The secondary structure of a protein is primarily maintained by hydrogen bonds. These bonds form between the backbone atoms of the polypeptide chain, specifically between the hydrogen atom of an amide group (N-H) and the oxygen atom of a carbonyl group (C=O).

Here's a breakdown:

* Alpha-helix: The hydrogen bonds form between the carbonyl oxygen of one amino acid residue and the amide hydrogen of the amino acid residue four positions down the chain. This creates a helical structure.

* Beta-sheet: The hydrogen bonds form between carbonyl oxygens and amide hydrogens of adjacent polypeptide chains that run in a parallel or anti-parallel fashion, resulting in a sheet-like structure.

While hydrogen bonds are the primary driving force for secondary structure, other interactions can also contribute, such as:

* Ionic interactions: These occur between charged amino acid side chains.

* Hydrophobic interactions: These occur between nonpolar amino acid side chains, pushing them together and away from water.

* Van der Waals forces: These are weak attractions between molecules, but they can collectively contribute to the stability of secondary structures.

It's important to note that these interactions are weaker than covalent bonds, but they are still crucial for maintaining the specific shape and stability of the protein's secondary structure.