The structural, electronic and chemical bonding properties of biopolymer chitosan in tetramer structures are investigated by using density functional theory. These polymer structures exhibit structural activity relationship because the activity of these polymer isomers varies with the structural modification and molecular weight. In this approach the Generalized Gradient Approximation (GGA) is used for the exchange-correlation potential. In density of states analysis the calculated forbidden energy gap is reduced by 2.23eV for cis-chitosan structure, deduces its semi-conducting behaviour with decreased crystallinity and improved bioactivity. Hirshfeld population and charge density techniques are used to analyse electrostatic interactions and the charge distribution within these molecules. Crystal orbital overlap population shows ionic interactions and provide chemical bonding information. Modification of chitosan improves the properties and provides the better functionally improved bio-polymer. This study is aimed to be used in various chemical industries, pharmaceutical and biomedical fields.