In this Letter, the electronic and structural properties of the -COOH, -NH2, and -CONH2 groups, used as sidewall functionalizations by interacting with an (8, 0) single-wall carbon nanotubes, were investigated by using ab initio calculations. The results show that the systems have a very stable interaction with the nanotubes, presenting similar properties. The binding energies and the charge transfers between the functional groups and the tube are calculated and compared. A common feature for the electronic structures of these groups is a half-filled level in the gap region. These covalently functionalized nanotubes are promising systems for attaching biomolecules aiming bio-related applications. (c) 2006 Elsevier B.V. All rights reserved.