This week I looked at Science, Vol. 317, No. 5844, 9/14/07. Nothing really caught my interest, so I looked in the Journal of the American Chemical Society, JACS, Vol. 129, No. 37, 9/19/07.
Here, two articles caught my eye.
Modular Total Chemical Synthesis of a Human Immunodeficiency Virus Type 1 Protease
Human Immunodeficiency Virus Type 1 is more commonly known as HIV-1, the more widespread and contagious form of HIV. HIV-1 protease (HIV-1 PR) is a component of HIV-1 which is present in propagation/reproduction of the virus within the body. The exact catalytic mechanism of this is currently unknown. Being able to synthesize this protease would help us further understand it, and come closer to knowing exactly how the virus reproduces itself.
HIV-1 PR is composed of 2 99-residue polypeptide chains. This means that each of the 2 chains has 99 amino acids bound together in a certain sequence. In order to fully synthesize the protease, 4 smaller chains, about 25 peptides long were made by SPPS, solid phase peptide synthesis. The 4 chains were to be bound together with native chemical ligation, where they are connected by amide-thiol bonds. The thiols are later removed by Ramel nickel desulfurization.
However, when this was attempted, precipitation issues were encountered; the products would aggregate (clump up) and come out of the solution as a solid. In order to prevent this, another tactic was attempted; strings of the amino acid Arginine were bound to the end of one of the 4 small peptide chains. Arginine has been found to keep polypeptides in solution, though the reason is still unknown at this point. 10 Arginine units were bound to one of the 4 chains, and then each of the other chains was able to be connected one by one. The new issue arose of how to remove the Arginine once the 99 peptide chain was constructed. They thought that once constructed, the protease would act as it does when in the body and attempt to begin reproduction. The first step of this is to cleave the end of the chain and bind to another unit. This was successful.
The final product was made in 12% yield, which is a relatively high yield compared to other methods of creating peptide chains. The reaction took about 4 days, and was characterized (identified/verified) by LCMS (Liquid Chromatography Mass Spectroscopy) and MALDI (Matrix Assisted Laser Desorption/Ionization). These methods both take the mass of the product, which can be compared against the theoretical mass. An x-ray structure of the product was obtained by X-ray Crystallography and the product was seen to be bonded together as hoped.
This is a good step in the process, and the novel way that this was made keeps it free from contamination from enzymes which are encountered with other synthetic methods.
DOI: 10.1021/ja072870n
Paclitaxel-Functionalized Gold Nanoparticles
Paclitaxel is a drug which is used in cancer chemotherapy. Traditional drug delivery methods are not effective for all drugs; one major issue is that the hydrophobic nature of some drugs prevent it from being dissolved in the bloodstream and administered normally. A method to circumvent this problem is to enclose the drug in a hydrophobic surrounding, known as a micelle. However, a problem with this method is that this results in a very large product and this can be caught in the reticuloendothelial system, the RES, which is a system to fight foreign substances in the body, and is composed of part of the bone marrow, thymus, and liver.
Another approach is to make a smaller delivery system to avoid capture in the RES. This is done through nanoparticle delivery. Molecules of the drug are embedded onto the surface of a small metal nanoparticle. Gold is a common choice because of the ability to put organic groups onto it through an alkane-thiol bond. Even with this method, there are still problems - it can only be measured qualitatively because it is hard to characterize because of issues with solubility. They found that by using a 2 nm gold core, these issues don't arise.
Paclitaxel was bound to a hexaethylglycol (HEG) chain. A phenol thiol (4-mercapo-phenol) was bound the surface of the gold particle and these two were linked together by combining the carboxyl terminated end of the HEG with the phenol to form an ester. The HEG was chosen as a linker because it increases the solubility in water, and also minimizes destruction of the drug by the RES (which is called opsonization).
This was performed successfully. It is a 9 step process, of which many steps show 90%+ yield. The final product was characterized by NMR and (nuclear magnetic resonance) IR (infrared) spectroscopy. SEC (size exclusion chromatography) showed a good yield of the product with no side reactions resulting in small products. It was shown through TGA (thermogravimetric analysis) that there are approximately 73 (+/- 4) molecules per particle, which results in about the nanoparticle being composed of 60% Pacitaxel, which is an extraordinarily high organic to metal ratio. The product was also seen through TEM (transmission electron microscopy). This novel approach will be able to be used for lots of applications in drug delivery.
DOI: 10.1021/ja075181k
Godspeed.
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