Frequency exchange

Obviously the exchange frequency of the OH protons is small in comparison with the coupling constant (4.9 Hz), so coupling between the OH and CH2 protons also causes additional splitting of the H signals (5h = 3.58 and 4.72). 

Fig. 29. Observed and calculated 2H NMR spectra for the mesogenic groups of a) the nematic (m = 2), b) the smectic (m = 6) liquid crystalline polymer in the glassy state, showing the line shape changes due to the freezing of the jump motion of the labelled phenyl ring. The exchange frequency corresponds to the centre of the distribution of correlation times. Note that the order parameters are different, S = 0.65 in the frozen nematic, and S = 0.85 in the frozen smectic system, respectively...

Firstly, we are going to demonstrate how branch interference may be taken into account within the quasi-classical impact theory. Then we shall analyse a quasi-static case, when the exchange frequency between branches is relatively small. An alternative case, when exchange is intensive and the spectrum collapses, has been already considered in Chapter 2. Now it will be shown how the quasi-static spectrum narrows with intensification of exchange. The models of weak and strong collisions will be compared with each other and with experimental data. Finally, the mutual agreement of various theoretical approaches to the problem will be considered. 

Human cells exposed to various nickel compounds have an increased frequency of chromosomal aberrations, although sister chromatid exchange frequency is unaffected. Cells from nickel refinery workers exposed to nickel monosulfide (0.2 mg Ni/m3) or nickel subsulfide (0.5 mg Ni/m3) showed a significant increase in the incidence of chromosomal aberrations (Boysen et al. 1980 WHO 1991 USPHS 1993). No correlation was evident between nickel exposure level and the frequency of aberrations (USPHS 1993). 

Edwards, A.J., Moon, E.Y., Anderson, D. and McGregor, D.B. (1993). The effect of simultaneous exposure to bromodeoxyuridine and methyl methansulphonate on sister chromatid exchange frequency in culture human lymphocytes and its mutation research. Mutation Res. 247 117-125. 

Unscheduled DNA synthesis (UDS) in hepatocytes was not increased in rats exposed to chloroform at gavage doses 400 mg/kg in oil (Mirsalis et al. 1982). Exposure to 200 mg/kg/day chloroform in oil by gavage for 4 days increased sister chromatid exchange frequency in bone marrow cells of mice (Morimoto and Koizumi 1983). Other genotoxicity studies are discussed in Section 2.5. 

Metabolism and genetic toxicity have been reported to differ with the isomer of nitro-toluene. p-Nitrotoluene was not mutagenic in bacterial assays, but it did increase sister chromatid exchange frequencies and chromosomal aberrations in vitro-, in vivo it did not increase the frequency of micronuclei in bone marrow of treated rodents. Similar findings were reported for the ortho isomer, except that it did not induce chromosomal aberrations in vitro. Only the ortho isomer induces DNA excision repair in the in vivo-in vitro hepatocyte unscheduled DNA synthesis assay. Furthermore, ort/jo-nitrotoluene binds to hepatic DNA to a much greater extent than meta- or para-nitrotoluene, and investigators suggest that it may act similarly to the rodent hepatocarcino-gen 2,6-dinitrotoluene. ... 

In one of two studies, di(2-ethylhexyl) phthalate induced a small increase in sister chromatid exchange frequencies in Chinese hamster ovary cells cultured without but not with exogenous metabolic activation. In other studies conducted only without metabolic activation, it caused no increase in sister chromatid exchanges in either Chinese hamster Don cells or rat liver RL4 cells. Di(2-ethylhexyl) phthalate did not induce micronuclei in Chinese hamster ovary cells or in cultured rat hepatocytes, whereas the induction of micronuclei by di(2-ethylhexyl) phthalate in Syrian hamster embryo cells has been reported. 

Wiencke, J.K. Kelsey, K.T. (1993) Susceptibility to induction of chromosomal damage by metabolites of 1,3-butadiene and its relationship to spontaneous sister chromatid exchange frequencies inhuman lymphocytes. In Sorsa, M., Peltonen, K., Vainio, H. Hemminki, K., eds, Butadiene and Styrene Assessment of Health Hazards (lARC Scientific Publication No. 127), Lyon, lARC, pp. 265-273... 

Evans, E.L. Mitchell, A.D. (1981) Effects of 20 coded chemicals on sister chromatid exchange frequencies in cultured Chinese hamster cells. In de Serres, F.J. Ashby, J., eds. Evaluation of Short-Term Tests for Carcinogens. Report of the International Collaborative Program (Progress in Mutation Research. Vol. 1), Amsterdam, Elsevier, pp. 538-550... 

In a single study, epichlorohydrin bound to DNA of mice and rats treated in vivo. One study reported that sister chromatid exchanges were induced in the bone marrow of partially hepatectomized CBA/J mice treated with epichlorohydrin by a single intraperitoneal injection. Sister chromatid exchange frequencies in mice that did not receive partial hepatectomy before treatment with epichlorohydrin were comparable to the control frequencies. One of two studies reported that epichlorohydrin induced chromosomal aberrations in mouse bone marrow. Positive results were also reported for epichlorohydrin in the mouse host-mediated assay in one of three studies. In single studies, epichlorohydrin caused sperm head abnormalities in rats but not mice. It did not induce micronuclei or dominant lethal mutations in mice in vivo. 

Richer et al. (1993) exposed five male volunteers to 50 ppm [188.5 mg/m ] toluene in a controlled exposure chamber for 7 h per day for three days on three occasions at two-week intervals. Blood samples were taken before and after each three-day exposure. No effects upon sister chromatid exchange frequencies were observed. 

Popp, W., Vahrenholz, C., Yaman, S.. Muller, C., Muller, G, Schmieding, W.. Norpoth, K. Fahnert, R. (1992) Investigations of the frequency of DNA strand breakage and cross-linking and of sister chromatid exchange frequency in the lymphocytes of female workers exposed to benzene and toluene. Carcinogenesis, 13, 57-61... 

Fig. 8.12 NMR line shapes for a nucleus undergoing exchange between two different environments that are equally populated. The exchange frequency vtxch increases from left to right.

For nonmetallic substances, the electrons cannot move as freely as in the case of metals because their energy bands are essentially completely full or empty. The electrical conductivity in nonmetallic materials is dominated by another mechanism, i.e., the defect mechanism, instead of electron conduction. In ionic crystals such as salts (e.g., sodium chloride), two types of ions, cations and anions, are driven to move by the electrical force qE once an electrical field is applied. The ions can move only by the defect mechanism that is, they exchange position with a vacancy of the same type. At the room temperature, the fraction of vacancies for salt is very small (of the order of 10-17) with low exchange frequency (of the order of 1 Hz) so that electrical conductivity is extremely low. Although impurities and high temperature can affect electrical conductivity by a large factor, nonmetallic materials generally have very low electrical conductivity and these substances are widely used as electrical insulators. 

For a phase-separated region to exist, lipids have to move into and out of various phases. The lateral diffusion constant in liquid crystalline bilayers is about l(T8cm2/s, which corresponds to an exchange frequency between lipid-lipid nearest neighbors of about 106/s. A necessary precondition for the detection of phases by NMR technique is that the proportion of observable species in the phase is sufficiently large. 

Nagaya T. 1986. No increase in sister-chromatid exchange frequency in lymphocytes of chromium platers. Mutat Res 170 129-132. 

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