Proton Nuclear Magnetic Resonance
Four samples were prepared as follows:
a) ~1mg of Arginine + 1mL of D2O + 1mL of NaOD
b) ~1mg of Arginine + 1mL of D2O + 1mL of NaOD + Dried Gold Sample
c) ~1mg of N-capped arginine + 1mL of D2O + 1mL of NaOD + Dried Gold Sample
d) ~1mg of N-capped arginine + 1mL of D2O + 1mL of NaOD
Results
Figures 8 and 9 depict the H-NMR spectra of four samples; two of which contained just the arginine solution and the arginine-stabilized gold nanoparticle solution, while the next two contained just the acetyl-capped arginine solution and the acetyl-capped arginine gold nanoparticle solution. Each peak is assigned to the appropriate splitting pattern of the protons in the structure of arginine.
Figures 8a and 8b display the H-NMR spectrum of the arginine solution, containing no gold nanoparticles. Peaks were observed at ~3.35ppm, ~3.12ppm, ~2.89ppm and ~1.65ppm. An upfield shift was observed when comparing the H-NMR spectra of the arginine solution (without gold nanoparticles), to the arginine-stabilized gold nanoparticle solution (Figures 8c and 8d); peaks were now observed at ~2.55ppm, ~2.41ppm, ~1.91ppm and ~ 0.92ppm. This confirms the formation of gold nanoparticles, but does not localize the nitrogen that donates its lone pair of electrons to ultimately reduce gold from Au3+ to Au0.
Figure 9a depicts the H-NMR spectrum of the N-capped arginine solution (without gold nanoparticles), whereas Figure 9b displays the H-NMR spectrum of the N-capped arginine-stabilized gold nanoparticle solution. According to both spectra, there was no observed chemical shift, suggesting that no gold nanoparticles were formed. Peaks were observed at ~2.55ppm, ~1.91ppm and ~0.91ppm in both spectra. The H-NMR spectrum of the arginine-stabilized gold nanoparticle solution was then compared to the N-capped arginine-stabilized gold nanoparticle solution, to localize the oxidized nitrogen.
The chemical shift observed between the spectra of the arginine-stabilized gold nanoparticle and the N-capped arginine-stabilized gold nanoparticle solution is so small, that the amine group at the B-position can be eliminated as the electron donor. This leaves the three amine groups located on the left-side of the arginine structure. It was also observed that the peak at ~2.41ppm of the arginine-stabilized gold nanoparticle spectrum, did not appear on the N-capped arginine-stabilized gold nanoparticle spectrum; suggesting that the –NH group closest to the C-position of the arginine structure could be the potential electron donor. Since proton exchange of –NH groups is usually too rapid for NMR detection, future efforts should include using aprotic solvents such as DMSO, to slow down the proton exchange. Besides that, other arginine capping agents could also be used to be able to further pinpoint the nitrogen that acts as the electron donor.
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