During Fmoc synthesis of an analog, [Abu20,31, HOTic22]hEGF(20- 31), of a fragment, Cys-Met-Tyr-Ile-Glu-Ala-Leu-Asp-Lys-Tyr-Ala-Cys, of the B-loop of human EGF, conductivity measurements showed that increased time was necessary for coupling and complete deprotection of the residues Met21 and Abu20 which followed the HOTic22. Use of different active ester-forming reagents, including HOBt and BOP, did not increase the yield. Use of symmetrical anhydride with extended coupling time increased the yield but did not complete the coupling. It appears that inclusion of HOTic in place of Tyr to introduce conformational constraint to peptide analogs can cause or augment a tendency towards conformations with increasing occlusion of N- terminal amino groups and result in the need for altered coupling strategies for completion of analog synthesis.
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