We have investigated the adsorption of hydrophobic amino acid valine/valine radicals with carbon nanotube (5, 0) using density functional theory. Non-covalent interaction of valine with the nanotube exhibits differential binding strength with respect to the functional group of valine present close to the nanotube surface and it has enhanced the conductivity of the tube. Covalent interaction of valine radicals with the CNT introduced local sp(3) hybridization at the adsorption site. In some of the cases nitrogen atom of the valine radical has broken the C-C bond and introduces a seven member ring in the adsorption site to retain the sp(2) bonding network. (C) 2011 Elsevier B. V. All rights reserved.