Question

why amino acid may be acidic, alkaline or neutral ?

Answer 1

Amino acids can be any of those three options.  Every amino acid by definition has a carboxylic acid group and an amino group, which are acidic and basic in that order.  In solution, these functional groups allow amino acids to become what is called a zwitterion, which is a molecule containing both a positive and negative charge.  What determines whether an amino acid is acidic, neutral, or alkaline is the R group attached to its alpha carbon.  Some amino acids have additional carboxylic acid functional groups, leading to acidity.  This is why it helps to know the structures of amino acids and the basics of acidity and basicity in organic chemistry.  If you do not want to memorize all the structures, looking at a table of pKa values would also help.