Biocidal Mechanisms of Metallic Copper Surfaces - Use of Biocidal Surfaces for Reduction of Healthcare Acquired Infections - page 105

6.2 Copper the “Modern” Bioelement
6.2.1 General Chemistry Properties
Along with the transition metals silver and gold, copper belongs to the group 11 of
the periodic table and is referred to as a coinage metal due to the characteristic
color, corrosion resistance and value. The two stable isotopic forms present in the
Earth's crust is constituted primarily of form 63 (69.1 %), and form 65 (30.9 %).
The atomic number of copper is 29 with an electronic distribution of 1s 2 2s 2 2p 6 3s 2
3p 6 4s 2 3d 9 , yet, this distribution does not represent a low energy state. Naturally,
copper has one electron from the 4s orbital shifted to the 3d orbital (4s 1 3d 10 ). The
inner electronic layers (1s, 2s, 2p, 3s and 3p) are closer to the positively charged
nucleus permitting the 4s electron to “escape” to the 3d orbital, characterizing a low
energy state [ 40 ]. Copper can lose up to two electrons in one-electron step transfers
resulting in cuprous (Cu(I)), and cupric (Cu(II)). Altogether, these physical-
chemical features are of extreme importance for living organisms: copper can be
used in different types of reactions by controlling the mechanism and rate of
copper-catalysis. Copper importance and bioavailability was greatly enhanced
with the shift from an anaerobic to an aerobic atmosphere during the Great
Oxidation event, making it an essential micronutrient and multicellular life possible
[ 21 ]. Further details will be discussed in the next section.
6.2.2 How Organisms Use Copper
The primordial Earth atmosphere was anaerobic and early life forms used iron
given its bioavailability as water-soluble ferrous iron [Fe (II)] [ 15 , 32 , 68 ]. Redox
properties of iron were in the range of biological reduction potentials under
anaerobic conditions, making its presence essential for survival of early organisms.
Conversely, copper was not accessible for biological processes due to its existence
mostly in the form of water-insoluble cuprous sulfides [Cu(I)]. Soluble copper was
only present in acidic waters near hydrothermal vents, which are extreme environ-
ments that have not been representative of life on earth. In other words, copper was
not readily bioavailable under anaerobic conditions [ 15 , 32 , 68 ]. The aerobic
atmosphere started to be established about 10 9 years ago with the production of
oxygen as a by-product of prokaryotic (cyanobacterial) metabolism [ 15 , 32 , 68 ].
This fundamental change in the atmosphere resulted in one of the major alterations
(or pollution) of Earth's life conditions. The arrival of dioxygen was dramatic for
most living organisms because of its toxicity and its effect on the bioavailability of
metals such as iron and copper. The new atmosphere oxidized iron to the water-
insoluble ferric iron (III) state and, as a result, bioavailability of iron was lost.
Instead, copper became bioavailable due to the oxidation of insoluble Cu(I) to
soluble Cu(II). Metabolism under anaerobic conditions was designed to use