The two ethyl ester moieties of 2 do not allow the pyridyl nitrogen to participate in any intermolecular interactions, a key difference from 1, resulting in a different packing motif that forces the Br atoms to be oriented toward each other and producing the alternating chain of Br atoms.
This different behaviour of the free volume is due to a different packing efficiency of chains and a different dominant mechanism of the free volume generation, as well as a decrease in the flip dynamics of the phenyl rings.
On the other hand, solid-state samples of low MW P3ATs often exhibit also a different packing structure (form II) [3, 6, 9], in which the distance between the nearest-neighbor backbones increases up to 4.4 Å due to tilted and interdigitated alkyl chains [2, 3, 12 14].
Indeed, a different packing arrangement of α-Syn on the hydrophobic substrate results in a significantly higher density and a slight increase in shear modulus.
Three tetragonal anhydrate structures are sufficiently stable to appear on the crystal energy landscape within 13 kJ mol 1 of the most stable structure, each with a different packing arrangement and packing efficiency.
Subsequently, this model was used in molecular replacement to solve the structure of Pat-C at 3.1 Å resolution, with four molecules per asymmetric unit in a different packing environment (Rwork=24.8%, Rfree=28.1%, crystal form II; Supplementary Table S2 and Supplementary Figure S4A).
