Influence of N_ε‑Protecting Groups on the Protease-Catalyzed Oligomerization of l‑Lysine Methyl Ester

The direct oligomerization of l-lys-OMe by bromelain catalysis gave oligo­(l-lys) with DP_avg ∼ 3.6 and dispersity ∼ 1.1. Since higher chain length oligo­(l-lys) with lower dispersity values and one reactive amine for selective conjugation would be beneficial, we explored protease-catalyzed oligomerization of N_ε-protected l-lys monomers where N_ε-groups included <i>tert</i>-butoxycarbonyl (Boc) or benzyloxycarbonyl (Z) groups. By using N_ε-protected l-lys monomers, oligopeptide side-chains are hydrophobic-neutral which should dramatically alter enzyme kinetic and binding constants relative to nonprotected l-lys. Schechter and Berger’s conceptual model guided our choice of papain as the protease catalyst. Papain-catalyzed oligomerization of N_ε-Boc-l-Lys-OMe gave products with DP_avg values that were pH dependent and varied from 4.7 ± 0.2 to 7.5 ± 0.1. Similarly, oligo­(N_ε-Z-l-Lys) synthesis was pH dependent, and DP_avg values varied from 4.3 ± 0.2 to 5 ± 0.2. Oligo­(N_ε-Boc/Z-l-Lys) that precipitates from reaction media had a low dispersity (∼1.01). The relatively smaller N_ε-Boc group should increase propagating chain solubility enabling oligopeptides to reach higher DP_avg values prior to their precipitation. Since papain-catalyzed oligomerizations of N_ε-Boc/Z-l-Lys proceeded slowly at 0.54 mg/mL, higher enzyme concentrations were studied. By increasing the enzyme concentration in oligomerizations from 0.54 to 1.62 mg/mL for 3 h reactions, the %-yield and DP_avg of oligo­(N_ε-Z-l-lys) increased from 24 ± 0 to 88 ± 2 and 4.1 ± 0.7 to 5.7 ± 0.1, respectively. Furthermore, at 1.89 mg/mL papain, the %-yield of oligo­(N_ε-Z-l-lys) increased with time reaching 91% in 2 h. Acetonitrile at 20%-by-volume was a useful cosolvent that increased the oligopeptide yield and DP_avg relative to reactions run in pure buffer

Poly(butylene 2,5-furan dicarboxylate), a Biobased Alternative to PBT: Synthesis, Physical Properties, and Crystal Structure

This paper describes the synthesis, crystal structure, and physicomechanical properties of a biobased polyester prepared from 2,5-furandicarboxylic acid (FDCA) and 1,4-butanediol. Melt-polycondensation experiments were conducted by a two-stage polymerization using titanium tetraisopropoxide (Ti­[OiPr]₄) as a catalyst. Polymerization conditions (catalyst concentration, reaction time and second stage reaction temperature) were varied to optimize poly­(butylene-FDCA), PBF, and molecular weight. A series of PBFs with different <i>M</i>_w were characterized by DSC, TGA, DMTA, X-ray diffraction and tensile testing. Influence of molecular weight and melting/crystallization enthalpy on PBF material tensile properties was explored. Cold-drawing tensile tests at room temperature for PBF with <i>M</i>_w 16K to 27K showed a brittle-to-ductile transition. When <i>M</i>_w reaches 38K, the Young modulus of PBF remains above 900 MPa, and the elongation at break increases to above 1000%. The mechanical properties, thermal properties and crystal structures of PBF were similar to petroleum derived poly­(butylenes-terephthalate), PBT. Fiber diagrams of uniaxially stretched PBF films were collected, indexed, and the unit cell was determined as triclinic (<i>a</i> = 4.78(3) Å, <i>b</i> = 6.03(5) Å, <i>c</i> = 12.3(1) Å, α = 110.1(2)°, β = 121.1(3)°, γ = 100.6(2)°). A crystal structure was derived from this data and final atomic coordinates are reported. We concluded that there is a close similarity of the PBF structure to PBT α- and β-forms