The secondary structure of a protein is primarily maintained by hydrogen bonds between the backbone amide hydrogen (N-H) and carbonyl oxygen (C=O) groups of the polypeptide chain. These hydrogen bonds stabilize local folded structures such as alpha helices and beta pleated sheets, which are the most common secondary structural elements

• In alpha helices , hydrogen bonds form within a continuous stretch of amino acids, typically between the carbonyl oxygen of one amino acid and the amide hydrogen four residues ahead in the chain.
• In beta sheets , hydrogen bonds occur between carbonyl oxygens and amide hydrogens of adjacent, often non-continuous, beta strands.

Besides hydrogen bonds, other weaker forces such as van der Waals interactions and hydrophobic interactions contribute to the overall stability of secondary structures by helping maintain the geometry and packing of the polypeptide chain

. Environmental factors like temperature and pH can disrupt hydrogen bonding and thus destabilize the secondary structure, leading to protein denaturation

. In summary, the defining and maintaining force of protein secondary structure is the pattern of hydrogen bonds between backbone amide and carbonyl groups, supported by weaker interactions that help stabilize the folded conformation