Original View of Sickle Cell Hemoglobin.  The entire protein is made of alpha helices (red) and loops (white).
The protein is made of four differnt subunits.  Two are alpha chains (light and dark blue) and two are beta chains (yellow and green).
In sickle cell hemoglobin, a single mutation causes the 6th amino acid of the beta chains to be a valine instead of a glutamate (highlighted in red).
Notice that these valines are located on the outside surface of the protein.  In normal hemoglobin, the glutamate in this position hydrogen bonds with water to help keep the protein in solution.
In sickle cell hemoglobin, the non-polar valine embeds itself in a hydrophobic pocket of an adjacent hemoglobin, forming a hemoglobin dimer (two hemoglobins stuck together).
Because each hemoglobin protein has two beta chains, the valines on the other ends will bind to other hemoglobin proteins, forming long chains which can't transport oxygen effectively.  The chains deform the red blood cells, causing the sickle shape of the red blood cell..