Mitochondrial DNA damage as a biomarker for ultraviolet radiation exposure and oxidative stress.

The skin is regularly exposed to the harmful effects of sunlight, such as ultraviolet radiation (UVR), which leads to ageing effects as well as clinical precancerous lesions and skin cancer. The accumulation of mitochondrial DNA (mtDNA) damage has been strongly associated as an underlying cause of the general ageing process in tissues and mtDNA damage has been associated with cancer development in many tissues including human skin. This scenario is linked to the key roles of mitochondrial function and mtDNA both in terms of energy production and also oxidative stress production as well as a mediator of apoptosis. We and others have pioneered the use of mtDNA damage as a highly sensitive biomarker of UVR exposure and oxidative stress in human skin; furthermore, ageing-dependent mtDNA mutations can be accelerated by exposure to sunlight. In addition, this review will also highlight useful applications of mtDNA as a biomarker of UVR-induced oxidative stress including effects of antioxidants.

Direct observation of molecular cooperativity near the glass transition.

The increasingly sluggish response of a supercooled liquid as it nears its glass transition (for example, refrigerated honey) is prototypical of glassy dynamics found in proteins, neural networks and superconductors. The notion that molecules rearrange cooperatively has long been postulated to explain diverging relaxation times and broadened (non-exponential) response functions near the glass transition. Recently, cooperativity was observed and analysed in colloid glasses and in simulations of binary liquids well above the glass transition. But nanometre-scale studies of cooperativity at the molecular glass transition are lacking. Important issues to be resolved include the precise form of the cooperativity and its length scale, and whether the broadened response is intrinsic to individual cooperative regions, or arises only from heterogeneity in an ensemble of such regions. Here we describe direct observations of molecular cooperativity near the glass transition in polyvinylacetate (PVAc), using nanometre-scale probing of dielectric fluctuations. Molecular clusters switched spontaneously among two to four distinct configurations, producing random telegraph noise. Our analysis of these noise signals and their power spectra reveals that individual clusters exhibit transient dynamical heterogeneity and non-exponential kinetics.