Reactivity in Chemistry
Reduction Potentials of Metal Ions in Biology
MB7. Solutions for Selected Problems
At neutral pH, lysine would normally be protonated. It would noy have a lone pair on the nitrogen to coordinate a metal ion.
The pKa of an α-position is typically about 20, although it can vary depending on what other groups are nearby. That is too high to build up a significant amount of the deprotonated species in water, which has a pKa (in water) of 14.
a) The pKa of the side chain of histidine is about 6.0; upon changing from pH 7 to pH 5, this group would become protonated and positively charged. The positive charge would be more stabilising (or less destabilising) toward Fe2+ than Fe3+; the reduction potential would increase.
b) The pKa of the side chain of histidine is about 3.7; upon changing from pH 4 to pH 3, this group would become protonated and positively charged. The positive charge would be more stabilising (or less destabilising) toward Fe2+ than Fe3+; the reduction potential would increase.
c) The pKa of the side chain of histidine is about 4.2; upon changing from pH 4 to pH 5, this group would become deprotonated and negatively charged. The negative charge would be more stabilising (or less destabilising) toward Fe3+ than Fe2+; the reduction potential would decrease.
Ti4+ and Ca2+
F- and HO-
a) Co2+ with NO2-
b) Mg2+ with CH3CO2-
c) Cu+ with RS-
d) Fe2+ with N2
e) Zn2+ with imidazole
Choose the best match for the following metal ions.
a) Fe3+ with asp
b) Cu+ with met
c) Zn2+ with his
d) Cu2+ with his
e) Co3+ with glu
Choose the amino acid residue that would have the effect on the metal ion as described, based on HSAB principles.
a) Increase the reduction potential of Cu2+: met.
b) Decrease the reduction potential of Fe3+: glu.
c) Make Cu+ easier to oxidise: his.
d) Make Fe2+ easier to oxidise: asp.
e) Make Fe3+ easier to reduce: cys.
a) 0 β b) 1.73 β c) 2.83 β d) 3.87 β e) 4.89 β f) 5.92 β
a) approx. 4 β b) approx. 4 β c) approx. 2 β d) approx. 3 β
e) approx. 5 β f) approx. 0 β g) approx. 6 β
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Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.
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