Insulin+and+Gold+Nanoparticle+Bonding+Interactions

Home

**__Covalent Bonding Interactions:__**

The nature of the binding of insulin to gold nanoparticles provides an important aspect that affects the stability as well as the conditions under which release of insulin from gold nanoparticles occurs. Current research has proposed nanoparticle formulations where convalent linkages of insulin are bound to the gold nanoparticles, as well as formulations through interaction of hydrogen bonding with amino acid capped gold nanoparticles (Scheme 1).



​ Figure 5: Loading of Bare capped nanoparticles and aspartic acid capped nanoparticles

Direct covalent binding of insulin to the gold nanoparticle surface occurs through amine or thiol groups which appear on the gold nanoparticle surface. In contrast, formulations in which gold nanoparticles are capped with aspartic acid produce weaker hydrogen bonding interactions, causing facile release of insulin. Research has shown that insulin bound to bare gold nanoparticles through covalent linkages has a lower success rate (86%) in comparison to aspartic acid capped gold nanoparticles (95%) (Langmuir, 2006). The weaker hydrogen bonding and electrostatic interactions are responsible for the faster and larger release of insulin.


 * __Insulin Interactions:__**

Insulin is composed of 51 amino acids, including cysteine. Chain A, containing 21 amino acids, and C, containing 30 amino acids, of insulin are held together by disulfide bonds that are most likely afforded by the cysteine amino acid. Cysteine contains a thiol functional group, and an oxidation reaction would result in the removal of hydrogens from -SH to form S-S bonds. Sulfur is am important element in insulin, the loading of insulin on gold nanoparticles, as well as, the reaction that occurs when insulin reaches the receptive site, which is speculated to be composed of a sulfur-containing functional group.

Figure 6: Schematic representations of reactions on surfaces of metallic gold and elemental sulfur

For the purpose of this project, it is speculated that the receptive site of insulin across the transmucosal site is composed of a sulfur-containing functional group. The reason for this speculation is that gold-sulfur interactions are quite strong, with a high bond enthalpy of 418 kJ/mol ( Damge et al. 2008 ). The bond energy makes this interaction desirable and a robust attachment mechanism because sulfur has a high affinity for gold. However, the sulfur-containing functional group across the transmucosal site has not been confirmed, and thus, further research is needed to better understand the delivery of insulin via gold nanoparticles.

__**Gold Nanoparticle Interactions:**__

Gold nanoparticles are thermally and air stable since the monodispersed gold nanoparticles are capped by densely packed monolayers. Self-assembled monolayers (SAMs) are amphiphilic organized monolayers, with a hydrophilic head group and a hydrophobic tail group. The monolayers are organized based on the reactivity of the head group towards the hydrophobic substrate. Also, SAMs control the interfacial properties of the synthesis gold nanoparticles solids surfaces (Zhou et al. 2008). Thiol group is the most commonly selected anchor groups for adsorption onto the surface of the gold nanoparticle because of the strong chemical bond formed between gold and sulfur. In molecular scale, the formation of ordering alkanethiols is driven by strong affinity between sulfur and gold, the van der Waals interactions between the tethered alkyl chains, and the dipole interactions between the polar end groups.



Figure 7: Self-Assembly of thiol monolayers on gold surface

In the present project, gold monolayers are formed by having molecules with a sulfur-containing functional group reacting with the hydrophobic gold nanoparticle. Therefore, when synthesizing gold nanoparticles, the hydrophobicity of the particle plays a major role, as it dictates the reactivity and binding interactions between gold and sulfur ( Damge et al. 2008 ). To ensure that a hydrophobic nanoparticle is synthesized, the used of hydrophobic polymer is exploited.

In addition, hydrophobic nanoparticles are generally taken up more extensivey by the intestinal epithelium than hydrophilic nanoparticles ( Damge et al. 2008 ). It has been found that hydrophobic nanoparticles have a higher affinity for M cells than for absorptive cells. Hydrophobic nanoparticles poorly penetrate into the mucus, which is dense with epithelial cells. For this reason, it is important to have a balance of hydrophobic/hydrophilic properties inside the SAM to ensure the uptake of insulin across the intestinal epithelium.

__**Acid/Base Interactions:**__

In terms of the Hard Soft Acid Base (HSAB) theory, amines are generally hard bases, while gold(I) is a soft acid. Thus, if the gold nanoparticles present come from gold (I), then this binding is not expected to happen. The HSAB theory does have its limitations, in that it does not take into account other factors that contribute to the stabilization of the bond strength between the acid and base of the compounds. Such factors are the sizes of the charges, the electronegativities, and the orbital overlap between the acid and the base (Housecroft and Sharpe, 2005).

For the gold core-amine complex, the size of the charges is approximately the same, which would allow for excellent overlap between the s-orbitals of the complex. In addition, the electronegativities of nitrogen and gold elements are 3.0 and 2.5, respectively. The similarity of the electronegativities of the elements is thought to positively influence the reaction, and aid in its occurance. Thus, for this reason, the HSAB theory cannot be the only factor in determining the stability of the gold-amine complex. In addition, taking into consideration that the reaction between the amine functional group and the gold nanoparticles is taking place in vivo, where target specific enzymes are present, there is reason to believe that such enzymes are affording the occurance of this reaction.

Insulin in neutral solutions can bind to nanoparticles in the present of NaOH by physical adsorption (Shimkunas, et. al. 2008). Due to van der waals interaction, hydrogen bonds form between -NH3+ and-COO-, and other CO- containing surface groups. The charge amino acid on the surface of the insulin molecule attract to the nanoparticles’ surface. The van der waals and hydrogen bonding interactions between nanoparticles functional groups and amine groups of insulin introduces a attractive interaction which makes insulin adsorbs onto the surface of the nanoparticles. (Shimkunas, et. al. 2009)  __**Entropy Considerations:**__   <span style="font-family: Arial,Helvetica,sans-serif; font-size: 137.28%;">Entropy is a measure of disorder in a thermodynamic system (Atkins and DePaula, 2006). In drug synthesis and drug delivery systems, it is favourable for a molecule to have order, because for chemical reactions taking place in vivo with specific enzymes present, the molecule is desired to have little flexibility. This lack of flexibility ensures that the chemical reaction preferred will take place.

By synthesizing gold nanoparticles of a certain size and of unique dimensions, there is a “prepaid” entropy price. The entropy cost is prepaid in the sense that the molecule synthesized is relatively rigid and has more order. A rigid molecule does not have many rotatable bonds. This quality is desirable in the drug synthesis/drug delivery systems industry because only one specific reaction can take place at a given time. Thus, the prepaid entropy cost and the thermodynamic factors of a given compound are important when considering them in terms of drug delivery systems. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 14.4pt;">Specific to the topic, the gold nanoparticles synthesized expose large surface areas to allow for bioconjugation with insulin. Insulin, being a protein, also has a fixed backbone protein structure in the form of a helix. The helix is held together by hydrogen bonding and other acid/base interactions occurring between the different amino acids in insulin.

The fixed backbone of insulin results in a loss of entropy, which is thermodynamically unfavourable (Glomm, 2005). However, the entropy effect is compensated for by the gain in free energy from hydrophobic interactions between the amino acids. Therefore, conducting a reaction between a fixed-sized gold nanoparticle and the rigid protein backbone of insulin is temporarily thermodynamically unfavourable, but results in a great gain of free energy that can be used to drive the reaction forward in the spontaneous direction.

Home Transport of Insulin through chitosan reduced gold nanoparticles