pulse is applied to the system that has an energy matching the energy dif-
ference for one of the spins, the populations of those spins will equilibrate.
After the pulse, the spins in the uppermost level will relax down to the
lowest level. When a second radiofrequency pulse probes the second spin,
the population levels are now greater and the NMR signal is enhanced.
Thus, the pulse sequence uses the dipole–dipole interactions between two
coupled spins to enhance the NMR signals of those spins, allowing their
detection as cross peaks in the two-dimensional spectrum.
NMR spectra of amino acids
An NMR experiment is usually a COSY measurement in which the cross
peaks arise from nuclei that interact through chemical bonds. For indi-
vidual amino acid residues, the resulting spectrum results in a pattern
that is characteristic of the bonding arrangements. Consider the idealized
COSY spectra of several amino acid residues (Figure 16.9). The assignment
of the peaks begins with a peak in a region that is free of overlapping
peaks, with the others following from the COSY connectivity. In assign-
ing the spectrum of the amino acid residues, the protons associated with
the carboxyl and amino groups are not included, as in a polypeptide chain
they would not contribute to the spectrum.
The simplest amino acid residues are Gly and Ala. Glycine has one CαH
proton present and so shows a single peak at 3.97 ppm. Alanine has one
proton at the Cαposition and three at the Cβpositions. The three protons
of the methyl position are all magnetically equivalent and contribute one
peak that has an integrated area 3-fold larger than the peak from the Cα
position. Since these protons are bonded together there is a cross peak
showing that connectivity. For threonine, the methyl protons are magnet-
ically equivalent as was found for alanine. Since the methyl protons are
connected only to the CβH there is only one cross peak involving the methyl
protons. Likewise, the CαH proton is connected only to the CβH proton,
resulting in only one cross peak involving the CαH proton. Thus, the two
peaks near 4 ppm can be assigned through the cross peaks. For valine,
the spectrum has contributions from four sets of protons. The methyl groups
each contribute near 1 ppm whereas the CαH proton is at 4.18 ppm
and the CβH proton is at 3.13 ppm. The connectivity gives a cross peak
between the CαH proton and only the CβH proton whereas CβH proton
is also coupled to each of the methyl groups.
Research direction: development of new NMR techniques
The primary use of NMR is to determine the structures of molecules.
For biological systems, NMR provides the means to determine protein
352 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY
