To obtain structural information on the active site of thiamin-dependent enzymes in solution, we have studied the interactions of Cu2+ ions with 2-(alpha -hydroxyethyl)thiamin pyrophosphate (HETPP), the pentapeptide Asp-Asp-Asn-Lys-IIe surrounding the thiamin pyrophosphate moiety in the transketolase enzyme, and the tertiary CU(2+)pentapeptide-HETPP system in aqueous solutions at various pH values. In the binary Cu2+-pentapeptide system around physiological pH, the bonding sites were the terminal NH2 group, the aspartate beta -carboxylates, and a deprotonated peptide nitrogen, while, in the Cu2+-HETPP system at the same pH, the Cu(II) was coordinated to the pyrophosphate group and to the pyrimidine N(1') atom. It is found that, in the tertiary system at physiological pH, the, peptide bone 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. The stability constants in the, tertiary system are higher than those in the simpler CU2+-HETPP and CU2+-peptide systems. The present data show that the coenzyme adopts the so-called S conformation in solution. The importance of our findings concerning the N(1') coordination and the S conformation in the tertiary system is discussed in conjunction with the role of HETPP as an intermediate of thiamin catalysis.