Step 3: Building a peptide chain

As another example, we will build a small peptide chain.

Select File → New
Select the  Structures → Amino Acids → AA Backbone structure
Place it in the drawing area 

There appears a subunit (or actually two) of a basic peptide chain. Notice that one of the atoms is selected, namely the terminal nitrogen. This atom is, again, the 'replacing' atom. In order to extend the peptide backbone, you now have to choose the right atom to be replaced. The obvious choice is the (non double-bonded) terminal oxygen.

Press the space bar, which activates the last used option of the 'Structure' tool ('AA backbone' here). 
Double click on the terminal oxygen.
You may want to use View → Reset View.

You can now replace the hydrogens on the backbone by amino acid side groups of your choice. These can be found in the Structures → Amino Acid → AA Side Groups submenu.