Polypeptides are chains of amino acids. They can coil/fold into different shapes. Some 'R'-groups are either hydrophobic, hydrophilic, or neutral in their charge. Suppose you have a very long polypeptide, where a part of the chain is hydrophobic and another part is hydrophilic. I go ahead and drop this long polypeptide in water and it coils up. How do you think it would coil? Why? What can you infer about the proteins in our bodies regarding this situation

The hydrophilic part of the polypeptide chain would be oriented towards the surrounding water while the hydrophobic portion would be arranged away from the water and would make the hydrophobic interior resulting in coiling of the polypeptide chain. The body proteins assume the shapes to organize their hydrophobic parts away from the watery fluid.

Explanation:

Amino acids are the monomers of proteins. Each amino acid, present in proteins, has a central carbon atom. Four functional groups are attached to the central carbon atom of amino acid. These are the carboxyl group (COOH), amino group, a hydrogen atom (H) and a side chain (called R group). Amino acids differ from each other for the side chain present in them. Based on the nature of the side chain, amino acids are hydrophobic or hydrophilic or neutral. Sidechains of some amino acids are charged and have positive or negative charges on them.

The extracellular fluid of the body is a watery medium. Blood also has liquid plasma that is 99% water. The intracellular fluid, called cytosol, is also a water-based liquid.  

The hydrophilic side chains of the amino acids interact with the watery surroundings. They can make hydrogen bonds with the water. The charged hydrophilic R groups make electrostatic interactions with the surrounding medium. The hydrophobic R groups of the amino acids tend to arrange themselves away from the watery medium. This makes the part of the polypeptide chains with hydrophobic amino acids to fold so that these R groups are oriented away from the water.

Similarly, when a long polypeptide chain is dropped in the water, the hydrophobic R groups are arranged away from the water while the hydrophilic ones are arranged towards the surface of the chain facing the water. It is done to adopt a stable shape where maximum attraction forces are established.  

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