Small-angle x-ray scattering from solution of rodlike particles with length distribution

The scattering function of rods with a constant radius, 8 Å, and a length distribution of the Schulz–Zimm type was calculated on a computer as a model of helical synthetic polypeptide. The influence of length and length distribution on the scattered intensity of small-angle x-ray scattering was clarified. As length grows and length distribution broadens it is difficult to obtain reliable values of molecular weight and radius of gyration from a Zimm plot. The influence of length distribution on the mass per unit length, M_q, and the radius of gyration of the cross section, 〈S〉^1/2, from the Guinier plot of the cross-sectional factor decreases as the length increases, and reliable values of M_q and 〈S〉^1/2 can be obtained even for rods with wide distribution for rods more than 600 Å long. In particular, it is pointed out that the value of 〈S〉^1/2 is little influenced by length and length distribution.     

Rate of spontaneous deactivation of styryl and α-methylstyryl carbanions

The deactivation of α-methylstyryl and styryl carbanions followed by spectral change and titration was studied kinetically. First-order rate constants were obtained for the deactivation of oligo-α-methylstyryl lithium, disodium, and dipotassium in tetrahydrofuran and of polystyryl potassium in varying degrees of polymerization in benzene that contained a small amount of tetrahydrofuran and n-hexane. It was observed that the addition of cryptate [222] exerts a significant effect on the system of oligo-α-methylstyryl disodium in tetrahydrofuran. The effect of dilution, degree of polymerization of the polymer, and counterions on the rate of deactivation of carbanions was discussed.     

Krafft temperature and enthalpy of solution of<Emphasis Type="Italic"> N</Emphasis>-acyl amino acid surfactants and their racemic modifications: effect of the counter ion

The Krafft temperatures and enthalpies of solution of N-hexadecanoyl alaninate and valinate, and N-tetradecanoyl phenylalaninate were obtained from differential scanning calorimetry. The Krafft temperature of N-acyl amino acid surfactant increased with decreasing size of the counter ion, with some exceptions. The enthalpy of solution was endothermic and increased with decreasing size of the counter ion except for the cases of lithium salt. The results showed that the L-L interaction in the solid state of N-hexadecanoyl amino acid surfactant salt was superior to the D-L interaction for both the alanine and valine systems when the counter ion size increased. However, the D-L interaction was still advantageous for the phenylalanine system with Cs⁺ as a counter ion. Both Fourier transform infrared spectroscopy studies and theoretical calculations suggested that the difference in magnitudes of the interactions between peptide and counter ion was a dominant factor for the chiral effect.     

Synthesis and characterization of titanium alkyl, oxo, and diene complexes bearing a SiMe2-bridged phenoxy-cyclopentadienyl ligand and their catalytic performance for copolymerization of ethylene and 1-hexene

A series of titanium complexes bearing a SiMe₂-bridged phenoxy-cyclopentadienyl ligand were synthesized and characterized, and their catalytic behavior for copolymerization of ethylene and 1-hexene was investigated. Treatment of dimethylsilyl(2,3,4,5-tetramethylcyclopentadienyl)(3-tert-butyl-5-methyl-2-phenoxy)-titanium dichloride (1) with appropriate nucleophiles afforded dimethoxy complex 2, dimethyl complex 3, and dibenzyl complex 4. Standing a toluene solution of 2 in air afforded a dinuclear μ-oxo complex 5 as a single isomer. 1,3-Diene complexes 6-8 were prepared by reaction of 1 with the corresponding 1,3-dienes in the presence of 2 equiv. of n-BuLi. X-ray analysis of 1,4-diphenyl-1,3-butadiene complex 6 revealed that the diene ligand coordinates to titanium in s-cis fashion with a prone orientation. The newly prepared titanium complexes were applied to copolymerization of ethylene and 1-hexene upon activation with AlⁱBu₃ and [C₆H₅NMe₂H][B(C₆F₅)₄]. It was found that the alkyl complexes 3-4 and the diene complexes 6-8 showed higher activities than 1 at elevated temperature.     

Structure and conductance of aromatic and aliphatic dithioacetamide monolayers on Au(111)

The structure and electrical properties of self-assembled monolayers of cyclic aromatic and aliphatic dithioacetamides (1,4-bis(mercaptoacetamido)benzene and 1,4-bis(mercaptoacetamido)cyclohexane) and of mixed dithioacetamide/alkanethiol monolayers are characterized by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and contact angle goniometry. Both dithioacetamides are found to pack densely on Au(111), however the monolayers are poorly ordered as a result of hydrogen bond formation between the amide groups. The coassembly and the insertion method are compared for the formation of mixed dithioacetamide/alkanethiol monolayers. By coassembly, islands of dithioacetamides in a dodecanethiol matrix can only be obtained at a low dithioacetamide/dodecanethiol concentration ratio in solution (1/10) and by thermal annealing of the resulting monolayers. Small and well defined dithioacetamide domains are realized by insertion of dithioacetamides into defect sites of closely packed octanethiol monolayers. These domains are used to determine the molecular conductance by means of STM height profiles and molecular lengths resulting from density functional theory (DFT) calculations. The difference in the tunneling decay constant beta measured for aromatic dithioacetamides (beta = 0.74-0.76/A) and for aliphatic dithioacetamides (beta = 0.84-0.91/A) highlights the influence of the conjugation within the cyclic core on molecular conductance.     

A role for internal water molecules in proton affinity changes in the Schiff base and Asp85 for one-way proton transfer in bacteriorhodopsin

Light-induced proton pumping in bacteriorhodospin is carried out through five proton transfer steps. We propose that the proton transfer to Asp85 from the Schiff base in the L-to-M transition is accompanied by the relocation of a water cluster on the cytoplasmic side of the Schiff base from a site close to the Schiff base in L to the Phe219-Thr46 region in M. The water cluster present in L, formed at 170 K, is more rigid than that at room temperature. This may be responsible for blocking the conversion of L to M at 170 K. In the photocycle at room temperature, this water cluster returns to the site close to the Schiff base in N, with a rigid structure similar to that of L at 170 K. The increase in the proton affinity of Asp85, which is a prerequisite for the one-way proton transfer in the M-to-N transition, is suggested to be facilitated by a structural change which disrupts interactions between Asp212 and the Schiff base, and between Asp212 and Arg82. We propose that this liberation of Asp212 is accompanied by a rearrangement of the structure of water molecules between Asp85 and Asp212, stabilizing the protonated Asp85 in M.     

Effects of Si−Al composition on the fluorescence spectra of 1-naphthol during the sol-gel-xerogel transitions

A systematic study has been carried out on the characteristic changes in the fluorescence spectra of 1-naphthol doped in the sol-gel-xerogel transition systems comprised of tetraethyl orthosilicate and diisobutoxyaluminium triethylsilicate catalyzed by a small amount of HCl, NH₄OH, as well as under uncatalyzed conditions. In the systems containing large amounts of silicon, the fluorescence of 1-naphthol shifts to the red (a predominant emission from the ¹L_a state) during the first stage of the reaction. This red shift indicates an increase in the polarity of the matrix surrounding 1-naphthol. In the second stage of the reaction, the spectrum shifts to the blue (a predominant emission from the ¹L_b state), reflecting an increase in the micro-viscosity around 1-naphthol. In the systems containing relatively large amounts of aluminum, however, the spectrum just after mixing shows a larger red shift than that originating from the ¹L₂ emission. This large red-shifted fluorescence reflects the formation of a complex between 1-naphthol and the −O−Al−O−Si−O-network. The spectrum then shifted to the blue. The spectral behaviours observed indicate that there is a large and dynamic molecular-level change in the physicochemical properties of the matrix surrounding the 1-naphthol molecules during the sol-gel-xerogel transitions of the systems while the gelation phenomenon reflects macroscopic inflexibility although it is completely different from the restriction of movement at the molecular level.     

High chelation control of three contiguous stereogenic centers in the Reformatsky reactions of indium enolates with alpha-hydroxy ketones: unexpected stereochemistry of lactone formation

[reaction: see text] A boat-type of chelated bicyclic transition state involving highly diastereoselective construction of three contiguous stereogenic centers in the Reformatsky reaction of indium enolates with alpha-alkoxy/hydroxy ketones is proposed. alpha-Hydroxy ketones with indium enolates furnished highly diastereoselective lactones, while alpha-alkoxy ketones gave acyclic esters in moderate selectivities. X-ray structure analyses of key products unequivocally revealed the unexpected stereochemistry of products and the reaction pathway.     

Adsorption of anions to zirconium(IV) and titanium(IV) chemically immobilized on gel-phase

Adsorption behaviors of 25 anions to zirconium(IV) and titanium(IV) chemically immobilized on a gel-phase were studied by ion chromatography and by batch adsorption experiments. The affinities of Zr(IV) to iminodiacetate as an anchoring group and to anions as a sample are much stronger than those of Ti(IV). On a Zr(IV) column, fourteen anions showed no retention, four anions showed pH-dependent retention in a low pH region, and seven anions were irreversibly adsorbed at pH <7. In the last group, fluoride was adsorbed both by the ligand exchange mechanism and the addition mechanism, while phosphate, arsenate and selenite were only by the ligand exchange mechanism. The structures of the adsorbed species are discussed.     

Direct coupling reaction between alcohols and silyl compounds: enhancement of Lewis acidity of Me3SiBr using InCl3

The combination of InCl3 and Me3SiBr provided an enhanced Lewis acid system that can be used to promote a wide range of direct coupling reactions between alcohols and silyl nucleophiles in non-halogenated solvents, such as hexane or MeCN. The enhanced Lewis acidity of this system was measured by the 13C NMR in terms of the coordination to an alcohol. Moreover, the interaction between Me3SiBr and the In(III) species was revealed by 29Si NMR spectral analysis. Highly chemoselective allylations toward a hydroxyl moiety over ketone and acetoxy ones have been demonstrated.