Chemistry Reference
In-Depth Information
A shift in k
max
to shorter wavelength is called a
hypsochromic
effect,
or blue shift, and usually occurs when compounds with a basic auxochrome
ionise and the lone pair is no longer able to interact with the electrons of
the chromophore. Hypsochromic effects can also be seen when spectra are
run in different solvents or at elevated temperatures. Spectral shifts of
this type can be used to identify drugs that contain an aromatic amine
functional group, e.g. the local anaesthetic benzocaine (see Figure 7.9).
O
H
2
N
C
OC
2
H
5
Figure 7.9
The structure of benzocaine.
Bathochromic and hypsochromic effects are seldom seen in isolation.
Bathochromic effects are usually associated with increases in the intensity
of light absorbed, while hypsochromic effects usually occur with decreases
in absorbance. An effect that causes an increase in light absorbance is called
a
hyperchromic
effect, while a decrease in the intensity of light absorbed is
termed a
hypochromic
effect. The four words used to describe shifts in k
max
could almost have been chosen to cause maximum confusion among students
trying to remember the terms. Perhaps the best way to remember the terms
is to say that hyper- means an increase, hypo- a decrease, and that a shift to
longer wavelength is a red shift while a shift to shorter wavelength is a blue
shift, or, alternatively, commit to memory Figure 7.10. Hyperchromic effects
are used in anticancer drug research to measure the extent of drug binding
to DNA. If a solution of duplex, or double-stranded, DNA is gently heated,
the double helix will start to unwind, exposing the heterocyclic bases in the
centre of the duplex. This can be observed experimentally as the absorbance
of the DNA solution at 260 nm will increase, causing a hyperchromic effect.
Drugs that bind to DNA stabilise the molecule and reduce the extent of the
observed hyperchromicity.
Drugs that contain phenolic groups, e.g. paracetamol (see Figure
7.11), also show spectral shifts on ionisation. In the case of phenols, which
are weak acids with a p
K
a
of approximately 10, ionisation increases the
intensity of light absorption and the position of k
max
moves to longer wave-
length. This is because ionisation and loss of the H atom as an H
ion
results in a full negative charge on the oxygen (a phenoxide ion), which can
interact with the ring more effectively than the lone pair of electrons present
in the unionised molecule. This is shown for phenol in Figure 7.11.
