Synthesis and characterization of novel sulfur-containing 2-(1H-pyrrolyl) carboxylic acids and their effects on garlic greening

Abstract

It has been established that protein amino acids are involved in garlic greening by reacting with thiosulfinates to form pigment precursors (PP), a class of 2-(1H-pyrrolyl) carboxylic acids. This opens an interesting question of whether specific sulfur-containing non-protein amino acids naturally occurring in garlic such as derivates of cysteine and their corresponding sulfoxides also play a role in garlic greening through the same way. To answer this question, four new sulfur-containing model compounds having a 2-(1H-pyrrolyl) carboxylic acid moiety were synthesized by reacting four non-protein amino acids with 2,5-dimethoxytetrathydrofuran, respectively, to study their effects on garlic greening, the structures of which are similar to that of a previously proposed pigment precursor for garlic greening. The puree of freshly harvested garlic bulbs turned green after being soaked in solutions of P-methyl, P-allyl and P-propyl, and with both increasing concentrations and incubation time, the green color of the puree became deeper. These results provide evidences that the sulfur-containing non-protein amino acids likewise participate in garlic greening by a similar pathway to their analogues, protein amino acids. Additionally, the three compounds exhibited a good relationship between structure and activity of garlic greening, namely, the larger the size of the R group, the larger the contribution. In contrast, P-methyl-O, P-methyl’s sulfoxide cannot turn the puree of freshly harvested garlic bulbs green within the 13 days time frame of the experiment under the same conditions.