Tumbling frequency

Methyl radicals formed on a silica gel surface are apparently less mobile and less stable than on porous glass (56, 57). The spectral intensity is noticeably reduced if the samples are heated to —130° for 5 min. The line shape is not symmetric, and the linewidth is a function of the nuclear spin quantum number. Hence, the amplitude of the derivative spectrum does not follow the binomial distribution 1 3 3 1 which would be expected for a rapidly tumbling molecule. A quantitative comparison of the spectrum with that predicted by relaxation theory has indicated a tumbling frequency of 2 X 107 and 1.3 X 107 sec-1 for CHr and CD3-, respectively (57). 

Morris and coworkers have studied trans-[OsH(H2)(depe)2] (depe = diethylpho-sphinoethane) using several spectroscopic methods, and in particular by Ti(H2) inversion-recovery methods in solution between 190 and 300 K [200]. They conclude that if the motion of the H2 ligand is much faster than the tumbling frequency, Eq. (11) applies. If the motion of the H2 ligand is slower than the tumbling frequency. 

Plots of J o>) versus oj for different values of Tc are shown in Fig. 8.1. The values of rc should be compared with the reciprocal of the nuclear Larmor frequency w0. For either a very short or very long rc the value of J 0 is relatively small. J(to) reaches its maximum when rc = l/o)0, that is, when the average molecular tumbling frequency is equal to the nuclear precession... 

Bacteria sense spatial gradients of chemoattractants by measurements separated in time. A bacterium sets off in a random direction and, if the concentration of the chemoattractant has increased after the bacterium has been swimming for a period of time, the likelihood of tumbling decreases and the bacterium continues in roughly the same direction. If the concentration has decreased, the tumbling frequency increases and the bacterium tests other random directions. The... 

Figure A5-1 Plot of spectral density vs. logarithm of tumbling frequency.

The following equation describes the spectral density in terms of the tumbling frequency and the correlation time ... 

Several attempts have been made to relate the surface stability of the CH radical with its tumbling frequency as determined by the dependence of the linewidth on the nuclear spin quantum number (52). Thus Gardner and Casey... 

It is obvious that a more consistent set of data for tumbling frequency of the CHj radical on the surface would be highly desirable. 

The tumbling frequency determination must be resolved with respect to the second-order splitting. The values of the parameters used in the Klvelson equation or its equivalent must be stated if direct comparison between studies is to be possible. 

Again, in solution this motion will be very Jerky and severely damped by collisions with surrounding solvent molecules (see rotational diffusion. Section 4.5). However, this sort of tumbling frequency may be typical for macromolecules in a vacuum, such as when being analysed in a mass spectrometer. 

The interpretation of ROESY experiments is not devoid of complications and hence, if the molecular tumbling frequencies allow it, NOESY experiments are to be preferred. One of the complications of ROESY experiments is the possible appearance of cross-peaks originating from scalar coupling instead of cross-relaxation. These crosspeaks have the same origin as the ones that are observed in TOCSY (total-correlation spectroscopy) experiments and can be distinguished from the cross-relaxation peaks because they have opposite sign. However, TOCSY crosspeaks may cancel with NOESY peaks involving the same protons. 

E. coli mutants defective in the chemotactic response to all stimuli have been isolated. Lesions in eight genes (designated che) result in such behaviour (Table 4.3) falling into two broad phenotypic classes. cheB and cheZ mutants tumble incessantly. Mutation in cheA, cheX, cheYand cheD loci leads to reduced tumbling frequency. cheC mutants can exhibit either... 

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