Download Topic #2 - U of L Class Index

CHEMISTRY 2000
Topics of Interest #2:
Fluorescent Molecules in Medicine
How Fluorescence Works

Fluorescent molecules like ethidium bromide
(shown at right) are widely used in biological
applications as stains.
NH2
Br
-
+

N
Many fluorescent molecules have large
extended pi systems. The three fused rings in
ethidium bromide form one large pi system:
NH2
one of the many pi MOs of ethidium cation
How Fluorescence Works

Molecules with large extended pi systems can absorb light fairly
easily because they have many MOs with similar energies:
NH2
+
N
NH2

The energy level diagram above was generated for the ethidium
cation using HyperChem. Notice how close in energy the valence
MOs are. In some places, they’re starting to look almost band-like.
How Fluorescence Works

Light is absorbed to excite an electron
from an occupied MO into an empty MO
in the same molecule. (see yellow arrow
at right)


Exciting the electron into a higher energy
MO can also put the molecule in a higher
energy state for vibration/rotation. As
such, the molecule will release some
energy as heat when it relaxes into a
lower energy vibrational/rotational state.
Later, when the electron drops back into
a lower energy MO, a photon is emitted.
Since some of the energy originally
absorbed has already been released, the
photon is lower in energy. So, if UV light
is absorbed (as is common), coloured
light can be emitted. (see image at right)
Time-Sensitive Fluorescence

A couple of interesting advances in fluorescent applications were
reported in the last decade:

Recently, a group at the Albert Einstein College of Medicine in New
York have developed fluorescent “timers” – molecules that gradually
change the colour of light they emit with time. This would let
researchers track the movement of proteins in a cell as they age. At
first, the tagged proteins would emit relatively high energy blue light.
With time, the fluorescent molecule would start emitting lower energy
red light instead of blue.
F.V. Subach, O.S. Subach et al Nature Chem. Biol. (2009) 5 pp.118-126
as reported in Nature (2009) 457 p.238 and http://www.biologyblog.com/blogs/permalinks/1-2009/fluorescent160timers.html
A “Brainbow”

A few years ago, researchers at Harvard University engineered mice
that would produce fluorescent molecules attached to some of the
proteins in their nervous system. Originally, they expected to be able
to produce only a few colours of interconnecting neurons:
They were pleasantly surprised to find that, when the mice were
interbred, mixed coloured neurons were also produced. The images on
the next page show the rather spectacular results.
J. Lehrer Nature (2009) 457 pp.524-527.
A “Brainbow”
Neural proteins attached to fluorescent molecules allow for detailed
mapping (neuron-by-neuron) of a mouse’s brain:
J. Lehrer Nature (2009) 457 pp.524-527.