Surface-Induced Keto–Enol Tautomerization of DNA Base Molecules and Consequent [4 + 2]-like Cycloaddition on Si(111)7×7

Abstract

Adsorption and film growth of deoxyribonucleic acid (DNA) base molecules (cytosine, guanine, thymine, and adenine) on Si(111)­7×7 have been studied by combining X-ray photoelectron spectroscopy (XPS) with ab initio calculations based on the density functional theory (DFT). Multiple tautomeric forms and keto–enol tautomerization are revealed by the O 1s, N 1s, and C 1s XPS spectra of the O-containing DNA bases: cytosine, guanine, and thymine. While the carbonyl group-containing keto tautomer is more stable in a thick film and in powder, the hydroxyl group-containing enol tautomer is found at the interface. The keto–enol tautomerization, as induced by the reactive Si(111)­7×7 surface, leads to the formation of a conjugated aromatic six-membered ring with a delocalized π electron system and to the consequent [4 + 2]-like cycloaddition between the enol tautomer and the 7×7 surface. The DFT calculation suggests that the enol tautomer exhibits a kinetic advantage over the keto one for the [4 + 2]-like cycloaddition. Among the several plausible pathways for the cycloaddition provided by the enol tautomer, the experimentally determined one involves a ring N and ring C atom (a polar pair), rather than two ring C atoms (a nonpolar pair), to better match the polar Si adatom–restatom pair of the 7×7 surface. Furthermore, the reacted ring C atom does not have any attached terminal functional group (e.g., −NH2 and −OH). Further deposition leads to continuous film growth in the keto tautomeric form for cytosine and guanine. For the only O-free DNA base molecule, adenine, dative bonding N → Si, rather than the [4 + 2]-like cycloaddition, is observed on the 7×7 surface. Of the four DNA base molecules, adenine is also the only one with its aromaticity maintained when adsorbed on the Si(111)­7×7 surface. A reactive surface like the 7×7 surface could therefore provide a new control to trigger tautomerization that is often associated with genetic mutation.

A COF-coated polyamide membrane fabricated by vapor-assisted conversion for water-in-oil emulsion separation

“…2j). 28,29 Compared with the O 1s XPS profile of the original PA membrane, the proportion of aldehyde-ketone groups in TFB-BD(Me) 2 -0.05 is 85.85% (Fig. 2k).…”