Interaction Of Hydrophobic Molecules With Water – New Insights

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Researchers now report how water repelling substances interact with water at the molecular level.  This finding may have far-reaching implications in the science of protein chemistry, drug design, biofouling and nanomaterials.

Chemistry textbooks teach us how grease can be removed using detergent. Detergent molecules have a water soluble (hydrophilic) head and a water-phobic (hydrophobic) tail. The hydrophobic tails trap the water repelling grease and form particles called micelles, with grease-filled core and hydrophilic exteriors.  The result: grease ‘dissolves’ into the detergent solution.

Hydrophobicity describes the property of an object that does not prefer to associate with water. As mentioned above, examples of hydrophobicity include the separation of water and oil and the formation of water droplets on the surface of wax. Hydrophobicity is also one of the most important driving forces in chemistry and biology such as the folding, assembly and function of proteins.

On the macroscopic scale, oil forms spherical droplets in water to minimize the unfavorable energy associated with the interface between water and oil. This explanation works for macroscopic objects but cannot explain the anomalous behaviors of small hydrophobic molecules in the nanoscopic world.

polymer hydrophobicity and water
 The microscopic AFM cantilever directly applies a force to stretch a single polystyrene  molecule from its naturally collapsed state to an extended state in water. Image provided to by Isaac  Li and Dr. Gilbert  Walker.

Much of the developments on small molecule hydrophobicity in the past few decades came from theoretical studies that relied on existing solubility data of small hydrophobic molecules such as methane (CH4). However, people were unable to perform bulk experimental measurements on polymer hydrophobic interactions primarily due to their insolubility in water. The experiments, performed by Drs Li and Walker in the Department of Chemistry, University of Toronto, on single polymers bridged experimental evidence with theoretical predictions of hydrophobic hydration in the nanoscopic world.

The paper was published on September 30, 2011 in the peer-reviewed journal Proceedings of the National Academy of Sciences (PNAS).

"To bypass the limitation that we cannot dissolve hydrophobic polymers in water, we  used Atomic Force Microscopy (AFM) to study the hydrophobic interactions inside a single, hydrophobic polymer molecule," said  Li and Dr Walker.  The microscopic AFM cantilever directly applies a force to stretch a single polystyrene  molecule from its naturally collapsed state to an extended state in water. They were able to measure the energy to “dissolve” this single polystyrene molecule by a physical force.

“By measuring the hydration energy of the polymer at different temperatures, we uncovered the signature of hydrophobic hydration of the polymer, which is strikingly similar to that of small molecules. We found that although polymers span hundreds of nanometers in length (therefore belonging to the class of macromolecules), its hydration behavior is actually defined by the length scale of individual monomers on the sub-nanometer scale.” explained  Li, who is a graduate student with Dr Walker.

A better understanding of how polymers (and macromolecules in general) interact through hydrophobic interaction could potentially advance the understanding of protein folding, materials science and biotechnologies.

According to the authors, this principle can be used to better design the hydrophobic core that contribute to the stabilities of proteins and to better understand interaction between drug and protein if the nature of the interaction involves hydrophobic interaction. In the field of nanomaterials, the finding may have significance in designing systems that self-assemble better at molecular scale.  This principle may also be used to study the adhesive proteins, responsible for biofouling and thereby finding strategies to counteract.

Source Article: Signature of hydrophobic hydration in a single polymer. Isaac T. S. Li and Gilbert C. Walker. PNAS. Published online September 12, 2011, doi: 10.1073/pnas.1105450108.

Additoinal Source: Drs Isaac Li and Gilbert Walker, Department of Chemistry, University of Toronto, ON,Canada.

Link to Walker Labortory

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