Other studies

Other Studies.—Studies on the alkylation of homopolynucleotides by a number of alkylating agents, some of which are potent carcinogens (e.g. ethylnitrosoguanidine, methyl- and ethyl-nitrosourea) indicate that an appreciable degree of alkylation takes 

At this pH, the phosphotriesters formed mainly lose alcohol to re-form the phosphodiester, but at higher pH, hydrolysis with chain scission takes place. Alkylation also proceeds at nucleophilic centres on the base and sugar moieties, and the modification likely to cause most damage in vivo has not been defined. Studies involving aflatoxin Bi and benzanthracene derivatives have tended to concentrate on the modification of the bases, to the exclusion of possible effects at the phosphate group, but there is good evidence that the latter should not be ignored.  

Other Studies.— The kinetics and mechanisms of reactions of tptz (5) complexes of cobalt(ii), copper(ii), and nickel(u) with water and with hydroxide have been established and compared. Covalent hydrates are believed to be important intermediates in aquation of the [M(tptz)(OH2)3] + complexes. Kinetics of aquation of the anions [M(acac)a] (M = Co, Cr, or Ru) have been studied in pulse radiolysis experiments. All three steps were monitored for the cobalt(n) complex, but the first step for the chromium(ii) complex was too fast to follow and, predictably, all steps for the ruthenium(ir) complex were too slow to follow by this technique. The mechanism of acac loss is thought to involve equilibrium 

Lawrance and S. Suvachittanont, Inorg. Chim. Acta Lett., 1979, 32, L13. 

5 Other Studies. - In addition to these studies, other QM/MM studies include the study of catalysis by HIV protease,171-173 the mechanism of carbonic anhydrase,174,175 tyrosine phosphatase,176,177 papain,178,179 citrate synthase180 and the mechanism of serine protein kinases.181 

145 fs was used in a simulation of total length 6 ps, of which the last 3 ps were used for data analysis. Experimentally it is found that the a-conformer is more stable in the gas phase due to the anomeric effect but the / -conformer is more stable in solution. The ratio is about 36 64% corresponding to about 

35 kcal mol-1. The total simulation length was too short to study conformational changes or to calculate the free energy difference between the two anomers but was sufficiently long to analyse the water structure. For the /2-anomer it was seen that the water molecules flow more freely, i.e. in a more disordered manner, around the anomeric site while for the a-anomer the water molecules were bound more tightly and in a more ordered manner. This may well underlie the stability of the / -anomer in aqueous solution (see also Section 6). 

Benjamin and Wilson used classical molecular dynamics to study the photodissociation of model ICN in the gas phase and Xe solution. One of the novel features of this study involved the treatment of molecular dynamics on coupled surfaces in solution using the Meyer-Miller method.i For sufficiently high excitation (Benjamin and Wilson use 266 nm), ICN may be excited to a linear state that correlates with an I product. However, this excited state may couple to another state that has a bent configuration and correlates to ground state I. Benjamin and Wilson developed an extension of the Meyer-Miller method, which takes into account the interaction between the ICN and the Xe solvent. The result is a Hamiltonian that describes motion on a self-consistent average of the two excited state potentials. 

Benjamin and Wilson discuss many more features of this ICN photodissociation than we have space to summarize. Throughout their work is a theme which we previously discussed there is a timescale in the solution phase dynamics during which there is little deviation from what is observed in the gas phase. In this system, the rapidity with which the choice of final state is made gives an extra importance to this gas phase epoch. Furthermore, as Benjamin and Wilson stress, there are close parallels between photodissociation processes, which are more easily studied experimentally, and the thermally activated process in that gas-phase like periods as well as periods of dissipation of energy back to the solvent occur in both, and also that the time inverse of photodissociation can be compared approximately with the barrier climbing of the reactants in a thermally activated process. 

A different type of process has been studied by Ohmine,i the photoisomerization of ethylene in either Ar or water solvents. The ethylene begins on the planar ground state and is excited through a Franck—Condon process to the triplet excited state. This excited state has a minimum energy configuration that is twisted rather than planar. The excess energy present in the system in the initial configuration on the excited state surface (what Ohmine calls the 

A novel feature of Ohmine s calculation involved the calculation of the intramolecular ethylene forces on the triplet state surface.Rather than attempting to determine an analytic form for the potential energy surface from which the necessary forces could be calculated, Ohmine calculated the ethylene internal forces at each time step of the molecular dynamics from a semiempiri-cal quantum mechanical (MINDO/3) energy gradient for the triplet surface. This method will not produce as accurate forces as a fully ab initio treatment would, but it may suffice for the qualitative arguments of the type that Ohmine has presented. 

Ohmine has observed that the dissipation of the excess ethylene energy occurs considerably faster in water than it does in liquid Ar. He has shown that this rate of energy loss is consistent with the overlap between the velocity power spectrum of the ethylene motions and the power spectrum of the forces that the solvent exerts on the ethylene molecule. The water solvent is able to exert forces that have a much greater range of frequencies than do the forces from the Ar solvent, and is therefore able to dissipate energy more efficiently from the higher frequency motions of the ethylene molecule. (A similar effect can be seen in simulations of the vibrational relaxation of diatomic molecules in rare gas solution by Chesnoy and Weis o and by Whitnell, Wilson, and Hynes in water.13 135) Ohmine also found when the depth of the Ar-Ar attractive well was increased by a factor of 50, that the rate of dissipation of energy into the solvent increased markedly, as did the power spearum of the forces that the solvent exerts at all frequencies. 

Ito et al. (1995) examined the combined dietary administration to rats of 19 organophosphate pesticides and 1 organochlorine pesticide, aU permitted for use in Japan, each at its ADI level. The dietary exposure at this level did not enhance the development of diethyl nitrosamine initiated pre-neoplastic lesions whereas at 100 times the ADI, the number and area of lesions were increased. The authors concluded that the study provided direct support for the present use of the safety factor approach in the quantitative hazard evaluation of pesticides. 

In rats treated orally by gavage with 100- and 1000-fold the ADI of three pesticides, indication of immune suppression was found at 1000-fold the ADls. No immunological effects were reported at the 100-fold ADI level as well as after treatment with the individual pesticides. The authors suggest that one of the pesticides had a slight antagonistic effect on the two others at high doses (Akay et al. 1999). 

Wade et al. (2002) studied effects (systemic, immune, and reproductive) of a 70-day exposure to a complex mixture of persistent contaminants in sexually mamre rats. Each chemical was included in the mixture at the MRL, RfD, or TDI as determined by ATSDR or US-EPA, and the rats were exposed to the mixture at 1, 10, 100, and 1000 times the estimated safe levels daily for 70 days (see Table 10.7). 

Composition of Contaminant Mixture Administered to Male Rats 

The authors concluded that the mixture induced effects on the liver and the kidney, and on the general metabolism at high doses but caused only minor effects on immune function, reproductive hormone levels, or general indices of reproductive function measures. The results suggested that additive or synergistic effects of exposure to contaminants resulting in residue levels representative of contemporary human tissue levels are unlikely to result in adverse effects on immune function or reproductive physiology in male rats. 

The apparent anomaly in Porter s results probably reflected the fact that he was oxidizing triplet CF3COCF3 rather than CF3 radicals. That Porter failed to find CO is consistent with this explanation. To check this hypothesis Heicklen71 photolyzed a mixture of 86 torr of CF3COCF3 and 591 torr of 02 with radiation mainly at 3130 A. These conditions favor triplet-state formation.30 As products Heicklen found CFaO with a quantum yield of 0.030, CF3CFO with a quantum yield of about 9 x 10  

Franke and Schumacher61 subsequently performed the analogous experiment in the gas phase with CCl3Br and either Br2 or Cl2 in the presence of 02. The reaction scheme was 

The oxidation of perhalomethyl radicals always leads to the respective carbonyl halide as the sole oxidation product. There are two possible routes. The first is the direct step 

At this time, the details of the oxidation of perhalomethyl radicals are not known. This author feels that the evidence favors reaction (3-7b), but considerably more work is needed to establish the mechanism unambiguously. 

Tang et al fabricated CNCs (4-24% v/v) and epoxy resin-based thermosetting thin film nanocomposites by using solvent casting method followed by curing. CNCs with aspect ratios of 10 and 84 were isolated from cotton and trmicates, respectively. The 

Elanthikkal et al. investigated the effect of CNCs (from banana waste fibers) (0-10% w/w) on the morphological, thermal, and mechanical properties of poly(ethylene-co-vinyl acetate) (PEVA)/cellulose composites. The produced composites showed superior thermal and mechanical properties as compared to that of the EVA copolymer alone. In this study, three different theoretical models (Halpin-Tsai model, the Kemer model and the Nicolais-Narkis model) have been employed to compare the results with observation of tensile data from mechanical tests. Here, experimental results showed better agreement with the prediction given by the use of the Halpin-Tsai model, assuming that there is perfect wetting of filler by the polymer matrix [217]. 

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Other studies have also been made on the dynamics around a conical intersection in a model 2D system, both for dissociahve [225] and bound-state [226] problems. Comparison between surface hopping and exact calculations show reasonable agreement when the coupling between the surfaces is weak, but larger errors are found in the shong coupling limit. 

I hc choice of the O0 method depends on scvcrni faclors including your previous experience and preferences. If you want Lo compare the results lo other studies, you must use ihe same semi-empirical melhod. Since some tn elli ods can converge much more quickly than olh ers, you m igh l wan t to use a fast meth od lo obtain an approximation of the final answer, and then a more accurate method for the final lesulL,... 

Likewise, Grieco, while working with amphiphile-like reactants, observed an enhanced preference for endo-adduct in aqueous solutions, which he attributed to orientational effects within the micelles that were presumed to be present in the reaction mixture ". Although under the conditions used by Grieco, the presence of aggregates cannot be excluded, other studies have clearly demonstrated that micelle formation is not the reason for the improved selectivities . Micellar a peg tes even tend to diminish the preference for endo adduct. ... 

It is worth noting here that the results of some other studies of aromatie substitutions, sueh as the Friedel-Crafts benzylation and iso-propylation of alkylbenzenes, and the bromination of alkylbenzenes with bromine, eatalysed by ferrie ehloride, are under suspicion as depending upon slow mixing. As regards halogenation eatalysed by Lewis aeids, positive evidenee to support this eritieism has been obtained. ... 

These and other studies of the relative substituent effects of X and CH X in nitration were considered in terms of the transmission factor a of the methylene group. To avoid complications from conjugative interactions, attention was focussed mainly on substitution at the meta-position, and ct was defined in terms of partial rate factors by the equation ... 

Numerous other studies have shown that alkyl groups directly attached to the pos itively charged carbon stabilize a carbocation Figure 4 13 illustrates this generalization for CH3+ CH3CH2 " (CH3)2CH+ and (CH3)3C" Among this group CH3+ is the least stable and (CH3)3C the most stable... 

Tobacco smoke contains a variety of air pollutants. In a survey of 80 homes ia an area where the outdoor TSP varied between 10—30 lg/m, the iadoor TSP was the same, or less, ia homes having ao smokers. la homes having oae smoker, the TSP levels were betweea 30—60 lg/m, while ia homes having two or more smokers, the levels were betweea 60—120 lg/m (64). la other studies, iadoor TSP levels exceeding 1000 lg/m have beea fouad ia homes with aumerous smokers. la additioa to TSP, burning tobacco emits CO, NO formaldehyde [50-00-0] bea2opyreaes, nicotine [54-11-5] pheaols, and some metals such as cadmium [7440-43-9] and arsenic [7440-38-2] (65). 

In the area of municipal and iadustrial wastewater treatment, the principal environmental issue is the toxicity of residual flocculating agents ia the effluent. Laboratory studies have shown that cationic polymers are toxic to fish because of the iateraction of these polymers with giU. membranes. Nonionic and anionic polymers show no toxicity (82,83). Other studies have shown that ia natural systems the suspended inorganic matter and humic substances substantially reduce the toxicity of added cationic polymer, and the polymers have been used successfully ia fish hatcheries (84—86). Based on these results, the EPA has added a protocol for testing these polymers for toxicity toward fish ia the presence of humic acids (87). The addition of anionic polymers to effluent streams containing cationic polymers to reduce their toxicity has been mentioned ia the patent Hterature (83). 

Other studies which have been reported describe unusual chemistry such as HSO F—Nb(S02F) systems (42). Also the unique properties of fluorosulfuric acid have been found to provide unusual solvent systems, which can vary properties such as acidity, heats of solution, enthalpy, and heats of neutralization (43). 

Oral LD q levels have been deterrnined in the mouse at 470 mg/kg (21) and the guinea pig at 7750 mg/kg (22). Several other studies (23—25) have shown that large quantities of both synthetic and natural glycerol can be adniinistered orally to experimental animals and humans without the appearance of adverse effects. Intravenous adniinistration of solutions containing 5% glycerol to animals and humans has been found to cause no toxic or otherwise undesirable effects (26). 

The stoichiometric reaction of y -diisopropenylbenzene [3748-13-8] with two moles of j -butyUithium in the presence of triethylamine has been reported to produce a useful, hydrocarbon-soluble dilithium initiator because of the low ceiling temperature of the monomer (78,79) which is analogous in stmcture to a-methylstyrene however, other studies suggest that oligomerization occurs to form initiators with functionahties higher than two (80). 

Infrared absorption studies have shown that correlates with an absorption at 3 p.m associated with an OH-stretching frequency (20). Indeed, infrared absorption provides a useful tool for evaluation in rapid production quaUty control. Infrared and other studies show that degradation is caused by proton inclusion in the grown quartz. 

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