Final answer:
Glycine acts as a secondary structure breaker in proteins due to its small size and the resulting flexibility it provides to the peptide chain, disrupting the regular hydrogen-bonding patterns required for the maintenance of alpha helices and beta sheets.
Step-by-step explanation:
The reason why glycine is known as a secondary structure breaker within protein structures is largely due to its small size and the flexibility it confers to the polypeptide chain. Secondary structures such as the alpha-helix and beta-sheet are stabilized by hydrogen bonds that form between the carbonyl oxygen and amino hydrogen in the polypeptide backbone, conveniently excluding the R-group side chains.
In the case of glycine, its R-group is a single hydrogen atom, which is much smaller compared to the larger side chains of other amino acids. This diminutive size allows the peptide chain greater flexibility, disrupting the regularity needed to maintain secondary structures like alpha helices or beta sheets. Therefore, in a protein's sequence, the presence of glycine can lead to a 'kink' or a flexible turn, interrupting the pattern of hydrogen bonding necessary for these structures. Pauling's pioneering work on protein secondary structures highlights the importance of specific amino acid properties in dictating a protein's overall structure and function.