Analysis of accordion DNA stretching revealed by the gold cluster ruler.

A promising method for measuring intramolecular distances in solution uses small-angle x-ray scattering interference between gold nanocrystal labels [Mathew-Fenn, Science 322, 446 (2008)]. When applied to double-stranded DNA, it revealed that the DNA length fluctuations are strikingly strong and correlated over at least 80 base pair steps. In other words, the DNA behaves as accordion bellows with distant fragments stretching and shrinking concertedly. This hypothesis, however, disagrees with earlier experimental and computational observations. This Rapid Communication shows that the discrepancy can be rationalized by taking into account the cluster exclusion volume and assuming a moderate long-range repulsion between them. The long-range interaction can originate from an ion exclusion effect and cluster polarization in close proximity to the DNA surface.