Insulin was the first protein to have its primary structure determined.
Image
available at the Protein Data Bank. Primary citation is Chang, X., Jorgensen,
A.M., Bardrum,
P., Led,
J.J. 
Solution
structures of the R6 human insulin hexamer.
Biochemistry
v36
pp.9409-9422 , 1997
Chemistry 108: Essentials of organic and biochemistry, Spring 2006