With the use of first-principles total energy calculations, the optimized geometrical structures of seven different junctions constructed from different carbon nanotube units (metallic and semiconducting) and different covalent linkages (peptide bonds, -CONH-C6H4-CONH-, and >CON-C6H4-CON<) are investigated. By performing a set of nonequilibrium Green's function calculations combined with density functional theory, the I-V characteristics of some of the designer junctions are obtained. The results show that the junction made of two different types of carbon nanotube units (metallic and semiconducting) acts as a rectifying diode. On the other hand, the junctions made of carbon nanotube units with the same chirality (zigzag or armchair) show almost symmetric I-V curves under positive and negative applied voltages. Our results are expected to contribute in the design and implementation of various electronic logic functions based on carbon nanotubes for applications in the field of nanoelectronics.