To obtain structural information on the active-site of thiamin-dependent enzymes in solution, the ternary Cu2+-[Asp-Asp-Asn-Lys-Ile]-[2-(alpha -hydroxyethyl)thiamin pyrophosphate (HETPP)] system has been synthesized and studied by pulsed EPR (ESEEM and HYSCORE) spectroscopy in aqueous solution at physiological pH. HYSCORE proved to be especially useful in elucidating the coordination environment of the Cull ion. The present data show that, in the ternary Cu2+-[pentapeptide]-[HETPP] system at physiological pH, the peptide backbone offers three coordination sites to the metal ion and the coordination sphere is completed by two additional phosphate oxygens and the nitrogen N(1 ') of the thiamin coenzyme. Thus the synthetic ternary system offers the first example of a reliable structural model of the active site of thiamin-dependent enzymes in solution. The importance of our findings concerning the N(1 ') coordination in the Cu2+-[HETPP]-[pentapeptide] system is discussed in conjunction with the role of HETPP as an intermediate of thiamin catalysis.