Cis/trans Proline vs. Double bond character I am learning about proteins and we have learned that the C-N bond is rigid because of it's partial double-bond character. We have also learned that proline is commonly found in beta turns (type 1) because it can isomerize from trans to cis (which helps make the 180 degree turn).

I don't understand how these 2 things fit together--how can it change from trans to cis if it can't rotate around the C-N bond?

I found this which talks about the double bond being broken (in this case, it would be the partial double-bond, I guess) and then reforming...is that what is going on here?
The change from cis to trans (called isomerization) is not spontaneous. There are proteins called prolyl isomerases (see Wikipedia Prolyl Isomerase) that assist in the isomerization process. This happens when the protein is being folded into its final secondary and tertiary structures.