Solution Liquid-Phase Methods for Peptide Synthesis

3 Solution ( = Liquid-Phase) Methods for Peptide Synthesis 

The major disadvantage of solid-phase peptide synthesis is the fact that ail the by-products attached to the resin can only be removed at the final stages of synthesis. Another problem is the relatively low local concentration of peptide which can be obtained on the polymer, and this limits the turnover of all other educts. Preparation of large quantities ( 1 g) is therefore difficult. Thirdly, the racemization-safe methods for acid activation, e.g. with azides, are too mild (= slow) for solid-phase synthesis. For these reasons the convenient Menifield procedures are quite generally used for syntheses of small peptides, whereas for larger polypeptides many research groups adhere to classic solution methods and purification after each condensation step (F.M. Finn, 1976). 

We shall describe here one step in the total synthesis of a protected heptatetracontapep-tide ( = 47 amino acids) by K. Hofmann (H.T. Storey, 1972), and compare the techniques and the results with those of solid-phase synthesis. 

The example clearly demonstrates the main advantages of solution chemistry over the solid-phase approach  

On the other hand, a lot of material is lost in the chromatography and work-up procedures ( 40%), and the analytical data provided are not very convincing, either (except for TLC). 

A protected serine hydrazide was condensed by the azide method to an S-protected tripeptide H-Asn-Cys-Tyr-NHNH-Cbz to form a protected tetrapeptide Boc-Ser-Asn-Cys-Tyr-NHNH-Cbz 1.02 g of N-rerr-butoxycarbonylserine hydrazide (Boc-Ser-NH-NHj) n DMF containing HCI in dioxane was mixed at -20 °C with reri-butyl nitrite. This mixture containing the azide Boc-Ser-Nj was neutralized with triethylamine, and a solution of 3.4 g asparaginyl-S-(ethylcarbamoyl)cysteinyl-tyrosinyl 2-(benzyloxy-carbonyl)hydrazide trifluoroacetate was added. After 72 hours at 4 °C a simple work-up procedure and precipitation from methanol-petroleum ether yielded 3 g of impure protected retrapeptide hydrazide. It 

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Fridkin, M. Polymeric Reagents in Peptide Synthesis, in The Peptides, vol. 2, Gross, E., Meienhofer, J. eds. pp. 333-363, New York, Academic Press 1980 Kisfaludy, L. Repetitive Methods in Solution, ibid. pp. 417-440 Mutter, M., Bayer, E. The Liquid-Phase Method for Peptide Synthesis, ibid. pp. 285-332... 

Diphthalimidocarbonate is a new reagent for the synthesis of active esters. A new photolabile support and a new technique for the hold-in-solution method provide improvements in the liquid-phase synthesis of peptides. The carboxy-group of an A -acylated amino-acid is protected by conversion into an oxazole ring photo-oxygenation regenerates the carboxy-group as a triamide, which is activated for peptide synthesis (Scheme 148). ... 

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