❌ is the order (sequence) of the amino acids forming a protein chain. The amino acids are held together by ❌ and the chain is called a ❌.

Secondary Structure refers to when the protein chain changes into a coil (called ❌), pleat (called ❌), or a ❌. These are formed by a ❌.

The R-groups can form ❌ types of bonds.
❌ -> Hydrophobic Interaction
❌ -> Hydrogen
❌ -> Ionic Bond
❌ -> Disulfide Bridge
*Exceptions
1. The amino acid cysteine is polar. However, it forms a disulfide bridge instead of a hydrogen bond. This is because it has a sulfur molecule.
2. Methionine also contains sulfur and is non-polar. It forms a hydrophobic interaction instead of a disulfide bridge.*

Polar amino acids usually have an N, F, or ❌ in their R-groups. ❌ is an exception. It is polar; however, it doesn't follow the common criteria. Another exception is ❌. It has an N in its R-group, but it is non-polar as it has an equal pull on each side.