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.

Many fluorescent molecules have large extended pi systems. The three fused rings in ethidium bromide form one large pi system:

N

NH2

NH2

+

Br-

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:

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.

N

NH2

NH2

+

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.biology-blog.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.

Neural proteins attached to fluorescent molecules allow for detailed mapping (neuron-by-neuron) of a mouse’s brain:

A “Brainbow”

J. Lehrer Nature (2009) 457 pp.524-527.