  We investigate the neutron number $(N)$ dependence of root mean square radii
of point proton distribution (proton radii) of Be, B, and C isotopes with the
theoretical method of variation after spin-parity projection in the framework
of antisymmetrized molecular dynamics (AMD). The proton radii in Be and B
isotopes changes rapidly as $N$ increases, reflecting the cluster structure
change along the isotope chains, whereas, those in C isotopes show a weak $N$
dependence because of the stable proton structure in nuclei with $Z=6$. In
neutron-rich Be and B isotopes, the proton radii are remarkably increased by
the enhancement of the two-center cluster structure in the prolately deformed
neutron structure. We compare the $N$ dependence of the calculated proton radii
with the experimental ones reduced from the charge radii determined by isotope
shift and those deduced from the charge changing interaction cross section. It
is found that the $N$ dependence of proton radii can be a probe to clarify
enhancement and weakening of cluster structures.
