Isotope shift carbon

The overall biosynthetic pathway to the tetracychnes has been reviewed (74). Studies (75—78) utilising labeled acetate and malonate and nmr analysis of the isolated oxytetracycline (2), have demonstrated the exclusive malonate origin of the tetracycline carbon skeleton, the carboxamide substituent, and the folding mode of the polyketide chain. Feeding experiments using [1- 02] acetate and analysis of the nmr isotope shift effects, led to the location of... 

Figure 5.3. Model describing expected carbon isotopic shifts in direction of matrix values for browsers and grazers. Large arrows represent large shifts, small arrows the converse.

Carbon isotope fractionation was examined during the aerobic degradation of TCE by Burkholderia cepacia strain G4 that possesses toluene monooxygenase activity (Barth et al. 2002). There were substantial differences in values of isotope shifts during degradation, from 57 to 17 ppm, and when the data were corrected to correspond to the same amount of substrate reduction the Releigh enrichment factor was 18.2. 

The extreme negative values for the carbon isotope shifts (up to around 5-60 in some of the banded iron(ll)-formations), strongly indicate that microbial processes were involved in the formation of these deposits (Canheld et al. 2000). One explanation of the low values is that the bacteria involved could have used carbon sources that were already metabolized, e.g., by methanogens, and thus already had undergone a significant isotope shift (i.e., 5 values for methane are already negative however, the values for the associated C02/carbonates are positive, between -i-5 to -i-15%o) (Coleman et al. 1993 Horita and Berndt 1999). 

The ultraviolet absorption spectrum of formaldehyde consists of many sharp discrete bands of Doppler width. The isotopic shifts due to C and O atoms are sometimes 5 to 10 cm-1 in the 3000 to 3100 A region [see Moore (715)]. Hence, it is possible to selectively excite a specific carbon or oxygen isotopic species in mixtures of other isotopic species. 

INADEQUATE spectrum, however, may become difficult, even in the case of smaller molecules At a tertiary carbon for example, three doublets will appear, usually overlapping due to similar coupling constants. Further, isotope shifts and AB effects will remove the 13C — 13C satellites from the shift position of the known 13C 12C signal. Thus, it was not until a second dimension was introduced to the experiment - that INADEQUATE became a practical method of structure elucidation. 

The AB and AX systems of all 13C —13C bonds appear in one spectrum when the INADEQUATE pulse sequence (Fig. 2.48) is applied. Complete interpretation usually becomes difficult in practice due to signal overlapping, isotope shifts and AB effects (Section 2.9.4). A separation of the individual 13C— 13C two-spin systems by means of a second dimension would be desirable. It is the frequency of the double quantum transfer (d e) in Fig. 2.48 which introduces a second dimension to the INADEQUATE experiment. This double quantum frequency vDQ characterizes each 13CA — I3CX bond, as it depends on the sum of the individual carbon shieldings vA and vx in addition to the frequency v0 of the transmitter pulse located in the center of the spectrum if quadrature detection is applied [69c, 71] ... 

The Clemmensen reduction was accompanied by D/H exchange via acid-catalysed enolization of the carbonyl group, resulting in the production of deuteriated compounds 57 and 58 with various numbers of deuterium atoms. The mixture of the compound 59 obtained by the Wolf-Kishner reduction was predominantly labelled at position 2. This has been proved by the 13C-NMR spectrum. Isotope shift and loss intensivity on substituted C(2) carbon signals have been observed54,55. 

The isotope shift observed with chemisorbed C130 indicated that the force constant, k, of the platinum-carbon stretching vibration was 4 X 106 dynes/cm. and the carbon-oxygen force constant, k2, was 16 X 106 dynes/cm. Use of these values of the force constants in the equation (60)... 

Quade J, Cerling TE, Bowman JR (1989b) Development of the Asian Monsoon revealed by marked ecological shift in the latest Miocene in northern Pakistan. Nature 342 163-166 Quade J, Cater JML, Ojha TP, Adam J, Harrison TM (1995) Dramatic carbon and oxygen isotopic shift in paleosols from Nepal and late Miocene environmental change across the northern Indian sub-continent. Geol Soc Am Bull 107 1381-1397... 

We speculate that process changes could cause changes in sequence distribution. C NMR spectroscopy has been used to study similar copolymers (10,11,12) and hence should be of value in correlating process with sequence distribution, nmr is a means of looking at changes in the electronic environment of nucleii of the isotope of carbon, 13C. There are a number of texts on % NMR spectroscopy and its application to organic molecules, and polymers (13,14,15). The chemical shifts which one observes as characteristic of different carbons can be caused by... 

Whereas introduction of, 3C or deuterium onto carbon atoms requires more-or-less laborious syntheses, O-deuteration of hydroxyl groups or N-deuteration of amino groups is readily achieved by exchange of protons by deuterons with D20. In the deuterated carbohydrates thus obtained, only small isotopic-shifts are observed in the l3C-n.m.r. spectra however, when measured under appropriate conditions, these shifts are very useful for the assignment of 13C-sig-nals.50,57-61... 

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