All reagents used were of pro-analysis (p.a.) grade, except for the
solvents used in HPLC, which were of HPLC grade. Trichloroacetic
acid (TCA) from Sigma-Aldrich (St. Louis, MO, USA) and toluene,
acetonitrile and sodium bicarbonate from Merck (Darmstadt, Germany)
were used.