  The project aims to investigate most important weak interaction nuclei in the
presupernova evolution of massive stars. To achieve the goal, an ensemble
containing 728 nuclei in the mass range A = 1--100 was considered. We computed
the mass fractions of these nuclei using Saha's equation for predetermined
values of \textit{$T$}, \textit{$\rho$} and \textit{$Y_e$} and assuming nuclear
statistical equilibrium. The nuclear partition functions were obtained using a
newly introduced recipe where excited states, up to 10 $MeV$, were treated as
discrete. The weak interaction rates (electron capture (\textit{ec}) and
$\beta$-decay (\textit{bd})) were calculated in a \textit{totally} microscopic
fashion using the proton-neutron quasiparticle random phase approximation
(pn-QRPA) model and without assuming the Brink-Axel hypothesis. The calculated
rates were coupled with the computed mass fractions to investigate the time
rate of change of lepton to baryon fraction of the stellar matter. We compare
our results with the previous calculations reported in the literature.
Noticeable differences up to orders of magnitude are reported with previous
calculations. These differences may influence the evolution of the star in the
later stages of presupernova. We present a list of top 50 \textit{ec} and
\textit{bd} nuclei which have the largest effect on $Y_e$ for conditions after
silicon core burning. The competition between the \textit{ec} and \textit{bd}
rates in stellar core was investigated and it was found that $Y_e$ =
0.424--0.455 is the interval where the \textit{bd} results are bigger than the
\textit{ec} rates.
