L allysine ethylene acetal

All reactants and reagents were of the highest purity. HPLC-grade acetonitrile was obtained from EMD Millipore (Billerica, MA, USA). Water was purified with a Milli-Q system (Millipore, Billerica, MA, USA). L-Allysine ethylene acetal (>98%), potassium hydrogen phthalate (>99%) and fluorescein (99%) were supplied by Sigma (Sigma-Aldrich, Saint Louis, MO, USA). Trifluoroacetic acid (99.5%)was purchased from Alfa Aesar (MA, USA), formic acid (99.0%) from Fisher Scientific (Fisher Scientific, Fair Lawn, NJ, USA) and sodium 2-naphthol-7-sulfonate (>98%) from TCI America (PA, USA). Analytical grade methanol and chloroform were obtained from VWR (PA, USA).

P6C/AASAL was prepared using a modified version of the previously
reported procedure.^{3 (link),9 (link)} A 50 mg sample of L-allysine ethylene
acetal (Sigma-Aldrich) was stirred with 800 mg of Amberlyst-15 (dry)
ion-exchange resin (Sigma-Aldrich) in water (5 mL) for 30 min at 45 °C.
The reaction mixture was filtered through a sintered funnel, and the resin was
washed with water (5 mL). P6C/AASAL was eluted from the resin with 4 mL of a
28% ammonia solution. The filtrate was evaporated to dryness under a
stream of nitrogen, producing a pale yellow solid.
P6C/AASAL was quantitated by reaction with
o-aminobenzaldehyde (oAB). The synthesized
P6C/AASAL sample was dissolved in 1 mL of 100 mM sodium pyrophosphate buffer (pH
8.0). A P6C/AASAL dilution series was incubated with 200
μM oAB in a total volume of 1 mL
for 1 h at 37 °C. The concentration of the
P6C–oAB reaction product was determined from the
absorbance at 465 nm using an extinction coefficient
(ε) of 2.8 mM⁻¹cm⁻¹.^{10 (link)}