The study analyses the effect of adjacent C-13 nuclei on the relaxation properties of the spins suitable for the relaxation studies in nucleic acids, namely C1' in sugars, C8 in purines and C5 and C6 in pyrimidines. Numerical simulations show that the additional 13C nuclei increase R1 and R2 relaxation times and decrease the values of heteronuclear NOEs. As a result, the order parameters and correlation times for the internal motions are overestimated if the relaxation data obtained from fully labeled samples are analyzed using a standard modelfree approach.