A Fundamental Study of Quantitative Desulfurization of Sulfur Containing Amino Acids by Raney Nickel and its Character

Raney nickel catalyst alone removes sulfur in reasonable yield from organic sulfur compounds, such as thiols, thioethers, disulfides, hemithioacetals, hemithioketals, diketals, thioamides, thiolesters, thiophenes, thioazoles, sulfoxides, sulfones, isothiocyanates, thioureas, and also organoselenium and tellurium compounds. This desulfurization reaction, therefore, has been used for the solution of structure, and synthetic work. Elaborate, and excellent reviews were published by Hauptmann and Waltner [1], and Pettit and Tamelen [2] for desulfurization reactions, and Bonner and Grimm [3] for its mechanisms. In 1947, Vogler [4] reported that 5 g methionine was desulfurized in a 60% yield by 150 g Raney nickel at 100 °C for 90 min. In a series of studies by Wieland, Amanitin and Phalloidin, poisonous cyclopeptides from mushrooms, were subjected to the desulfurization reaction by Raney nickel [5-8]. In these reports, large excess of Raney nickel (1 g) did not quantitatively desulfurize 20 mg of these peptides even under drastic condition. In the course of our studies on the structure of new sulfur-containing amino acids [9, 10] it was noticed that neither Raney nickel W-6 [11] nor W-2 [12] could efficiently cleave thioether bond of the new sulfur-containing amino acids and methionine. For example, 20 mg of these amino acids were desulfurized in an only 15% yield by 100 mg of these catalysts under the described condition [2], and the increased quantities of Raney nickel did not sufficiently cleave them. However, it was found that Al-Ni alloy incompletely treated with 5 N NaOH or Al-Ni alloy itself could effectively desulfurize them [10]. After several years, Perlstein et al. [13] reported that a modified Raney nickel also did not desulfurize methionine essentially, while cysteine and cystine were completely converted to alanine. Recently, Otieno [14] reported that W-6 Raney nickel catalyst degraded cysteine rapidly, but methionine up to 53% at 25 °C in 6 hours. We desired to celave thioether bonds of some glutathione-S-conjugated peptides quantitatively for their determination. For this reason, it was sought such condition that as little amount of Raney nickel as possible is able to desulfurize the sulfide bond thoroughly under mild conditions.