ANGLE-DEPENDENT X-RAY PHOTOELECTRON-SPECTROSCOPY AND ATOMIC-FORCE MICROSCOPY OF SULFUR-MODIFIED DNA ON AU(111)

Angle-dependent X-ray photoelectron spectroscopy (ADXPS) was used to i nvestigate the role of sulfur in the binding of a 7250-base backbone-s ulfur-modified deoxyribonucleic acid (DNA) nucleotide on a Au(111) fil m on mica. The quantitative depth profiles for carbon, nitrogen, oxyge n, phosphorus, sulfur, and gold in the sample were calculated from the ADXPS intensity ratios using a novel algorithm. The results show an e levated concentration of sulfur and phosphorus between a multielement layer of the DNA molecule and the Au(111) surface, suggesting the DNA was bound to the Au(111) surface at the sulfur, as was our intention w ith this engineered polynucleotide. DNA surface coverage measurements made by atomic force microscopy (AFM) are compared to those made by th e spatially averaged technique of XPS. The apparent coverage of DNA me asured by AFM was several orders of magnitude less than that measured by XPS, suggesting that a large amount of DNA was swept out of the ima ge area. The images presented are some of the first of DNA bound to a Au(111) surface as a direct result of the chemical modification of the DNA.