Local ordering in liquids: solvent effects on the hyperfine couplings of the cyclohexadienyl radical

Ordering of solvent molecules in the vicinity of a dipolar free radical affects its hyperfine coupling constants (hfcs). Specifically, it is demonstrated how the variation of the experimental methylene proton and muon hfcs of the muoniated cyclohexadienyl radical in several solvents and solvent mixtures of varying polarity can be accounted for by a dipole-dipole reaction field model that is based on the model of Reddoch and Konishi (J. Chem. Phys. 1979, 70, 2121) which was developed to explain the solvent dependence of the 14N hfc in the di-tert-butyl-nitroxide radical. Ab initio calculations were carried out with the cyclohexadienyl radical in an electric field to model the electric field arising from the electric dipole moments of the surrounding solvent molecules. An extension of the model that includes the dipole-quadrupole interaction can account for the larger hfc in benzene compared with that in octadecane, and it is predicted that the hfc will be proportional to the concentration of quadrupole moments to the 4/3 power. The influence of hydrogen bonding between the radicals' pi electrons and the OH groups of the solvent on the hfcs is also discussed. Comparison with gas-phase data permits a separation of vibrational effects and reveals that approximately 28% of the temperature dependence in water is due to increasing solvent disorder.     

Quantifying the intrinsic effects of two point mutation models of pro-pro-pro triamino acid diamide. A first-principle computational study

Stabilities and conformational properties of two Pro --> Thr point mutation models were computed at the B3LYP/6-31G(d) level of theory for the parent triamino acid diamide Pro-Pro-Pro (HCO-Pro-Pro-Pro-NH2). Geometrical parameters for the amino acid sequences, used in the molecular orbital computations for Pro-Pro-Thr and Pro-Thr-Pro, were retrieved from the Protein Data Bank. Thermodynamic functions (S, H, G) were computed for the fully optimized geometries. To assess the stabilization energetics of these mutant models, relative to the parent Pro-Pro-Pro reference conformer epsilon(L) epsilon(L) gamma(L), isodesmic reactions were constructed to calculate DeltaS, DeltaH, and DeltaG. The importance of intramolecular hydrogen bonds involving the -OH group of the Thr side chain, which emerged after the point mutations, was also examined to determine the internal stabilization of these peptide models. This study describes an approach to analyzing a point mutation at the center of a peptide chain and compares its stability to that of a point mutation at a terminal end in a small peptide model.     

On some relations between advancing, receding and Young's contact angles

Problems of experimental determination and theoretical verification of equilibrium contact angles are discussed basing on the literature data. A relationship between the advancing and receding contact angles versus the equilibrium contact angle is described and then verified using the literature contact angles determined on paraffin wax and polypropylene. Using the proposed relationship and experimentally determined equilibrium contact angles, obtained by plotting the advancing and receding contact angles versus the contact angle hysteresis or by applying vibration of the system liquid drop/solid surface, it is found that the same value of the surface free energy for paraffin wax is calculated from the contact angles of water and ethylene glycol. However, in the case of polypropylene some inconsistency appears between the equilibrium contact angles of the probe liquid used and the calculated surface free energy. More experimental data of the equilibrium contact angle are needed to verify further the relationship.     

Label-free voltammetric detection of single-nucleotide mismatches recognized by the protein MutS

MutS, a protein involved in DNA mismatch repair, recognizes mispaired and unpaired bases in duplex DNA. We have previously used MutS in an electrochemical double-surface technique (DST) for in-vitro detection of point mutations in DNA. The DST involved binding of unlabeled MutS to DNA heteroduplexes at the surface of magnetic beads followed by a highly sensitive electrochemical determination of the protein by measurement of a catalytic protein signal (peak H) at mercury electrodes. Detection of MutS using a peak resulting from oxidation of tyrosine and tryptophan residues of the protein at a carbon-paste electrode (CPE) was also possible but was approximately three orders of magnitude less sensitive. In this work we present an optimized technique for ex-situ voltammetric determination of MutS at a CPE. Choice of optimum experimental conditions (pH of supporting electrolyte, square-wave voltammetry settings, etc.) resulted in substantial improvement of the sensitivity of the assay, enabling detection of approximately 140 pg (1.6 fmol protein monomer) MutS in a 5-μL sample. The sensitivity was increased further by acid hydrolysis of the protein before measurement. The hydrolyzed protein was detectable down to 5 pg (approx. 56 amol) MutS in 5 μL solution. By using the DST combined with determination of the bound unlabeled MutS at the CPE we demonstrated selective interactions of the protein with single-base mismatches and discrimination among different base mispairs in 30-mer or 95-mer DNA duplexes. In agreement with previous studies, binding of the protein to the 30-mer substrates followed the trend G:T>>C:A>A:A>C:T>homoduplex. The electrochemical data were confirmed by use of an independent technique—a quartz-crystal microbalance for real-time monitoring of MutS interactions with DNA duplexes containing different base mispairs. By using the electrochemical DST a G:T mismatch was detectable in up to 1000-fold excess of homoduplex DNA.     

Esterification of palmitic acid with epichlorohydrin on anion exchange resin catalyst

The esterification reaction of palmitic acid with epichlorohydrin catalyzed by an anionic macroporous resin was studied. Purolite A-500 resin proved to be a very effective catalyst in the synthesis of 3-chloro-2-hydroxypropyl palmitate. The effects of certain parameters such as speed of agitation, catalyst particle size, catalyst loading, temperature, initial molar ratio between reactants on the rate of reaction were studied. It was found that the overall rate is intrinsically kinetically controlled. The structure of synthesized ester was confirmed by FTIR and ¹H NMR analyses.      

Transition state infrared spectra for the trans-->cis isomerization of a simple peptide model

Trans-->cis isomerization of N-methylacetylamide (MeCO-NHMe) has been studied at the G3MP2B3 level of theory and the vibration spectrum has been calculated as a function of the torsional mode of motion along the peptide bond. Noticeable spectral differences have been observed for the transition state interconnecting the cis and trans isomers.     

An improved synthesis of 4-chlorocoumarin-3-sulfonyl chloride and its reactions with different bidentate nucleophiles to give pyrido[1',2':2,3]- and thiazino[3',2':2,3]-1,2,4-thiadiazino[6,5-c]benzopyran-6-one 7,7-dioxides

An improved synthetic method affording 4-chlorocoumarin-3-sulfonyl chloride (4) in very good yield (ca. 85%) is reported. This compound was reacted with various bidentate nucleophiles such as 2-aminopyridines and 2-aminothiazoles in order to obtain substituted pyrido- and thiazino-1,2,4-thiadiazino-benzopyranone dioxides (potential anticancer and anti-HIV agents). These reactions occurred rapidly at room temperature giving yellowish precipitates, which are insoluble in common organic solvents, making the purification process challenging. Further investigation has shown that these fused heterocycles are not stable and decompose with opening of the 1,2,4-thiadiazine ring.     

Polarization invariance in beam propagation

It has been known for some time that the degree of polarization of a light beam may change on propagation, even in free space. In this Letter we derive sufficiency conditions for the degree of polarization of a beam generated by a uniformly polarized stochastic, electromagnetic source of a wide class to be the same throughout the far zone and in the source plane.     

Beam criterion for atmospheric propagation

A criterion is introduced for testing whether a beam retains its beam-like form after it propagates any particular distance through the turbulent atmosphere. The criterion applies to monochromatic as well as to partially coherent beams and is illustrated by examples.     

Definitions of the degree of polarization of a light beam

A necessary and sufficient condition is derived for certain ad hoc expressions that are frequently used in the literature to represent correctly the degree of polarization of a light beam.