Practice problems on the NMR of amino acids
Test your ability to correlate NMR spectra withstructure by trying the following problems.
Use the correlation tables, the relative sizes of thesignal, electronegativity and the number of signals and
logic to connect the structures with the spectra.
These are the signals of protons (1H NMR). We think of the position (in ppm)of the NMR signals in the same way we viewed the frequency of IR signals.
The 1H NMR signals are characteristic of certain types of protons in molecules
NMR units are ppm (parts per million). We discuss where this ppmunit comes from in class.
These are the position of the signals of carbon atoms (13C) in ppm. Wethink of the position of a NMR signal in ppm in the same way we viewed thefrequency of IR signals. The 13C NMR signals are characteristic of certain
types of carbon atoms in molecules.
024681012
PPM
N H2
C H3
C H3
O
O H
HH v a lin e
The 1H NMR of the amino acid valine is given below. Assignthe protons of valine to the signals (a, b, c, d and e) in the
NMR spectrum.
a
bcdd
The answer is at the end of the ppt
Leucine and isoleucine are isomers.Two 13C NMR spectra A and B are shown below.
One is leucine the other is isoleucine. Which is which?
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PPM
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PPM
A
B
CH2
CH3
CH3O
HO
NH2
H
isoleucine
H
NH2
CH2 CH3
CH3
O
HO
H
H
leucine
The 13C NMR spectrum of valine and leucine are shown.Which is valine and which is leucine?
020406080100120140160180
PPM
NH2
CH3
CH3O
OH
HH
valine
NH2
CH2 CH3
CH3
O
HO
H
H
leucine
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PPM
signal
signal
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PPM
020406080100120140160180
PPM
Two signalsVery closetogether
NH2
CH2O
HO H
NH2
CH2' OHO
HO H
tyrosine
phenylalanine
The 13C NMR spectrum of phenylalanine and tyrosine (both possessaromatic rings) are shown. Which is phenylalanine and which is
tyrosine?
024681012
PPM
N H2
C H3
C H3
O
O H
HH v a lin e
The 1H NMR of the amino acid valine is given below. Assignthe protons of valine to the signals in the NMR spectrum.
a
b
c
de
a
b
a
de
c
Leucine and isoleucine are isomers.Two 13C NMR spectra A and B are shown below.
One is leucine the other is isoleucine. Which is which?
020406080100120140160180
PPM
020406080100120140160180
PPM
A
B
CH2
CH3
CH3O
HO
NH2
H
isoleucine
H
NH2
CH2 CH3
CH3
O
HO
H
H
leucine
Two equivalentcarbon atoms(the two CH3
groups)
All the carbonatoms areunique
The 13C NMR spectrum of valine and leucine are shown.Which is valine and which is leucine?
020406080100120140160180
PPM
NH2
CH3
CH3O
OH
HH
valine
NH2
CH2 CH3
CH3
O
HO
H
H
leucine
020406080100120140160180
PPM
signal
signal
An essentialdifference betweenvaline and leucine isthat leucine has onemore carbon atom. Inthe bottom spectrumthe CH2 carbon atomof leucine is just tothe left of the CH3
carbon atom.
020406080100120140160180
PPM
NH2
CH2' OHO
HO H
tyrosine
020406080100120140160180
PPM
Two signalsVery closetogether
NH2
CH2O
HO H
phenylalanine
The 13C NMR spectrum of phenylalanine and tyrosine (both possessaromatic rings) are shown. Which is phenylalanine and which is
tyrosine?
The most significantdifference between the twostructures is the OH on thebenzene ring of thetyrosine. This substituentwill cause the carbons ofthe benzene ring oftyrosine to be much moredifferent chemically thanthose of phenylalanine.This difference will also bereflected in thedifferences in theseparation of the carbonsignal in the NMR