEDIATES

A series of photo-CIDNP (chemically induced dynamic nuclear polarization) experiments were performed on pyrimidine monomers and dimers, using the electron-donor Nα-acetyltryptophan (AcTrp) as a photosensitizer. The CIDNP spectra give evidence for the existence of both the dimer radical anion, which is formed by electron transfer from the excited AcTrp* to the dimer, and its dissociation product, the monomer radical anion. The AcTrp spectra are completely different from those obtained with an oxidizing sensitizer like anthraquinone-2-sulfonate, because of different unpaired electron spin density distributions in pyrimidine radical anion and cation. In the spectra of the anti (1,3-dimethyluracil) dimers, polarization is detected that originates from a spin-sorting process in the dimer radical pair, pointing to a relatively long lifetime of the dimer radical anions involved. Although the dimer radical anions of the 1,1′-trimethylene-bridged pyrimidines may have a relatively long lifetime as well, their protons have only very weak hyperfine interaction, which explains why no polarization originating from the dimer radical pair is detected. In the spectra of the bridged pyrimidines, polarized dimer protons are observed as a result of spin sorting in the monomer radical pair, from which it follows that the dissociation of dimer radical anion into monomer radical anion is reversible. A study of CIDNP intensities as a function of pH shows that a pH between 3 and 4 is optimal for observing monomer polarization that originates from spin-sorting in the monomer radical pair. At higher pH the geminate recombination polarization is partly cancelled by escape polarization arising in the same product.     

PHOTOSENSITIZATION OF PYRIMIDINE DIMER SPLITTING BY A COVALENTLY BOUND INDOLE

Abstract— Photosensitized pyrimidine dimer splitting characterizes the enzymatic process of DNA repair by the DNA photolyases. Possible pathways for the enzymatic reaction include photoinduced electron transfer to or from the dimer. To study the mechanistic photochemistry of splitting by a sensitizer representative of excited state electron donors, a compound in which an indole is covalently linked to a pyrimidine dimer has been synthesized. This compound allowed the quantitative measurement of the quantum efficiency of dimer splitting to be made without uncertainties resulting from lack of extensive preassociation of the unlinked dimer and sensitizer free in solution. Irradiation of the compound with light at wavelengths absorbed only by the indolyl group (approximately 280 nm) resulted in splitting of the attached dimer. The quantum yield of splitting of the linked system dissolved in N₂0-saturated aqueous solution was found to be 0.04 ± 0.01. The fluorescence typical of indoles was almost totally quenched by the attached dimer. A splitting mechanism in which an electron is efficiently transferred intramolecularly from photoexcited indole to ground state dimer has been formulated. The surprisingly low quantum yield of splitting has been attributed to inefficient splitting of the resulting dimer radical anion. Insights gained from this study have important mechanistic implications for the analogous reaction effected by the DNA photolyases.     

TRANSIENT INTERMEDIATES IN INTRAMOLECULARLY PHOTOSENSITIZED PYRIMIDINE DIMER SPLITTING BY INDOLE DERIVATIVES

Abstract— Intramolecularly photosensitized pyrimidine dimer splitting can serve as a model for some aspects of the monomerization of dimers in the enzyme-substrate complex composed of a photolyase and UV-damaged DNA. We studied compounds in which a pyrimidine dimer was covalently linked either to indole or to 5-methoxyindole. Laser flash photolysis studies revealed that the normally observed photoejection of electrons from the indole or the 5-methoxyindole to solvent was diminished by an order of magnitude for indoles with dimer attached (dimer-indole and dimer-methoxyindole). The fluorescence lifetime of dimer-indole in aqueous methanol was 0.85 ns, whereas that of the corresponding indole without attached dimer (tryptophol) was 9.7 ns. Similar results were obtained for the dimer-methoxyindole (0.53 ns) and 5-methoxytryptophol (4.6 ns). The quantum yield of dimer splitting for the dimer-methoxyindole (φ₂₈₇K7 = 0.08) was only slightly greater than the value found earlier for the dimer bearing the unsubstituted indole (4>_2K7= 0.04). Transient absorption spectroscopy also revealed lower yields of indole radical cations following laser flash photolysis of dimer-indole compared to the indole without attached dimer. Dimer-methoxyindole behaved similarly. These results are interpreted in terms of an enhanced rate of radiationless relaxation of the indole and methoxyindole excited singlet states in dimer-indoles. The possible quenching of the indole and methoxyindole excited states via electron abstraction by the covalently linked dimer is discussed.     

Functional Hydride Transfer by a Thiolate‐Containing Model of Mono‐Iron Hydrogenase featuring an Anthracene Scaffold

We report the synthesis, X‐ray structure and functional biomimetic activity of a model complex of mono‐iron hydrogenase (Hmd). To achieve the desired biomimetic fac‐CNS(thiolate) ligation motif, an anthracene framework is used to provide the requisite donors in a single chelate. A bulky aryl thiolate (ortho dimethylphenyl) is included to achieve mononuclearity. In addition to exhibiting structural (X‐ray) and spectroscopic (NMR, IR) similarity to the enzyme, the complex is competent for H₂ activation (heterolysis) and hydride transfer to a model substrate—mimicking the functional behavior of the enzyme in a biomimetic CNS coordination sphere for the first time.     

Statistical mechanics of the nonlinear Schrödinger equation

We investigate the statistical mechanics of a complex fieldø whose dynamics is governed by the nonlinear Schrödinger equation. Such fields describe, in suitable idealizations, Langmuir waves in a plasma, a propagating laser field in a nonlinear medium, and other phenomena. Their Hamiltonian $$H(\phi ) = \int_\Omega {[\frac{1}{2}|\nabla \phi |^2 - (1/p) |\phi |^p ] dx}$$ is unbounded below and the system will, under certain conditions, develop (self-focusing) singularities in a finite time. We show that, whenΩ is the circle and theL ² norm of the field (which is conserved by the dynamics) is bounded byN, the Gibbs measureυ obtained is absolutely continuous with respect to Wiener measure and normalizable if and only ifp andN are such that classical solutions exist for all time—no collapse of the solitons. This measure is essentially the same as that of a one-dimensional version of the more realisitc Zakharov model of coupled Langmuir and ion acoustic waves in a plasma. We also obtain some properties of the Gibbs state, by both analytic and numerical methods, asN and the temperature are varied.