Background

Home __Amino Acid Background:__ **  Over 700 amino acids occur naturally, 20 of these amino acids are considered 'standard' amino acids, which can further be divided into essential or non-essential amino acids (Young, 1994). Each of the 20 amino acids contain a basic structure including a carboxylic acid group, an amine group and a substituent side chain which differs in each of the amino acids. The properties of each amino acid vary as the structure of the substituent varies, allowing classification of the amino acids into several categories based on the properties of the substituent. Such categories include hydrophilic, hydrophobic, acidic side chains, basic side chains, as well as hydrogen bond donors and hydrogen bond acceptors. Such chemical properties play an important role in determining the kinds of interactions amino acids can undergo.

 Figure 1: Cysteine

Sulfur containing amino acids, cysteine and methionine are generally non polar and hydrophobic. Methionine is one of the most hydrophobic amino acids and is therefore most often found on the interior of the helix structure in proteins. Cysteine is able to ionize to form a thiolate anion, therefore despite its negative charge, it is most often also found on the interior portion of the protein helix. The thiol group present in cysteine is able to react with other thiol containing compounds through an oxidation reaction in order to form a disulfide bond, serving as an important reactivity and structural functions in proteins (Hell and Wirtz 2008). Cysteine residues are able to crosslink proteins which form through disulfide bridges between the cysteine residues. Insulin is an example of a protein with cysteine crosslinking in which two separate peptide chains are connected by a pair of disulfide bonds (Hell and Wirtz 2008).

Figure 2: An Insulin molecule formed by two chains (A and B) linked by a disulfide bond

__** Insulin Background: **__

Insulin is a protein, composed of a 51 amino-acid peptide chain (Banting, 1922). The structure of insulin contains two chains, the A chain contains 21 amino acids, the B chain contains 30 amino acids (Figure 1). It is produced inside the body by pancreatic cells to maintain blood glucose levels by enabling the cells to absorb glucose, which can then be turned into energy. High blood glucose levels cause the disease known as diabetes mellitus as a result of the body either not producing enough insulin or the cells not correctly responding to the insulin being produced. Diabetes can lead to many complications including neuropathy, nephropathy, blindness, cardiac failure and many others human illnesses (Daneman, 2006).

Diabetes is classified into two types, Type 1 and Type 2. In type 1 diabetes there is auto-immunization of B cells that decreases the production of the insulin, and hence decreases the metabolism of the glucose present in the blood. Meanwhile, in type 2 diabetes, the binding of the glucose to its receptor on the target cell, which is mediated by insulin, is disturbed due to insulin resistance and leads to an increase in the blood glucose level (Damgé, 2008). Due to insulin resistance in type 1 diabetes, insulin is currently only used as a treatment in type 1 diabetes, most commonly through subcutaneous injections.

__** Gold Background: **__

Gold is a third row transition metal in the periodic table of elements. For millennia, it has been used industrially and for decorative purposes (Schmidbaur, et.al., 2005). Understanding the chemistry of gold is very important when considering its uses and reactivity. Gold has been compared to copper and silver in terms of reactivity, but gold was of particular interest to scientists because it has unique chemical properties. As an atom, gold has the atomic number of 79, with only one stable isotope at 197 g / mol atomic mass. With an electron configuration of [Xe][4f14][5d10]6s1, gold contains 14 f-electrons and 10 d-electrons. These electrons are the farthest from the nucleus, and therefore, can be easily removed when reacting with another molecule.

Since there is only one s-electron present, it is more energetically favourable for gold to lost that electron and become gold (I), making it the most abundant of the gold oxidation states. Gold (III) is also possible when dissolving elemental gold in strong oxidants. Referring to the Crystal Field Theory (Housecroft and Sharpe, 2005), gold (III) is expected to be a coloured substance because the configuration would become [Xe][4f14][5d8], and d-electrons are capable of absorbing ultraviolet-visible (UV-Vis) light when promoted to a higher energy orbital. The wavelength at which gold absorbs UV-Vis light can be determined by using a UV-Vis spectrometer, and the energy of gold (I) can be calculated using the wavelength obtained. This energy would be the energy it takes to promote an electron from a low energy orbital to a high energy orbital.

Of the other elements in the periodic table, gold can form thermodynamically favourable binary combinations with very few of these elements (Schmidbaur, et.al., 2005). Sulfur and oxygen, however, do not form thermodynamically stable binary combinations with gold. Hence, it is believed that sulfur and gold form kinetically stable combinations. A kinetically stable reaction is one that has a low activation energy, and can form an intermediate or product at a fast rate. Meanwhile a thermodynamically stable reaction has a high activation energy, forms the product at a slower rate, but it the most stable of the two products due to less steric hindrance, and possibly other factors.

From a molecular orbital point of view, gold contains s, p, d and f orbitals. Using a relativistic concept, it has been determined that the s-orbital undergoes contraction and the d-orbital undergoes expansion during reactions. This information provides insight and qualitative description of the electronic state in the frontier orbitals when compared to a non-relativistic atom. Molecular orbital analyses can also attribute the yellow colour of elemental gold to the small gap between the full 5d-orbital band, the expanded orbital, and the Fermi level of 6s-orbital band, the contracted orbital. The small band between the two orbitals allows for the calculation of the energy of the complex, as mentioned above.

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