Behavior of drug excited states within macromolecules: binding of colchicine and derivatives to albumin.

The aqueous solutions of colchicine (COL) and analogues such as colchiceine (CEI), and N-deacetylcolchiceine (DCEI) have been studied by laser flash photolysis (LFP) with and without the presence of human serum albumin (HSA) to determine the possible interactions between the drugs and the albumin. When irradiation of these drugs was performed in aqueous media, transient absorption species were not detected. However, triplet excited states of CEI and DCEI were detected when the experiments were carried out in the presence of albumin. Contrary to expectations, in the case of COL, no transient absorption species was observed. A deep study of COL triplet excited-state properties has revealed that intersystem crossing quantum yield (φ(ISC)) decreases from organic media such as dichloromethane to water with φ(ISC) values ca. 0.035 and <0.001 respectively. This fact together the inappreciable reactivity of (3)COL with tyrosine and tryptophan agree with the unbinding of COL to HSA. This study will show for the first time a way to determine the association constant (K(a)) using the LFP technique. Thus, for CEI and DCEI binding to HSA, K(a) values of 8 ± 3 × 10(4) M(-1) and 2 ± 1 × 10(4) M(-1) were obtained, respectively. Moreover, it was also established that these drugs are mainly placed into site II of the albumin using this technique. Therefore, this study validates the LFP as a useful methodology to study the interactions of COL and its analogues with serum albumins, and consequently it could be applied to others proteins such as tubulin.