6-mercapto hexyl-oligonucleotides bind to a gold surface strongly enough to permit imaging by a scanning tunneling microscope (STM). STM images showed worm-like chains that were approximately 12-Å-wide for single-stranded DNA and 20-Å-wide for double-stranded DNA. The chain lengths corresponded to 3.4 ± 0.4 Å per basepair for double-stranded DNA and 2.2 ± 0.4 Å per base for single-stranded DNA. This unexpectedly short length for single-stranded DNA was confirmed using oligomers with both single- and double-stranded regions. When the attachment of the samples was weakened (by imaging in water or scraping with the STM tip) the images changed to pairs of 'blobs,' apparently reflecting the attachment points of the molecules to the gold surface. Given this interpretation, images of DNA containing a five-base bulge imply that the bulge bends the oligomer by 90° ± 20°.
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