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 o f the sample molecule has also enabled direct observation of the fluorescen ce intensity fluctuations generated as the molecule switches between two co nformations. As a probe for conformational fluctuations we use tetramethylr hodamine linked to a 217-bp DNA oligonucleotide. The measurements on this a nd similar systems reveal the existence of a distribution of reaction rates between the conformations. Here we report 37 detected single-molecule conf ormational fluctuations collected with the same immobilisation method as de scribed earlier. Within the time window of observation the reaction rates d iffer between the molecules, but stay constant within a single molecule. Th e distribution of the relaxation rates between the molecules correspond to the distribution seen in a bulk measurement on a similar system. We therefo re conclude that within the observation time window the single DNA molecule s behave in a non-ergodic way. (C) 1999 Elsevier Science B.V. All rights re served.