Amino acid derivatives of naphthalenediimide (NDI) form non-covalent polymers, which assemble into helical nanotubes through hydrogen bonding. The two enantiomers possess distinct circular dichroism (CD) spectra, but the bands could not be entirely ascribed to the effects of the monomer or a supramolecular structure. We calculate the CD of oligomers, using the (exciton) matrix method, based on ab initio results for the monomer. Several features in the experimental spectrum could be reproduced well and allow assignment of the electronic states of the oligomeric structure. The calculations provide evidence that the oligomer structures in the solid state and the solution phase are equivalent. The calculated spectra show a dependence on the oligomer length, which potentially could be exploited for the experimental characterization of the length of the helical nanotubes.