First-principles calculations to study (5,5) armchair and (5,0) zig—zag boron nitride nanotubes (BNNTs) functionalized with chitosan are done. The influence of point defects such as mono and di—vacancies as well as the substitutional doping with carbon atoms are also considered. It is shown that the local density approximation within the density functional theory is appropriate to describe these systems. The armchair BNNT with di—vacancies, originally semiconductors, evolve to semimetal in the presence of chitosan; in other cases its semiconductor character is maintained. Zig-zag nanotubes are semimetallic. The high polarity and the broad charge distribution predict its solubility and its possible dispersion in aqueous systems.