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Optimized structures of N7H adenine tautomer, Au 3 -adenine complex, and Fe 3 O 4 -adenine complexes formed at N3-/N9-and N10-/N7H-interaction sites, Fe 3 O 4 -Ade (A) and Fe 3 O 4 -Ade (B), respectively. Numbers in brackets indicate charge and multiplicity. The lengths of chemical bonds are indicated in pm. Coatings 2023, 13, x FOR PEER REVIEW

Optimized structures of N7H adenine tautomer, Au 3 -adenine complex, and Fe 3 O 4 -adenine complexes formed at N3-/N9-and N10-/N7H-interaction sites, Fe 3 O 4 -Ade (A) and Fe 3 O 4 -Ade (B), respectively. Numbers in brackets indicate charge and multiplicity. The lengths of chemical bonds are indicated in pm. Coatings 2023, 13, x FOR PEER REVIEW

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... Adenine may form several adsorption complexes depending on its tautomerism and ionic form. The characteristic SERS signature of adenine is the highly intense band near 735 cm − 1 , which is attributed to the ring breathing mode [81][82][83]. The Raman spectrum of solid adenine exhibits this band at a considerably lower wavenumber, 723 cm − 1 [83]. ...
... The characteristic SERS signature of adenine is the highly intense band near 735 cm − 1 , which is attributed to the ring breathing mode [81][82][83]. The Raman spectrum of solid adenine exhibits this band at a considerably lower wavenumber, 723 cm − 1 [83]. The mode is sensitive to the interaction of adenine with a surface, and accordingly, the DFT calculations predict interactions with Au and Ag surfaces to occur in N7H tautomeric form (see insert in Fig. 15a) [81,82]. ...
... However, at higher concentrations (10 − 5 M) adenine molecules replace anions from the surface as evidenced by the disappearance of an intense band near 242 cm − 1 and the appearance of a new lower-intensity band at 223 cm − 1 . This band is associated with metal-adsorbate vibrational mode and reflects the direct bonding of adenine with nanoparticle surface [83]. ...
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