  The conjugation of high-affinity cRGD-containing peptides is a promising
approach in nanomedicine to efficiently reduce off-targeting effects and
enhance the cellular uptake by integrin-overexpressing tumor cells. Herein we
utilize atomistic molecular dynamics simulations to evaluate key
structural-functional parameters of these targeting ligands for an effective
binding activity towards $\alpha_V\beta_3$ integrins. An increasing number of
cRGD ligands is conjugated to PEG chains grafted to highly curved TiO$_2$
nanoparticles to unveil the impact of cRGD density on the ligand's
presentation, stability, and conformation in an explicit aqueous environment.
We find that a low density leads to an optimal spatial presentation of cRGD
ligands out of the "stealth" PEGylated layer around the nanosystem, favoring a
straight upward orientation and spaced distribution of the targeting ligands in
the bulk-water phase. On the contrary, high densities favor clustering and
internalization of cRGD ligands in the inner region of the PEGylated layer,
driven by a concerted mechanism of enhanced ligand-ligand interactions and
reduced water accessibility over the ligand's molecular surface. These findings
strongly suggest that the ligand density modulation is a key factor in the
design of cRGD-targeting nanodevices to maximize their binding efficiency into
over-expressed $\alpha_V\beta_3$ integrin receptors.
