To create colloidal crystals of AP205 at pH values > 6.5 in PBS, pMETAC was synthesized with atom-transfer radical polymerization (ATRP) with similar molecular weight and net charge as selected antimicrobial peptides. A weight-averaged molecular weight (Mw) of 3.3 kDa and a number-averaged molecular weight (Mn) of 2.65 kDa (DI = Mw/Mn =1.26) was achieved (see Supporting Information for further information). The composition-triggered formation of pMETAC / AP205 VLP composites was then studied with SAXS. In our previous study, this polymer triggered the aggregation of Qbeta into colloidal crystals through specific adsorption events. In PBS at AP205 VLP/pMETAC weight ratios between 1:5 and 1:20, Bragg reflections and an upturn in the low-q < 0.2 nm -1 appear in the SAXS curves; see Figure . This upturn arises from attractive interactions among the AP205 VLP, and the Bragg patterns demonstrate the formation of internally geometrically ordered AP205 aggregates (supraparticles). Upon increasing the pMETAC concentration to 1:50, the Bragg peaks disappear. This likely results from specific adsorptions of pMETAC, leading to charge reversal and repulsive interactions of the polymer coils extending out from the 'saturated' surface of VLP.