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?

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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

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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

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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.

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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