Hydrophobic, Hydrophilic and Hydrogen Bonds

Oily Molecules are Unable to Bond with Water

Jul 1, 2009 Art Ayers

Molecules that form hydrogen bonds with water are hydrophilic and those that can't are hydrophobic.

Emotions (fear, love, attraction) are frequently attributed to atoms and molecules, but molecules just collide because of random motion (diffusion) and they either attach in a lower energy state, i.e. form a bond, or they bounce off. In most cases, the molecules aren’t even attracted and must impact for a bond to form.

Hydrogen Bond Formation is Essential for Interactions with Water

The oxygen atom of a water molecule (H20) pulls electrons away from each of the two hydrogens. As a result, the hydrogens each have a partial positive charge and can form hydrogen bonds with pairs of valence electrons on other atoms that have a partial negative charge. [Note that this is different than the sharing of one electron from each atom, as in a covalent bond.] The two pairs of valence electrons of water that are not involved in covalent bonds to hydrogens, have partial negative charges and can form additional H-bonds. Thus, each water molecule can form four H-bonds.

Hydrophilic Molecules Make Hydrogen Bonds with Water

Molecules that dissolve or interact with water, such as carbohydrates, are said to be hydrophilic, water loving, but these molecules just dissolve in water, because they form H-bonds with water molecules. Each hydroxyl (-OH) group on a carbohydrate can make hydrogen bonds to three different water molecules. The hydrogen can bond to a pair of valence electron on the oxygen of water and each of the two pairs of valence electrons of the hydroxyl can bond to a hydrogen of water. Most of the molecules within cells can form H-bonds and are hydrophilic.

Typical hydrophilic molecules include:

• proteins,
• carbohydrates,
• nucleic acids (DNA and RNA),
• salts (form ionic bonds),
• small molecules of metabolism (e.g. glucose, amino acids, ATP)

Hydrophobic Molecules Don’t Make Hydrogen Bonds with Water

In contrast, fats that float and don’t dissolve in water are called hydrophobic, but that doesn’t mean that fats hate water or are pushed away from water, it simply means that fats can’t form hydrogen bonds with water. Since bonding means that energy is released as molecules come into contact, then water molecules forming hydrogen bonds are in a lower energy state than water molecules in contact with a hydrophobic molecule, such as a fat. Random movement of a mixture of fat and water will eventually result in the water molecules sharing the minimal possible surface with the fat, because that is the lowest energy configuration.

Typical hydrophobic molecules include:

• fats,
• steroids,
• lipids,
• aromatic compounds (such as some drugs)

Molecules with Both Hydrophobic and Hydrophilic Regions Act as Detergents

Soaps are fatty acids that have a long hydrocarbon (fatty) tail at one end and a carboxyl group (-COOH, acid) at the other. The carboxyl group, because of the two oxygens, can form hydrogen bonds with five water molecules and is, therefore, hydrophilic. The fatty tail of the fatty acid is hydrophobic. Soaps are both hydrophilic and hydrophobic, i.e. amphipathic, and can cover the surface of a hydrophobic molecule with their own fatty tail and produce a new hydrophilic surface of carboxyl groups to dissolve oily materials in water. That is how soaps dissolve oily materials.

Cellular Membranes are Made of Amphipathic Phospholipids

Phospholipids are made from a glycerol with a phosphate and two fatty acids attached. The two fatty acids make the phospholipid primarily hydrophobic, but the phosphate provides three oxygen atoms that can make multiple hydrogen bonds. In water, the phospholipids form sheets consisting of two layers, i.e. a phospholipid bilayer. Each layer of phospholipids is highly structured so that all of the phosphates are on one surface and the hydrophobic tail of the fatty acids are on the other surface. In the membranes the hydrophobic faces of each layer are in contact and the phosphates are in contact with either the cytoplasm or the outside of the cell.

The Structure and Function of a Cell and Its Components Is Based on Hydrogen Bonding

Hydrophilic molecules of a cell literally shake into the lowest energy configuration that optimizes the number of total hydrogen bonds between the molecues and water. Hydrophobic molecules can't reach lower energy configurations by bonding with water, but rather hydrophobic molecules shake into a configuration with the minimum surface area exposed to water. Hydrophilic and hydrophobic molecules in their minimum energy configuration will stay that way unless they receive more energetic collisions form other molecules. Boiling for food preparation is an example of disruption of biological structures by increasing the kinetic energy of the water, i.e. increasing the temperature.

The copyright of the article Hydrophobic, Hydrophilic and Hydrogen Bonds in Chemistry is owned by Art Ayers. Permission to republish Hydrophobic, Hydrophilic and Hydrogen Bonds in print or online must be granted by the author in writing.

Membrane Hydrophobic and Hydrophilic Interactions

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