Abstract
Temperature-dependent dynamic processes in biological macromolecules can produce sharp and reversible transitions in spectroscopic properties that might be misinterpreted as evidence for thermally induced conformational changes. This provides a rational explanation for the paradoxical case of D-amino acid oxidase [D-amino-acid:oxygen oxidoreductase (deaminating), EC 1.4.3.3], for which a sharp fluorescence transition at 14 degrees C, not observed by sensitive calorimetry [Sturtevant, J. M. & Mateo, P. L. (1978) Proc. Natl. Acad. Sci. USA 75, 2584-2587], could be due to a dynamic quenching process of large activation energy, rather than a change in conformational state of the protein. Similar interpretations may be valid in other systems studied by experimental techniques that depend, directly or indirectly, on molecular relaxation processes.