The torsional potential energy scans of alanine, glycine and all sidechains are the same as those used in the development of the OPLS-AA/M force field, as described previously. In brief, structures were relaxed in the gas phase using Gaussian 09 with a ωB97X-D functional and a 6-311++G(d,p) basis set. Dihedral angles were scanned in 15 • increments from -180 • to 180 • . A single point energy calculation was then performed on the optimized structure using the double hybrid functional B2PLYP-D3(BJ) and the Dunning basis set aug-cc-pVTZ.