Structure determination of reaction products of pyrroloquinolinequinone (PQQ) with L-tryptophan in vitro and their effects for microbacterial growth

Abstract

The in vitro reaction of PQQ with L-tryptophan was investigated under an aerobic condition. Three different reaction products were isolated from the reaction solution in phosphate buffer (pH 6.5), and their chemical structures were investigated by UV, mass and NMR spectroscopies. By virtue of the structure of imidazolopyrroloquinoline (IPQ) of 4 isolated as the main product, which was determined by the NMR spectroscopic and X-ray single crystal analyses, the other two products of 5 and 6 were shown to be the indolyl and indolylmethyl derivatives of IPQ, respectively. Based on this information, a possible production pathway of 4–6 from PQQ and L-tryptophan was proposed. These products were shown to exhibit the potential growth-stimulating activity for the micro-organism, Acetobacter aceti, which was more active than PQQ when compared on a molar basis, indicating their biological significance in living cells.