Protonation of thymine,cytosine, adenine,and guanine DNA nucleic acid bases: Theoretical investigation into the framework of density functional theory

Gradient-corrected density functional computations with triple-zeta-type basis sets were performed to determine the preferred protonation site and the absolute gas-phase proton affinities of the most stable tautomer of the DNA bases thymine (T), cytosine (C), adenine (A), and guanine (G). Charge distribution, bond orders, and molecular electrostatic potentials were considered to rationalize the obtained results. The vibrational frequencies and the contribution of the zero-point energies were also computed. Significant geometrical changes in bond lengths and angles near the protonation sites were found. At 298 K, proton affinities values of 208.8 (T), 229.1 (C), 225.8 (A), and 230.3 (G) kcal/mol were obtained in agreement with experimental results. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 989–1000, 1998     

Gas-phase basicity of methionine

Proton affinity and protonation entropy of methionine (Met) were determined by the extended kinetic method from ESI-Q-TOF tandem mass spectrometry experiments. The values, PA(Met) = 937.5 +/- 2.9 kJ mol(-1) and Delta(p)S degrees (Met) = - 22 +/- 5 J mol(-1) K(-1), lead to gas-phase basicity GB(Met) = 898.2 +/- 3.2 kJ.mol(-1). Quantum chemical calculations using density functional theory confirm that the proton affinity of Met is indeed in the 940 kJ mol(-1) range and that a significant entropy loss, of at least - 25 J mol(-1) K(-1), occurs upon protonation. This last point is evidenced here for the first time and suggests revision of the tabulated protonation thermochemistry of Met. A comparison with previous experimental data allows us to propose the following evaluated thermochemical values: PA(Met) = 943 +/- 4 kJ mol(-1) and Delta(p)S degrees (Met) = - 35 +/- 15 J mol(-1) K(-1) and GB(Met) = 900 +/- 2 kJ mol(-1).     

Ab initio calculations of methionines and their protonated forms

Ab initio calculations of molecular and electronic structures of neutral molecules and protonated forms of methionine and its derivatives in the gaseous phase were carried out by the Hartree-Fock method using the 6–31G^* basis set with full geometry optimization. Proton affinities of methionine (1), methionine sulfoxide (2), and methionine sulfone (3) were calculated for different modes of coordination of the proton. The results of calculations demonstrated that in protonated forms of 1 and 3, bonding between the proton and the N atom is most favorable, while in protonated form of 2, bonding between the proton and the O atom of the SO group is most favorable. The proton affinities of the amino acids are as follows: 223.2 (1), 241.2 (2), and 221.5 (3) kcal mol⁻¹,i.e., methionine sulfoxide 2 exhibits the highest proton affinity in the series of the amino acids under consideration. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1487–1490, August, 1998.     

Gas-phase tautomers of protonated 1-methylcytosine. Preparation, energetics, and dissociation mechanisms

Tautomers of 1-methylcytosine that are protonated at N-3 (1+) and C-5 (2+) have been specifically synthesized in the gas phase and characterized by tandem mass spectrometry and quantum chemical calculations. Ion 1+ is the most stable tautomer in aqueous and methanol solution and is likely to be formed by electrospray ionization of 1-methylcytosine and transferred in the gas phase. Gas-phase protonation of 1-methylcytosine produces a mixture of 1+ and the O-2-protonated tautomer (3+), which are nearly isoenergetic. Dissociative ionization of 6-ethyl-5,6-dihydro-1-methylcytosine selectively forms isomer 2+. Upon collisional activation, ions 1+ and 3+ dissociate by loss of ammonia and [C,H,N,O], whose mechanisms have been established by deuterium labeling and ab initio calculations. The main dissociations of 2+ following collisional activation are losses of CH2=C=NH and HN=C=O. The mechanisms of these dissociations have been elucidated by deuterium labeling and theoretical calculations.     

Thermal stabilities,electronic properties and structures of metformin-metal complexes

The atomic superposition and electron delocalization molecular orbital (ASED-MO) theory was used to calculate structures and relative stabilities of metformin-metal complexes. The relative stabilities and decomposition pathways were discussed in terms of bond order, binding energy and the nature of charge on the central metal atom. The electronic transitions and their energy gaps were also studied. The optimization of the structures shows that the most stable state is distorted from planarity for Co^II and Ni^II complexes.     

Structure and properties of clay nano‐biocomposites based on poly(lactic acid) plasticized with polyadipates

The use of nano‐biocomposites based on plasticized poly(lactic acid) (PLA) has been proposed as a way to improve the polymer ductility and to expand PLA applications window. Novative nano‐biocomposites were elaborated with PLA plasticized by polyadipates (15 wt%) with different molar masses (from 1500 to 2500 Da), with 2.1 wt% of an organo‐modified montmorillonite (O‐MMT). These materials showed enhanced ductility and barrier properties. The clay was swelled in liquid polyadipates prior to their blending with PLA to facilitate chains intercalation and nanofiller exfoliation during melt‐blending. In certain processing conditions, quite homogenous and exfoliated structures were obtained, as shown by X‐ray diffraction (XRD) and transmission electronic microscopy (TEM) results. Irrespective of the average molar mass of the polyadipate, the clay addition induced a reduction in around 25% in oxygen transmission rate (OTR) without an important detriment in tensile properties. Nano‐biocomposites prepared with higher molar masses polyadipates showed the highest thermal stability as well as the lowest OTR, resulting in very promising and novative materials for different applications such as soft packaging. Copyright © 2010 John Wiley & Sons, Ltd.     

Quantum-chemical calculations of the electronic structure and reactivity of silylenes

The origin of the different reactivities of carbenes and silylenes has been discussed. Molecular orbital calculations have been carried out in order to estimate the effects of substituents on the singlet-triplet splittings E _ST in these molecules. The data ofab initio calculations have been used to establish the linear correlation between E _ST values for carbenes and silylenes. Both qualitative and quantitative appraisals of the applicability of the semiempirical MNDO method to calculations of E _ST values for carbenes and silylenes have been derived. E _ST values for a large number of substituted carbenes and silylenes have been computed by the PM3 method, one of the latest versions of the MNDO approach (Ref. 14).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1006–1008, June, 1994.     

退火过程中聚乳酸中间相的形成和结构演化

采用淬火法制备聚乳酸(PLA)非晶薄膜,并利用原位显微红外光谱在线研究PLA非晶薄膜在不同退火温度下的结构演化.结果表明,PLA非晶薄膜存在一个临界结晶温度,当退火温度高于临界结晶温度时,PLA非晶薄膜可以通过分子链的局部调整实现冷结晶,反之,不能发生冷结晶;在冷结晶过程中先出现中间相,随后发生中间相-晶体相的转变;中间相是通过分子链的构象调整和分子链间的堆砌调整产生的,退火温度越高,中间相出现得越早,最终得到的晶体结构越规整.     

Enhanced properties of poly(lactic acid) with silica nanoparticles

The objective of this article is to fabricate poly(lactic acid) (PLA) and nano silica (SiO₂) composites and investigate effect of SiO₂ on the properties of PLA composites. Surface‐grafting modification was used in this study by grafting 3‐Glycidoxypropyltrimethoxysilane (KH‐560) onto the surface of silica nanoparticles. The surface‐grafting reaction was confirmed by Fourier transform infrared spectroscopy and thermogravimetric analysis. Then the hydrophilic silica nanoparticles became hydrophobic and dispersed homogeneously in PLA matrix. Scanning electron microscope and Dynamic thermomechanical analysis (DMA) results revealed that the compatibility between PLA and SiO₂ was improved. Differential scanning calorimetry and polarized optical microscope tests showed that nano‐silica had a good effect on crystallization of PLA. The transparency analysis showed an increase in transparency of PLA, which had great benefit for the application of PLA. The thermal stability, fire resistance, and mechanical properties were also enhanced because of the addition of nano silica particles. Copyright © 2016 John Wiley & Sons, Ltd.     

Organically modified rectorite toughened poly(lactic acid): Nanostructures, crystallization and mechanical properties

To improve the toughness of PLA, poly(lactic acid) (PLA)/organically modified rectorite (OREC) nanocomposites were prepared via the melt-extrusion method. A partially exfoliated and partially intercalated structure was confirmed by WAXD and TEM. The crystallization behaviors of neat PLA and nanocomposite were studied by POM and DSC, and it was found that OREC had a great effect on the overall crystallization rate and spherulitic texture of PLA. The presence of OREC could toughen PLA greatly. For example, when 1 wt.% OREC was added, the elongation at break of the nanocomposite was increased to 210%. The toughening mechanism was analyzed through the observation of the inner structure of the tensile test bar using SEM.     

Thermal properties of poly(lactic acid)/organo-montmorillonite nanocomposites

Poly(lactic acid)/organo-montmorillonite nanocomposites were prepared by melt intercalation technique. Maleic anhydride-grafted ethylene propylene rubber (EPMgMA) was added into the PLA/OMMT in order to improve the compatibility and toughness of the nanocomposites. The samples were prepared by single screw extrusion followed by compression molding. The effect of OMMT and EPMgMA on the thermal properties of PLA was studied. The thermal properties of the PLA/OMMT nanocomposites have been investigated by using differential scanning calorimeter (DSC) and thermo-gravimetry analyzer (TG). The melting temperature (T _m), glass transition temperature (T _g), crystallization temperature (T _c), degree of crystallinity (χ_c), and thermal stability of the PLA/OMMT nanocomposites have been studied. It was found that the thermal properties of PLA were greatly influenced by the addition of OMMT and EPMgMA.     

Mechanical properties and structure of the zone‐drawn poly(L‐lactic acid) fibers

The zone‐drawing (ZD) method was applied three times to the melt‐spun poly(L ‐lactic acid) (PLLA) fibers of low molecular weight (M_v = 13,100) at different temperatures under various tensions. The mechanical properties and superstructure of the ZD fibers were investigated. The resulting ZD‐3 fiber had a draw ratio of 10.5, birefringence of 37.31 × 10⁻³, and crystallinity of 37%, while an orientation factor of crystallites remarkably increased to 0.985 by the ZD‐1. The Young's modulus and tensile strength of the ZD‐3 fiber respectively attained 9.1 GPa and 275 MPa, and the dynamic storage modulus was 10.4 GPa at room temperature. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 991–996, 1999     

Melt polycondensation of L‐lactic acid with Sn(II) catalysts activated by various proton acids: A direct manufacturing route to high molecular weight Poly(L‐lactic acid)

Poly(L ‐lactic acid) (PLLA) was produced by the melt polycondensation of L ‐lactic acid. For the optimization of the reaction conditions, various catalyst systems were examined at different temperature and reaction times. It was discovered that Sn(II) catalysts activated by various proton acids can produce high molecular weight PLLA [weight‐average molecular weight (M_w ) ≥ 100,000] in a relatively short reaction time (≤15 h) compared with simple Sn(II)‐based catalysts (SnO, SnCl₂ · 2H₂O), which produce PLLA with an M_w of less than 30,000 after 20 h. The new catalyst system is also superior to the conventional systems in regard to racemization and discoloration of the resultant polymer. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1673–1679, 2000     

Structure and mechanical properties of poly(L‐lactic acid) crystals and fibers

The elastic constants of poly(L ‐lactic acid) (PLLA) crystals are reported on the basis of a commercial software package and the published crystal structure of the α form. A chain modulus of 36 GPa and a shear modulus of 3 GPa have been obtained for cylindrically symmetric aggregates of perfectly oriented crystals. The helical conformation of the PLLA molecule reduces the stiffness in the chain axis direction because bond rotation plays a significant role in the deformation. X‐ray crystal strain measurements suggest that shear of the α crystal parallel to the helix axis is the easiest mode of deformation, in agreement with the expectations obtained from the low shear modulus of 3 GPa obtained from the theoretical calculations. A combination of small‐ and wide‐angle X‐ray scattering, differential scanning calorimetry, dynamic mechanical thermal analysis, and shrinkage measurements has been used to characterize the structure that develops and the crystal transformation that occurs during fiber processing. The structure that develops during processing very much depends on the crystal transformation, and a structural model is proposed for fibers at different degrees of plastic deformation. The transformation of the α crystal into the β form and vice versa is governed primarily by shear along the helix axis because the chains must shear past each other during the crystal transformation, disrupting the lamellar packing. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 892–902, 2007     

Influences of poly(lactic acid)‐grafted carbon nanotube on thermal,mechanical, and electrical properties of poly(lactic acid)

Poly(lactic acid)‐grafted multiwalled carbon nanotubes (MWNT‐g‐PLA) were prepared by the direct melt‐polycondensation of L ‐lactic acid with carboxylic acid‐functionalized MWNT (MWNT‐COOH) and then mixed with a commercially available neat PLA to prepare PLA/MWNT‐g‐PLA nanocomposites. Morphological, thermal, mechanical, and electrical characteristics of PLA/MWNT‐g‐PLA nanocomposites were investigated as a function of the MWNT content and compared with those of the neat PLA, PLA/MWNT, and PLA/MWNT‐COOH nanocomposites. It was identified from FE‐SEM images that PLA/MWNT‐g‐PLA nanocomposites exhibit good dispersion of MWNT‐g‐PLA in the PLA matrix, while PLA/MWNT and PLA/MWNT‐COOH nanocomposites display MWNT aggregates. As a result, initial moduli and tensile strengths of PLA/MWNT‐g‐PLA composites are much higher than those of neat PLA, PLA/MWNT, and PLA/MWNT‐COOH, which stems from the efficient reinforcing effect of MWNT‐g‐PLA in the PLA matrix. In addition, the crystallization rate of PLA/MWNT‐g‐PLA nanocomposites is faster than those of neat PLA, PLA/MWNT, and PLA/MWNT‐COOH, since MWNT‐g‐PLA dispersed in the PLA matrix serves efficiently as a nucleating agent. It is interesting that, unlike PLA/MWNT nanocomposites, surface resistivities of PLA/MWNT‐g‐PLA nanocomposites did not change noticeably depending on the MWNT content, demonstrating that MWNTs in PLA/MWNT‐g‐PLA are wrapped with the PLA chains of MWNT‐g‐PLA. Copyright © 2008 John Wiley & Sons, Ltd.     

Structural properties and potential reactivity of substituted 3- aroyldithiocarbazates

A series of substituted 3-aroyldithiocarbazates has been synthesized and studied. The corresponding acid dissociation constants have been determined potentiometrically. Semiempirical PM3 molecular orbital calculations suggest the existence of several tautomeric forms of the compounds. Geometrical parameters, proton affinities, and static reactivity indices have been examined. Structural properties and protonation sites are well described by calculations. The strong correlations between the pK _a values and the Hammett constants as well as the N(3) calculated proton affinities indicate that the N(3) atom is the most probable protonation site. The thermodynamics of the protonation process are mainly controlled by HOMO-LUMO rather than electrostatic interactions. According to PM3 results, 3-aroyldithiocarbazic acid should be quite stable in the gas phase, while a mechanism for its decomposition in solution is proposed.     

计算脂肪族胺、醇和醚的气相碱性的经验公式

本文给出计算脂肪族胺、醇和醚气相碱性的经验公式。由公式得出的PA值与ICR实验值接近。     

Gas permeation properties of poly(lactic acid)

The need for the development of polymeric materials based on renewable resources has led to the development of poly(lactic acid) (PLA) which is being produced from a feedstock of corn rather than petroleum. The present study examines the permeation of nitrogen, oxygen, carbon dioxide, and methane in amorphous films of PLA cast from solution. The properties of PLA are compared to other commodity plastics and it is shown that PLA permeation closely resembles that of polystyrene. At 30°C, N₂ permeation in PLA is 1.3 (10⁻¹⁰ cm³ (STP) cm/cm² s cmHg) and the activation energy is 11.2 kJ/mol. For oxygen the corresponding values are 3.3 (10⁻¹⁰ cm³(STP) cm/cm² s cm Hg) and 11.1 kJ/mol. The values for carbon dioxide permeation are 1.2 (10⁻¹⁰ cm³ (STP) cm/cm² s cmHg) and 6.1 kJ/mol. For methane values of 1.0 (10⁻¹⁰ cm³ (STP) cm/cm² s cmHg) and an activation energy of 13.0 kJ/mol are found. Studies with pure gases show that polymer chain branching and small changes in l:d stereochemical content have no effect on permeation properties. Crystallinity is found to dominate permeation properties in a biaxially oriented film. The separation factor for a CO₂/CH₄ mixed gas system is measured between 0 and 50°C and does not deviate significantly from the calculated ideal separation factor; at 0°C the separation factor is 16, a value that suggests continued studies of PLA as a separation medium are warranted.     

Processing of poly(lactic acid): Characterization of chemical structure, thermal stability and mechanical properties

The processing of poly(lactic acid) (injection and extrusion/injection) as well as annealing of processed materials were studied in order to analyze the variation of its chemical structure, thermal degradation and mechanical properties. Processing of PLA was responsible for a decrease in molecular weight, as determined by GPC, due to chain scission. The degree of crystallinity was evaluated by means of differential scanning calorimetry and X-ray diffraction. It was found that mechanical processing led to the quasi disappearance of crystal structure whereas it was recovered after annealing. These findings were qualitatively corroborated by means of FTIR. By analyzing ¹H NMR and ¹³C NMR chemical shifts and peak areas, it was possible to affirm that the chemical composition of PLA did not change after processing, but the proportion of methyl groups increased, thus indicating the presence of a different molecular environment. The thermal stability of the various materials was established by calculating various characteristic temperatures from thermograms as well as conversion and conversion derivative curves. Finally, the mechanical behaviour was determined by means of tensile testing (Young modulus, yield strength and elongation at break).