Proteins are expected23 to bind to the nanoparticles through those amino acids which form complexes with gold; among them sulfur containing cysteine, nitrogen containing histidine and tryptophan, and the basic amino acids lysine and arginine, at high pH above their isoelectric point (pI). The acids, aspartic and glutamic acids, would also be expected to adsorb when deprotonated, as would tyrosine through the oxygen atom side group following the loss of hydrogen. At the pH used in these experiments, 7.4, the deprotonated acids would be expected to bind to the nanoparticles, but not the bases. The results outlined in Table 1 confirm this hypothesis. The tentative peak assignments are taken from a variety of sources. Cysteine, histidine, tryptophan, tyrosine, aspartate, and glutamate peaks dominate the spectra below , while the high-frequency peaks are likely due to C─H and vibrations of these amino acids. In addition, among these amino acids, few characteristic peaks appear below . The appearance of low-frequency features arises from the binding of sulfur, nitrogen, and oxygen to the AuNP; for which the magnitude of charge transfer decreases in the same order, sulfur-nitrogen-gold. This has the effect of reducing the contribution of charge-transfer effects to the resonance enhancement in SERS, reducing the strength of these modes, in the order of 302, 397, and .