Fluorescence decays of poly(dA).poly(dT) at 420 nm; ex = 267 nm. Black circles: usual operating conditions (no stirring; pulse intensity: 200 KW/cm2). Colour lines correspond to ten successive measurement.
The investigation of model DNA helices using time-resolved absorption and fluorescence spectroscopy with UVB/UVC excitation knows currently an increasing interest due, in particular, to the use of femtosecond spectroscopy. The study of such complex and fragile systems presents specific difficulties which are not encountered in the experiments performed on the monomeric DNA units. They are related both to the quality of the DNA helices and their sensitivity towards UV radiation. The present paper tackles some of these problems (pitfalls) and describes experimental protocols developed in our laboratory in order to overcome them (tricks). We focus on experiments carried out by fluorescence upconversion spectroscopy, time-correlated single photon counting and nanosecond flash photolysis. We illustrate our experience with examples obtained for double helices containing only adenine-thymine base pairs. We consider this report of pitfalls and tricks, which is far from being complete, as a first step towards a codification of rules for time-resolved studies with model DNA helices. This codification has to be established by common agreement of the various groups working in the field.