exam4-2008 - Name _ Exam 4: CHEM/BCMB 4190/6190/8189 (120...

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Name _________________________ Exam 4, 2008: CHEM/BCMB 4190/6190/8189 1 Exam 4: CHEM/BCMB 4190/6190/8189 (120 points) Tuesday, 16 December, 2008 1 ). Shown (below, right) are results from 1D selective TOCSY NMR experiments (‘B’ and ‘C’) performed on the ± -methyl-3- O -methyl- cellobioside molecule, a disaccharide, shown below. In ‘A’ is shown the simple 1 H NMR spectrum of this molecule. The pulse sequence for the selective TOCSY experiment is shown (above, right). A long selective 90 ° pulse is applied at the frequency of a chosen signal, followed by the spin-lock sequence and then detection. In ‘B’ are shown the results of applying the selective pulse at the frequency of the signal for hydrogen 1 ² , and in ‘C’ are shown the results of applying the selective pulse at the frequency of the signal for hydrogen 1. In both ‘B’ and ‘C’, the TOCSY mixing time (spin-lock) was 200 ms. Explain the results in ‘B’, and explain the results in ‘C’. (Note: The signal for 1 is at the same frequency as water (HDO). This is unimportant, so you can ignore it. Also, ignore the very large spikes (singlets) in ‘A’ at 3.4 and 3.65 ppm, which result from impurities). ( 8 points ) The selective TOCSY experiment gives correlations between the nucleus whose frequency is selectively excited by the initial, selective 90 ° pulse and other nuclei in the spin system. If the mixing time is long enough, very long range correlations can be observed. In ‘B’, the frequency of 1 ± was selectively excited, and correlations to all other hydrogens in the sugar molecule on the left (2 , 3 , 4 , etc.) are observed. No correlations to nuclei in the other sugar (1, 2, 3, etc.) are observed, indicating that any couplings between 1 and these hydrogens are extremely small. Likewise, in ‘C’, correlations between 1 (whose frequency was initially excited) and the other hydrogens in the sugar on the right (2, 3, 4, etc.) are observed, but no correlations are observed to hydrogens in the sugar on the left. 1 H selective 90 ° pulse spin-lock
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Name _________________________ Exam 4, 2008: CHEM/BCMB 4190/6190/8189 2 2 ). Consider the populations N 1 , N 2 , N 3 and N 4 of the ±± , ±² , ²± and ²² states respectively for a 1 H- 1 H spin system (spins ‘A’ and ‘X’) without J coupling (no coupling between A and X). The energy diagram for this system is depicted (right), where the A’s represent the transitions of one of the 1 H nuclei, and the X’s represent the transitions of the other 1 H nucleus. We will define ³ H as the difference in the number of spins in ± and ² states for A, and ³ H will also represent the difference in the number of spins in ± and ² states for X. a. Write down the equilibrium values for N 1 -N 4 and the equilibrium population differences for the A and X transitions (please show how you calculate the A and X transition population differences). Assume that N 4 = N. ( 4 points ) N 4 = N N 3 = N + ± H A = N 2 – N 4 = N 1 – N 3 = H N 2 = N + H X = N 3 – N 4 = N 1 – N 2 = H N 1 = N + 2 H b. On the energy diagram above, draw dashed lines showing the W 0 (zero quantum) and W 2 (double quantum) relaxation pathways.
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This note was uploaded on 11/13/2011 for the course CHEM 4190 taught by Professor Staff during the Fall '08 term at University of Georgia Athens.

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exam4-2008 - Name _ Exam 4: CHEM/BCMB 4190/6190/8189 (120...

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