Nucleon spin structure, topological susceptibility and the $\eta$' singlet axial vector coupling

The observed small value of the first moment of the polarized nucleon spin structure function g_1 may be interpreted, in the Veneziano--Shore approach, as a suppression of the first moment \chi^\prime(0) of the QCD topological susceptibility. I give an extension of the Witten--Veneziano argument for the U(1) problem, which yields the O(1/N) correction to the N = \infty relation \chi^\prime(0)/F^2_0 = 1 (where F_0 is the \eta^\prime axial vector coupling).The correction, although negative, seems too small to account for the data. I further argue that the (\eta,\eta^\prime)\rightarrow\gamma\gam ma and J/\psi\rightarrow (\eta , \eta^\prime)\gamma decays indicate an enhancement rather than a suppression of F_0. A substantial gluon-like contribution in \langle 0\vert\partial^\mu j^{(0)}_{\mu_5} \vert\gamma\gamma \rangle \vert_{q^2=0},which could parallel a similar one in the corresponding nucleon matrix element,is suggested.