  Recent shell-model calculations of weak-interaction rates for nuclei in the
mass range A = 45 - 65 have resulted in substantial revisions to the hitherto
standard set of Fuller, Fowler, & Newman (FFN). In particular, key
electron-capture rates, such as that for Co60 are much smaller. We consider
here the effects of these revised rates on the presupernova (post-oxygen
burning) evolution of massive stars in the mass range 11 to 40 M_sun. Moreover,
we include, for the first time in models by our group, the effects of modern
rates for beta-decay in addition to electron capture and positron emission.
Values for the central electron mole number at the time of iron core collapse
in the new models are typically larger, by delta Y_e = 0.005 to 0.015, than
those of Woosley & Weaver 1995, with a tendency for the more massive models to
display larger differences. About half of this change is a consequence of
including beta-decay; the other half, result of the smaller rates for electron
capture. Unlike what might be expected solely on basis of the larger Y_e's, the
new iron core masses are systematically smaller owing to a decrease in the
entropy in the outer iron core. The changes in iron core mass range from zero
to 0.1 M_sun. We also observe, as predicted by Aufderheide et al. (1994), a
tendency towards beta-equilibrium just prior to the collapse of the core, and
the subsequent loss of that equilibrium as core collapse proceeds. We discuss
the key weak reaction rates, both beta-decay and electron-capture, responsible
for the evolution of Y_e and make suggestions for future measurements.
