Chemistry 202 
Peptide Secondary Structure
The Alpha-Helix.
Information on the alpha-helix can be found in your text and lecture notes.
The Beta-Turn
Beta-Turns are usually defined by four amino acids turning back on themselves. The figures below show three of the seven types of beta turns. Note that in each case there is one hydrogen bond and two amino acids at the "corners" of the turns. You should also notice that these structures are all very similar. The difference in confomation is not important at this level. But in biological systems, the correct conformation is the difference between life and death. The type of beta turn displayed is generally related to the identity of the amino acids found in the turn. The use of i, i+1, i+2, and i+3, are symbols used in peptide chemistry to define the first through last residue found in a turn as read from the N-terminus to the C-terminus.
If you have any questions, come to see me or send e-mail to krussell@umiami.ir.miami.edu
The Parallel Beta-Sheet is characterized by two peptide strands running in the same direction held together by hydrogen bonding between the strands. The bottom two strands on the figure represent a parallel beta sheet. The green lines represent hydrogen bonds between the strands. Without seeing the ends of the peptide it can be difficult to tell which direction a peptide strand is running. Then, how then does one tell if two adjacent strands are running in the same direction or opposite directions? That is done by counting the number of atoms in the the hydrogen bonded rings. Each hydrogen bonded ring in a parallel beta sheet has 12 atoms in it. Note that the hydrogen bonds holding together the two peptide chains are not 180 degrees. 
return