Final answer:
Alpha-helices and B-sheets are both stabilized by hydrogen bonding, however in alpha-helices the bonds are between every fourth amino acid, while in B-sheets the bonds are between segments that align parallel or antiparallel to each other.
Step-by-step explanation:
The difference between how the alpha-helices and B-sheets are stabilized is mainly in the patterns of hydrogen bonding. Alpha-helices are stabilized by hydrogen bonds forming between the oxygen atom in a carbonyl group of one amino acid and the hydrogen of an amino group four residues away. In contrast, B-sheets are stabilized by hydrogen bonds between carbonyl and amino groups that may be distant from each other in the sequence but are brought into proximity by the folding of the polypeptide chain. Both alpha-helices and B-sheets involve hydrogen bonding, but the R groups, or side chains, are oriented differently—protruding outward in helices and alternating above and below the sheet in B-sheets.