The interaction of Au55 nanoclusters with the major grooves in B-DNA was investigated. The nanoclusters are degraded to Au13 clusters by the transition of B-DNA to A-DNA in ultrahigh vacuum, and the resulting shrinkage of the major grooves. Mol. dynamics simulations were performed and provided further information on the mechanism by which the nanowires of Au13 clusters form and attempt to explain the interwire sepn. of 0.5 nm.