In an $$\alpha$$-helix structure, the polypeptide chain folds into a helix stabilized by hydrogen bonds between the oxygen atom of a carbonyl group in one amino acid and the hydrogen atom of the amino group located four amino acid residues further along the chain.

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Secondary protein structure refers to only the spatial arrangements of the polypeptide backbone that does not include the sided chain conformations. 
The most common secondary structures include:
-	Alpha helix
-	Beta sheets
- Turns

Secondary Protein Structure

A protein's secondary structure is the localized folding of a sufficiently long polypeptide chain into shapes such as helices and sheets. This folding is stabilized by hydrogen bonds between the amine and carbonyl functional groups of the peptide backbone, excluding the variable R side chain. The most common types of secondary structure are the $$\alpha$$-helix and $$\beta$$-pleated sheet.

Secondary Structure

Voet, D., Voet, J. G., & Pratt, C. W. (2013). Chapter 6: Proteins: Three-Dimensional Structure. In Fundamentals of biochemistry: Life at the molecular level (4th ed., pp. 127-175). Hoboken, NJ: Wiley. Retrieved January 05, 2021, from https://archive.org.

Fundamentals of biochemistry: Life at the molecular level, 4th edition: Chapter 6

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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

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Turns are polypeptide sequences that connect secondary structures (i.e. alpha helices and beta sheets). There are two types of arrangements for turns: 
- Type I
- Type II

Protein Turns

Alpha-Helix Structure

In a $$\beta$$-pleated sheet structure, the folds are stabilized by hydrogen bonds between continuous sequences of carbonyl and amino groups that are further separated on the backbone of the polypeptide chain.

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