Chemistry of Biuret, xanthoprotic, Millon's, and Molisch's tests

[chound]

What is the chemistry behind the following tests?
1)Biuret
2)Xanthoprotic
3)Millon's
4)Mollisch's test(Carbohydrates)

 

[chemisttree]

I find it hard to believe that a med student can't use google...