Light Absorption by Biomolecules

What causes light to interact with the molecules in cells? The most simple answer is that the electrons in conjugated double bonds (e.g. -C=C-C=C-C=C-, where double bonds are present between alternate pairs of carbon atoms) are able to absorb the energy from photons. Electrons occupy orbitals in molecules and are usually found in pairs. Each electron in the pair has an opposite ’spin’ so that they are considered to be balanced; two electrons with the same spin are not permitted to occupy the same orbital. Rather than trying to describe the interaction of light with biomolecules in words, it is usually easier for students if a graphic presentation is used. These are termed Jablonski diagrams. Orbitals are indicated as horizontal lines, with higher energy orbitals placed higher on the page (Figure: Orbitals). Electrons are indicated by arrows, with the direction of the arrow (up or down) showing the opposite nature of the spins. Such a Jablonski diagram is shown in Figure: Ground State. The electrons normally occupy the lowest energy level, termed the ground state. When energy (such as the energy in a photon) is available, one of the electrons may absorb it and move up to the excited orbital. The product of this energy absorption is shown in Figure: Excited State.

There are several characteristics of the absorption process that must be understood:

• An electron must absorb all of the energy in a photon; it can’t absorb only some of the energy.
• If the energy in a photon does not exactly match the energy difference between the ground state orbital and the excited state orbital, that photon cannot be absorbed and it continues to move through space away from the molecule.
• The absorption event is essentially instantaneous (about 10^-15 sec) and the photon ceases to exist upon absorption.
• The energy of the photon has been conserved in the excited electron and is potentially available for photochemical reactions.