Peptide Synthesis

Topics: Amino acid, Carboxylic acid, Peptide synthesis Pages: 12 (4020 words) Published: July 24, 2012
This experiment synthesized H-Gly-Phe-OH dipeptide using “Fmoc chemistry”. The first part of experiment was the synthesis of L-phenylalanine methyl ester hydrochloride. The methyl ester can be synthesised by reaction of thionyl chloride, SOCl2 and dry methanol with L-phenylalanine under reflux condition. The peptide bond was formed later in the experiment, where HBTU, DiPEA and a solution of Fmoc-Gly-OH in DMF were added to a solution of L-phenylalanine methyl ester hydrochloride in DMF. The product formed was the protected dipeptide, Fmoc-Gly-Phe-OMe. The deprotection then took place in which ethanolic sodium hydroxide was used as a base to remove the Fmoc group and thus the exposed amine was neutral. The final product was H-Gly-Phe-OH as colourless oil with crystals forming. Introduction:

In organic chemistry, peptide synthesis is the production of peptides in which multiple amino acids are linked via peptide bonds. Peptides, small polymers of amino acids are important synthetic targets as they represent a range of biologically active molecules such as artificial sweetener aspartame and antibiotic Gramicidin S. Peptides synthesis employs naturally occurring amino acids as building blocks for the stepwise elongation of an amino acid chain. They are synthesized by coupling the carboxyl group (C-terminus) of one amino acid to the amino group (N-terminus) of another amino acid. Due to the possibility of unintended reactions, protecting groups are usually necessary. Thus, a variety of amino acid protecting groups has been developed for the correct sequence to be formed. Solid-phase peptide synthesis (SPPS) is the approach to peptide synthesis. It is the cycle of coupling-wash-deprotection-wash. The free N-terminal amine of a solid-phase attached peptide is coupled to another N-protected amino acid unit. This unit is then deprotected, revealing a new N-terminal amine to which a further amino acid may be attached. Two common N-terminal protecting groups are – Fmoc (9-fluorenylmethoxycarbonyl) and Boc (tert-butoxycarbonyl). The N-terminal of amino acid monomers is protected and added onto a deprotected amino acid chain. Boc requires a moderately strong acid such as trifluoracetic acid (TFA) to be removed from the newly added amino acid, while Fmoc is a base-labile protecting group that is removed with a mild base. The other type of protecting group is C-terminal protecting groups which its use depending on the type of peptide synthesis used, either liquid-phase or solid-phase. Side chain protecting groups are known as permanent protecting groups, because they can withstand the multiple cycles of chemical treatment during the synthesis phase and are only removed during treatment with strong acids after synthesis is complete. In this experiment, H-Gly-Phe-OH dipeptide is produced using “Fmoc chemistry”. The Fmoc-group blocks the amine function of one amino acid, while leaving the carboxylic acid available for a condensation reaction with a second amino acid building block carrying a carboxylic acid protecting group. Methods and Observations:

Table of reagents:
Reagents| Mass, g| Mr, g mol¯¹| moles| Density, g cm¯³| Volume, mL| Thionyl chloride, SOCl2| -| 118.97| 0.014| 1.640| 1| Dry methanol| 7.1262| 32.01| 0.2224| 0.7918| 9| L-phenylalanine, C9H11NO2| 1.65| 165.19| 0.010| -| -| HBTU, C11H16F6N5OP| 1.90| 379.3| 0.005| -| -| DiPEA| 0.7755| 129.242| 0.006| 0.742| 1| DMF| 4.74| 73.09| 0.065| 0.948| 5| Fmoc-Gly-OH| 0.60| 297.32| 0.002| -| -| L-phenylalanine methyl ester hydrochloride, C10H14CINO2| 0.43| 215.68| 0.002|...
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