LANTHANIDE ION PROBES OF STRUCTURE IN BIOLOGY - LASER-INDUCED LUMINESCENCE DECAY CONSTANTS PROVIDE A DIRECT MEASURE OF THE NUMBER OF METAL-COORDINATED WATER-MOLECULES

Direct excitation of f-elcctron levels of Eu(III) and Tb(III) using a pulsed dye laser source results in luminescence emission which decays exponentially with an environmentally sensitive decay constant (reciprocal lifetime). The presence of OH oscillators in the first coordination sphere of the metal ion provides an efficient pathway for radiationless deexcitation via energy transfer to OH vibrational overtones. This pathway is virtually eliminated upon replacement of OH by OD oscillators. Decay constant measurements on both H2O and D2O solutions, or upon crystalline solids containing these molecules, allow the determination of the number of water molecules coordinated to the metal. The validity of the method is established by experiments on a variety of crystalline complexes of known X-ray structure involving from zero to nine coordinated water molecules. The technique is applied to a number of model chelate systems in solution and to the structurally well-characterized proteins thcrmolysin and parvalbumin. There is close correspondence between the solution and solid-state results. A particular advantage of the present method lies in the extreme simplicity of the experimental technique and in its applicability to dilute protein solutions. © 1979, American Chemical Society. All rights reserved.