Step 6: Decoupling and reaction with resin bound Gly to form dipeptide (Boc-Glu-Gly-Resin).Step 7: Orthogonal protection of second -NH2 group of Lys by benzyloxycarbonic acid treatment of already N’ terminal (αNH2-) protected Boc-Lys.Step 8: Linking such both NH2-group protected Boc-Lys derivative with Glu-Gly-Resin.Step 9: Addition of Boc-Gly to derivatized Lys-Glu-Gly-Resin.Step 10: Addition of Boc-Ala to Gly-derivatized Lys-Glu-Gly-Resin.Step 11: Universal decoupling and release of N’-Ala-Gly-Lys-Glu-Gly-C.’ According to the essay, gly has a vast pK range and can be covalently linked to the resin at any pH value. Glu has quite acidic isoelectric point (pI) (= 3.1), and for all reactions in aqueous solutions (Boc and DCCD derivatization) it is required to keep pH close to 3.0. Once Glu is linked, Lys has very alkaline pI (= 9.8) and thus the resin should be washed with a buffer of pH 10.0. All blocking reactions (Boc, DCCD and benzyloxycarbonic acid) need to be carried out in aqueous phase at this pH. Subsequently, for Gly and Ala, pH can be brought back to neutral or slightly acidic (pH 5.0-7.0). Orthogonal protection is a method to protect other than N’ and C’ terminals, the COOH, NH2, and other groups which may affect peptide bond formation or cause stearic hindrance such as –OH, -SH etc. Depending on the groups to be protected (here Lys) chemicals can be chosen. As coupling of amino acids through a peptide bond is enderogenic reaction, they need to be activated by DCCD before coupling.
Voet, D. & V