SERS evidence of urea‐disordered G‐quadruplex structure

Structural changes of the human telomeric d(TTAGGG)4 sequence (Tel24) in the presence of sodium ions and urea were studied using SERS spectroscopy. The SERS spectra indicated that Tel24 (4 × 10−5 mol/l) formed an anti‐parallel G‐quadruplex in a medium containing Na+ ions (100 mmol/l), which was disrupted by the subsequent addition of urea (7.2 mol/l). Intensity ratios of SERS bands originating from breathing vibrations of adenine (~730 cm−1) and guanine (~660 cm−1) rings clearly pointed to the urea‐induced unfolding of G‐quadruplex. In addition, SERS bands assigned to guanine implied the breaking of Hoogsteen hydrogen bonds, most likely due to the bonding of guanine bases with the urea molecules. Considering various experimental conditions applied, the structure‐related SERS response was more pronounced in the silver than in the gold colloid as well as upon NIR (785 nm) rather than Vis (532 nm) laser excitation. The CD spectra of the same colloidal samples supported structural changes of Tel24 obtained by SERS. SERS spectroscopy was used to study structural changes of DNA sequence Tel24 in the presence of sodium ions and urea. The SERS spectra indicated that Tel24 formed G‐quadruplex in the Na+‐containing medium, which was disrupted by the subsequent addition of urea. Intensity ratio of the SERS bands originating from breathing vibrations of adenine and guanine rings served as a marker of the folded and unfolded DNA structure.