The protonation state of the imidazole ring of histidine in the spectral calculations was taken to be neutral. Since the experimental pH is close to the pKa of histidine, we calculated the 20 cluster structures again with one of the three histidines protonated to HIP (red H1, yellow H4, purple H5) and with all three histidines protonated to HIP (green) in Figure . While the influence on the IR and ROA is limited, the Raman bands that are affected are those around 1600 cm -1 (imidazole stretching deformations: C=C stretching), around 1510 cm -1 (imidazole stretching deformations: C=N stretching), 1360 cm -1 (complex coupled vibrations, C-H deformations and imidazole stretching deformations), 1190 cm -1 (skeletal deformations, also in the protonated imidazole). 34 Proof-of-concept: randomized side chains