Olfactory Design: Smell and Spectroscopy

Our sense of smell is an incredible system designed to detect thousands of different chemicals. It serves various purposes, including warning us of potential dangers, such as rotting food. For instance, we can sense the presence of a specific component in rotten meat called ethyl mercaptan at an incredibly low concentration of 1/400,000,000th of a milligram per liter of air. Smell also plays a significant role in helping us distinguish between different types of foods and flowers. In fact, our sense of smell is responsible for most of the different tastes we experience when consuming food. Notably, animals often rely on this sense even more than humans do. For example, bees use their sense of smell to locate nectar.

The nose contains millions of receptors that come in 500 to 1000 different types. These receptors are found in the yellow olfactory epithelium, which covers about 2.5 square centimeters on each side of the inner nose. Each type of receptor is a protein that folds in a specific way so that it can bind to odor molecules with a particular shape. When an odor molecule binds to a receptor, it releases a g-protein, triggering a second messenger that stimulates a neuron to send a signal. This signal is transmitted by olfactory nerve fibers to specialized structures called olfactory bulbs located under the front part of the brain. The olfactory bulbs sort and transmit these signals to the brain for processing.

Recently, Luca Turin, a biophysicist at University College London, proposed an intriguing mechanism to explain how smell receptors work. According to his theory, an electron tunnels from a donor site to an acceptor site on the receptor molecule, causing it to release the g-protein. This tunneling process requires both the starting and finishing points to have the same energy level. Turin suggests that the donor site has a higher energy level than the receptor, and this energy difference matches the energy needed to excite the odor molecule into a higher vibrational quantum state. Therefore, when the odor molecule binds to the receptor, it can absorb the right amount of energy from the electron, enabling tunneling through its orbitals.

This means that smell receptors detect the energy of vibrational quantum transitions in odor molecules, a concept first proposed by G.M. Dyson in 1937. The energy of these transitions depends on factors such as the mass of the atoms involved, the strength of the chemical bonds, and the symmetry of the molecule. For diatomic molecules, the fundamental transition energy can be calculated using the formula:

E = h⁄ 2π (k/ µ )½

In this formula, h represents Planck's constant, k is the force constant of the bond, and µ is the reduced mass of the atoms. The reduced mass is determined by the masses of the two atoms involved. The frequency (ν) of incident electromagnetic radiation that can cause a transition is related to the energy by:

E = hν

Vibrational spectra are typically measured in wavenumbers, which are reciprocal centimeters (cm^-1). Wavenumber is related to energy by:

= E⁄ hc

Infrared absorption spectroscopy is commonly used to measure vibrational energies and bond strengths in molecules. This technique involves analyzing how molecules absorb infrared radiation.

The theory proposed by Turin suggests that groups of atoms with similar energies exhibit similar vibrational spectra. For instance, chemicals containing sulfur-hydrogen bonds tend to vibrate at around 2500 cm^-1. This vibration frequency often gives rise to a characteristic "rotten" smell. An example of such a compound is hydrogen sulfide (H2S), which produces a smell reminiscent of rotten eggs. Similarly, ethyl mercaptan, which is produced during the decomposition of meat, contains sulfur and hydrogen (C2H5SH) and is associated with a foul odor.

Turin's theory finds support in the case of decaborane (B10H14), a compound that smells similar to sulfur-hydrogen compounds despite having no chemical similarity apart from comparable vibrational energies. Although boron has a much lower atomic mass than sulfur, B-H bonds are weaker than S-H bonds, and these effects happen to balance each other out.

Further evidence comes from studying analogous compounds such as ferrocene and nickelocene. Both compounds have a divalent metal ion (iron or nickel) sandwiched between two cyclopentadienyl anions (C5H5-). The primary difference in their vibrational spectra is the vibration frequency of the metal ring bond. In ferrocene, this bond vibrates at 478 cm^-1, resulting in a spicy smell. In contrast, nickelocene exhibits a vibration frequency of 355 cm^-1 and smells like aromatic hydrocarbon rings. Turin proposes that below a threshold of 400 cm^-1, the vibrational signal is overwhelmed by "background noise," making it undetectable by our sense of smell.

Isotopes, which are atoms with different masses but similar chemical properties, can also affect vibrational energy. Replacing hydrogen (H) with deuterium (D), for example, doubles the difference in reduced mass. This change in mass affects the vibrational energy and can alter the perceived smell. Deuterated acetophenone, for instance, smells fruitier than ordinary acetophenone (C6H5COCH3). Additionally, compounds containing the cyanide or nitrile group (C≡N), which vibrate at around 2200 cm^-1, often have a slight bitter almond scent.

One challenge to Turin's theory arises from the different smells associated with enantiomers, which are molecules that are mirror images of each other. Despite having identical vibrational spectra, enantiomers can have distinct smells. For example, R-carvone smells like spearmint, while S-carvone smells like caraway. The explanation lies in the fact that the smell receptors in our nose are chiral, meaning they have a specific orientation. These receptors orient the two enantiomers differently, causing different vibrating groups to align with the tunneling direction in each enantiomer. Turin suggests that in the case of caraway S-carvone, a carbonyl (C=O) group lies in that direction and is detected by the receptors. In contrast, in minty R-carvone, the carbonyl group lies at right angles to the tunneling direction and is ignored. Turin supported this idea by creating a caraway scent by mixing minty carvone with butanone (C2H5COCH3), which contains a carbonyl group.

Whether or not Turin's theory is entirely accurate, the olfactory system exhibits what biochemist Michael Behe refers to as irreducible complexity. This means that the system requires multiple parts to function correctly, and removing any of these parts would render it non-functional. The machinery involved in chemical sensing relies on proteins with precise shapes to accommodate odor molecules effectively. Additionally, for our sense of smell to be useful, nerve connections must transmit the chemical information gathered by our nose to the brain for processing.

Understanding the intricacies of olfaction remains an ongoing challenge. However, Turin's theory offers valuable insights into the design and functionality of our sense of smell. It highlights the precise nature of receptor interactions with odor molecules and how vibrational energies play a crucial role in detecting and distinguishing various scents.

Why This Matters

The study of olfactory design and the mechanisms behind our sense of smell provides evidence for the intricate design of living organisms. The complexity of the olfactory system, with its specialized receptors, nerve connections, and processing in the brain, points to intentional design rather than mere chance. Exploring the sensory capabilities granted to us by our Creator enhances our understanding of the world around us and deepens our appreciation for His wisdom and creativity.

Think About It

1. Have you ever considered the incredible sensitivity of our sense of smell? Reflect on how this ability has an impact on your daily life.
2. How does the concept of irreducible complexity challenge the idea that complex systems can arise gradually through natural processes?
3. Consider how the design and functionality of our sense of smell point to a purposeful Creator. In what ways does this deepen your understanding of God's wisdom and creativity?