Non-ergodic behaviour in conformational transitions of single DNA molecules

Conformational fluctuations in single nucleic acid molecules have recently been observed through excited state lifetime measurements. Immobilisation of the sample molecule has also enabled direct observation of the fluorescence intensity fluctuations generated as the molecule switches between two conformations. As a probe for conformational fluctuations we use tetramethylrhodamine linked to a 217-bp DNA oligonucleotide. The measurements on this and similar systems reveal the existence of a distribution of reaction rates between the conformations. Here we report 37 detected single-molecule conformational fluctuations collected with the same immobilisation method as described earlier. Within the time window of observation the reaction rates differ between the molecules, but stay constant within a single molecule. The distribution of the relaxation rates between the molecules correspond to the distribution seen in a bulk measurement on a similar system. We therefore conclude that within the observation time window the single DNA molecules behave in a non-ergodic way. (C) 1999 Elsevier Science B.V. All rights reserved.