An unusual decarboxylative Maillard reaction between L-DOPA and D-glucose under biomimetic conditions: factors governing competition with Pictet-Spengler condensation.
In 0.1 M phosphate buffer at pH 7.4 and 37 degrees C, the tyrosine metabolite L-3,4-dihydroxyphenylalanine (L-DOPA) reacts smoothly with D-glucose to afford, besides diastereoisomeric tetrahydroisoquinolines 1 and 2 by Pictet-Spengler condensation, a main product shown to be the unexpected decarboxylated Amadori compound N-(1-deoxy-D-fructos-1-yl)-dopamine (3). Under similar conditions, dopamine gave only tetrahydroisoquinoline products 4 and 5, whereas L-tyrosine gave exclusively the typical Amadori compound 6. Fe(3+) and Cu(2+) ions, which accumulate in relatively high levels in parkinsonian substantia nigra, both inhibited the formation of 3. Cu(2+) ions also inhibited the formation of 1 and 2 to a similar degree, whereas Fe(3+) ions increased the yields of 1 and 2. Apparently, the formation of 3 would not be compatible with a simple decarboxylation of the initial Schiff base adduct, but would rather involve the decarboxylative decomposition of a putative oxazolidine-5-one intermediate assisted by the catechol ring. These results report the first decarboxylative Maillard reaction between an amino acid and a carbohydrate under biomimetic conditions and highlight the critical role of transition metal ions in the competition with Pictet-Spengler condensation.
Manini
P
P
Department of Organic Chemistry and Biochemistry, University of Naples Federico II, Complesso Universitario di Monte S. Angelo, Via Cinthia 4, I-80126 Naples, Italy.
d'Ischia
M
M
Prota
G
G
D002851:Chromatography, High Pressure Liquid; D003653:Decarboxylation; D005947:Glucose / Q000737:chemistry*; D007980:Levodopa / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008670:Metals / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet
D008670:Metals; D007980:Levodopa; D005947:Glucose