Why can DAP be used to elute a protein in affinity chromatography? A little background: as part of my bioinformatics degree I have to take Protein Chemistry course, but I miss chemistry basics (have CS background), so that's why I'm asking the following question.

We performed protein (Galanthus Nivalis L. agglutinin) purification in class, using affinity chromatography. 
The mannose column was loaded with protein (mixed with ammonium sulphate), washed with ammonium sulfate and then the protein was eluted using 20 mM DAP. While I understand the principle of affinity chromatography, I can't really figure out the chemistry of using DAP to elute the protein.

Would appreciate any help.
In this report, diaminopropane is used to elute a lectin from an affinity column. They mention the pH, which is the key to the question.


  The  bound  lectin was  eluted  with  20  mM diaminopropane  containing  0.15M NaCl,  pH ~11.


The diaminopropane is unbuffered, and the high pH weakens the interactions between lectin and carbohydrates.