XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1911
peak shift of 46.51 cm–1, signifying a high-intensity bind-
ing at the deprotonated pH.
Comparing shifts at isoelectric pH and deprotonation
pH revealed variations, particularly in the ν(COO–)anti
shift. This huge shift predict that at deprotonation pH the
intensity of binding of gold to alanine is very high com-
pared to the same binding at isoelectric pH.
Computational Study Results
Figure 5 illustrates the optimized structures of both depro-
tonated and neutral alanine complexes. Both can undergo
complex formation through N-Au-N, N-Au-O, and
O-Au-O bindings (refer to Additional Information), the
N-Au-N interactions were found to be the most stable due
to their highly negative complexation energy. Specifically,
the complexation energy for the gold-neutral alanine com-
plex is –345.5 kJ/mol, whereas the gold-deprotonated
alanine complex had a more pronounced stability with a
complexation energy of –441.5 kJ/mol. This difference
shows the likelihood of gold to form complexes with depro-
tonated alanine compared to its neutral counterpart, as evi-
denced by the difference in complexation energies.
The finding from this DFT study aligns with the
research conducted by Buglak and Kononov (2020), who
reported that neutral amino acids exhibited less negative
complexation energies compared to their deprotonated
counterparts. Notably, this trend was observed regardless
of the level of theory used in the DFT calculation, indicat-
ing a consistent preference for deprotonated amino acids in
forming the gold alanine complexes.
An intriguing observation from the geometric opti-
mization is the significantly shorter N-Au bond lengths in
the complex involving deprotonated alanine compared to
the neutral form (refer to Figure 5). This shortened bond
further corroborates the enhanced stability of the gold-
deprotonated alanine complex compared to its neutral
counterpart. Frequency calculations were performed on
both structures to predict the stability constants, revealing a
higher stability with log k=54.8 for the gold-deprotonated
complex as opposed to log k=43.7 for the gold-neutral ala-
nine complex. This indicates that it exhibits greater stabil-
ity despite the deprotonated alanine complex forming more
readily than the gold complex with neutral alanine. This
high stability implies a lower likelihood of the reverse reac-
tion (complex forming metal and ligand) during complex-
ation, setting it apart from its counterpart.
Dissolution Results
The dissolution experiments that were conducted to assess
the extent to which gold is dissolved by neutral alanine and
deprotonated alanine are shown in Figure 6. The experi-
ments were done at deprotonation pH 12.0, where alanine
is fully deprotonated, and at isoelectric pH 6.0, where it
exists fully as a zwitterion. The three experimental runs
done at deprotonation pH had an average gold dissolution
of 1.59 mg/L, whereas the other three experiments at iso-
electric pH had an average of 0.47 mg/L. This shows that
gold can be dissolved more effectively by the deprotonated
alanine molecule compared to the zwitterion alanine.
Several studies, including those conducted by Eksteen
and Oraby (2015), Brown (1982), and Sarvar et al. (2023),
have investigated the use of alanine in gold dissolution.
Brown’s (1982) investigation yielded intriguing findings,
demonstrating an increase in gold dissolution with an
increase in pH from 7.2 (near the isoelectric point) to 9.5,
which is closer to alanine’s deprotonation pH. This suggests
Figure 5. The optimized structure of gold with (a) neutral alanine (b) deprotonated alanine from Gaussian file and the
molecules opened using ChemCraft
peak shift of 46.51 cm–1, signifying a high-intensity bind-
ing at the deprotonated pH.
Comparing shifts at isoelectric pH and deprotonation
pH revealed variations, particularly in the ν(COO–)anti
shift. This huge shift predict that at deprotonation pH the
intensity of binding of gold to alanine is very high com-
pared to the same binding at isoelectric pH.
Computational Study Results
Figure 5 illustrates the optimized structures of both depro-
tonated and neutral alanine complexes. Both can undergo
complex formation through N-Au-N, N-Au-O, and
O-Au-O bindings (refer to Additional Information), the
N-Au-N interactions were found to be the most stable due
to their highly negative complexation energy. Specifically,
the complexation energy for the gold-neutral alanine com-
plex is –345.5 kJ/mol, whereas the gold-deprotonated
alanine complex had a more pronounced stability with a
complexation energy of –441.5 kJ/mol. This difference
shows the likelihood of gold to form complexes with depro-
tonated alanine compared to its neutral counterpart, as evi-
denced by the difference in complexation energies.
The finding from this DFT study aligns with the
research conducted by Buglak and Kononov (2020), who
reported that neutral amino acids exhibited less negative
complexation energies compared to their deprotonated
counterparts. Notably, this trend was observed regardless
of the level of theory used in the DFT calculation, indicat-
ing a consistent preference for deprotonated amino acids in
forming the gold alanine complexes.
An intriguing observation from the geometric opti-
mization is the significantly shorter N-Au bond lengths in
the complex involving deprotonated alanine compared to
the neutral form (refer to Figure 5). This shortened bond
further corroborates the enhanced stability of the gold-
deprotonated alanine complex compared to its neutral
counterpart. Frequency calculations were performed on
both structures to predict the stability constants, revealing a
higher stability with log k=54.8 for the gold-deprotonated
complex as opposed to log k=43.7 for the gold-neutral ala-
nine complex. This indicates that it exhibits greater stabil-
ity despite the deprotonated alanine complex forming more
readily than the gold complex with neutral alanine. This
high stability implies a lower likelihood of the reverse reac-
tion (complex forming metal and ligand) during complex-
ation, setting it apart from its counterpart.
Dissolution Results
The dissolution experiments that were conducted to assess
the extent to which gold is dissolved by neutral alanine and
deprotonated alanine are shown in Figure 6. The experi-
ments were done at deprotonation pH 12.0, where alanine
is fully deprotonated, and at isoelectric pH 6.0, where it
exists fully as a zwitterion. The three experimental runs
done at deprotonation pH had an average gold dissolution
of 1.59 mg/L, whereas the other three experiments at iso-
electric pH had an average of 0.47 mg/L. This shows that
gold can be dissolved more effectively by the deprotonated
alanine molecule compared to the zwitterion alanine.
Several studies, including those conducted by Eksteen
and Oraby (2015), Brown (1982), and Sarvar et al. (2023),
have investigated the use of alanine in gold dissolution.
Brown’s (1982) investigation yielded intriguing findings,
demonstrating an increase in gold dissolution with an
increase in pH from 7.2 (near the isoelectric point) to 9.5,
which is closer to alanine’s deprotonation pH. This suggests
Figure 5. The optimized structure of gold with (a) neutral alanine (b) deprotonated alanine from Gaussian file and the
molecules opened using ChemCraft