Sunlight as a Driver of Abiogenesis: The Quantum Mechanics of the Absorption of Light by Biomolecules

The physical basis of life is founded on the ability of living systems to harvest energy from their environment. In order for self-assembly to proceed from structure to function, there needed to be molecules capable of absorbing the main source of energy available on the Earth’s surface; that source was light from the Sun. Einstein developed the probabilistic equations for photon absorption by molecules. Feynman showed how to understand the quantum interaction between a photon and electron, a process that is at the heart of abiogenesis. In this chapter on the optical spectroscopy of biomolecules, we introduce the various molecular energy levels, including vibrational and rotational modes. Einstein’s A and B coefficients are derived along with Beer’s law and equations for the absorption cross sections of biomolecules through an elementary quantum mechanical treatment of the molecular absorption of light. Fluorescence lifetime, polarization, anisotropy, and dipole moment are introduced. Phototrophs, those first light-harvesting organisms, used the same chromophore, retinal, as later organisms utilize for sight. We calculate the wave functions for optical absorption by retinal. We also present photosynthesis based on chlorophyll, which evolved later.