Proton spin content and QCD topological susceptibility

The part of the proton spin $\mathrm{\Sigma }$ carried by $u,d,s$ quarks is calculated in the framework of the QCD sum rules in the external fields. The operators up to dimension 9 are accounted. An important contribution comes from the operator of dimension 3, which in the limit of massless $u,d,s$ quarks is equal to the derivative of QCD topological susceptibility ${\chi }^{\mathrm{\prime }}(0)$. The comparison with the experimental data on $\mathrm{\Sigma }$ gives $\chi^{\prime}(0)= (2.3 \pm 0.6) \times 10^{-3} ~ GeV^2$\mathrm{.}Thelimitson$\Sigma$and$\chi^{\prime}(0)$arefoundfromselfconsistencyofthesumrule,$\Sigma \ga 0.05,~~ \chi^{\prime} (0) \ga 1.6 \times 10^{-3} ~ GeV^2$\mathrm{.}Thevaluesof$g_A = 1.37 \pm 0.10$and$g^8_A = 0.65 \pm 0.15$ are also determined.