Sum-frequency-generation spectroscopy of DNA films in air and aqueous environments

Understanding the organization and orientation of surface-immobilized single stranded DNA (ssDNA) in aqueous environments is essential for optimizing and further developing the technology based on oligonucleotide binding. Here the authors demonstrate how sum-frequency-generation (SFG) spectroscopy can be used to compare the structure and orientation of model monolayers of ssDNA on gold in air, D(2)O, and phosphate buffered saline (PBS) solution. Films of adenine and thymine homo-oligonucleotides showed significant conformational changes in air versus aqueous environments in the CH stretching region. The thymine films showed changes between D(2)O and PBS solution, whereas the SFG spectra of adenine films under these conditions were largely similar, suggesting that the thymine films undergo greater conformational changes than the adenine films. Examination of thymine films in the amide I vibrational region revealed that molecules in films of nonthiolated DNA were lying down on the gold surface whereas molecules in films of thiol-linked DNA were arranged in a brushlike structure. Comparison of SFG spectra in the amide I region for thiol-linked DNA films in air and D(2)O also revealed substantial conformational changes.