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Journal articles made easy: Natural product herbicide, thaxtomin A

Description

This article looks at developing a natural product herbicide, thaxtomin A. It will help you understand the research the journal article is based on, and how to read and understand journal articles. The research article was originally published in our Organic & Biomolecular Chemistry journal.

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NMR spectroscopy data for (±)-thaxtomin A

1 H NMR data for TA, syn-1

The1H NMR spectra were recorded with a Bruker MHz 600 spectrometer for TA, syn-1.  Deuterated methanol (CD3OD) was used as a solvent.

1H NMR (600 MHz, CD3OD): d 7.84 (dd, J = 7.9 Hz, 1H), 7.69 (d, J = 8.0 Hz, 1H), 7.23 (t, J = 8.1 Hz, 1H), 7.18 (t, J = 8.0 Hz, 1H), 6.95 (s, 1H), 6.75 (m, 1H), 6.70 (m, 2H), 3.86 (dd, J1 = 6.4 Hz, J2 = 8.8 Hz, 1H), 3.31 (overlapping with solvent residual peak, 1H), 3.13 (d, J = 13.5 Hz, 1H), 3.03 (s, 3H), 2.82 (s, 3H), 2.60 (dd, J1 = 6.3 Hz, J2 = 14.1 Hz, 1H), 1.61 (dd, J1 = 8.9 Hz, J2 = 14.1 Hz, 1H)

Explanation

  • The NMR peaks are labeled according to the structure of thaxtomin A, while exactly how the hydrogens were labeled may be beyond the scope of this exercise.  It is important to keep in mind that each signal in a 1H NMR spectrum represents distinct hydrogen(s) present on the molecule undergoing the experiment. 
  • Values found directly above the peaks (blue) are related to how much electron density is surrounding the proton, higher number (downfield) means less electron density and lower number (upfield) means more electron density.  These values are called the chemical shift of the hydrogen(s).  Unfortunately Hc’s chemical shift is identical to the solvent residual peak of deuterated methanol (3.31), but if you look closely the edges of the Hc signal can be seen. 
  • The integration values can be found directly under each peak (# in red), these values represent the number of hydrogen atoms associated with the NMR signal. 
  • For example the peak at 3.03 ppm has an integration ratio right around 3, which makes sense because this singlet represents a methyl group that contains 3 hydrogens.
  • Integration ratios found in routine NMR seldom give the exact value but are usually within 10% of the actual number of hydrogen(s), therefore rounding to the nearest whole number is common. 

Explanation

  • This expansion represents the 8 aromatic hydrogens on thaxtomin A (labeled has HAr on the structure).

13 C NMR for TA, syn-1

The 13C NMR spectra were recorded with a Bruker 150 MHz spectrometer for TA, syn-1.  Deuterated methanol (CD3OD) was used as a solvent.

13C NMR (150 MHz, CD3OD): d 168.3, 166.8, 159.1, 143.6, 141.1, 137.4, 132.5, 131.2, 122.7, 121.0, 119.8, 119.3, 118.6, 118.4, 115.9, 110.5, 88.0, 64.6, 43.5, 34.2, 33.5, 28.5

Explanation

  • Carbon NMR (0-200 ppm) has a much larger scale then proton NMR (0-10 ppm).
  • The expanded spectrum above displays all of the aliphatic (non-aromatic) C atoms present in thaxtomin A.
  • There are 6 lines in this expansion and there are 6 aliphatic Cs on thaxtomin A, can you find the aliphatic carbons on the structure?

Explanation

  • This expansion represents all the aromatic and amide carbonyl signals.
  • The amide carbonyl signals are the furthest downfield at 166.80 and 168.33.
  • The aromatic signals of thaxtomin A can be found between 160-110 ppm, because there is no symmetry in the aromatic rings all 14 aromatic carbons can be accounted for.  

ADDITIONAL INFORMATION

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