Synthesis of peptide–cellulose conjugate mediated by a soluble cellulose derivative having β-Ala esters

A derivatization of cellulose was investigated for gaining up the solubility for subsequent conjugation reaction. Nβ-Boc-β-Ala, was introduced to the parent cellulose using carbonyl diimidazole. Degree of substitution towards the cellulose hydroxyls was 49%. Subsequent removal of Boc in trifluoroacetic acid (TFA) yielded β-Ala-cellulose·TFA salt. A protected hexapeptide, Boc-Ser(Bzl)-Gly-Tyr(Bzl)-Ser(Bzl)-Gly-Lys(Z) was synthesized via 9 steps of peptide elongation, and the C-end carboxyl groups of the peptide was coupled with the β-Ala-cellulose in a homogenous dimethyl sulfoxide solution, using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide monohydrochloride to ensure the Nβ-selective acylation. The degree of substitution of the protected peptide towards the β-amino groups of β-Ala-cellulose (DS%(/NH2)) was 52% for 1.0 eq.mol of the protected peptide feed, and in the presence of N-hydroxysuccinimide, DS%(/NH2) was increased to 61%. At 4.0 eq.mol feed, almost quantitative conjugation was observed as DS%(/NH2) = 98–99%. Deprotection of the conjugate using thioanisole–TFA resulted in complete removal of Boc, Bzl on Tyr, and Z on Lys, while a very trace amount of Bzl on Ser seemed to be left uncleaved.