In proteins, proline can act as a structural disruptor for (α) helices, and as a turning point in β sheets. Multiple prolines in a row can create a proline helix. Sequences of proline and 1-Aminosiobutyric acid (Aib) form a helical turn structure.
Incorporated into peptides proline forms a tertiary amide. It doesen't have a hydrogen on the amide group and can therefore not act as a hydrogen bond donor.