Biochemists studying the structure of collagen (a fibrous protein in connective tissue) found cross-links containing ,-unsaturated aldehydes

Question:

Biochemists studying the structure of collagen (a fibrous protein in connective tissue) found cross-links containing α,β-unsaturated aldehydes between protein chains. Show the structures of the side chains that react to form these crosslinks, and propose a mechanism for their formation in a weakly acidic solution.