X-ray Photoelectron Spectroscopy- and Surface Plasmon Resonance-Detected Photo Release of Photolabile Protecting Groups from Nucleoside Self-Assembled Monolayers on Gold Surfaces

Abstract

The formation of self-assembled monolayers (SAMs) on gold by 2-(5-iodo-2-nitrophenyl) propoxycarbonyl (I-NPPOC)-protected thymidine with an attached mercaptohexyl succinate linker and the kinetics of photochemical release of the I-NPPOC group were monitored using X-ray photoelectron spectroscopy (XPS) and surface plasmon resonance (SPR) detection. In the XPS spectra, the iodine peaks allowed for specific and accurate monitoring of the presence and loss of I-NPPOC groups on the surface. In the SPR experiment, the overall signal change on photoillumination is in accord with a theoretical estimation of the density of I-NPPOC groups in a dense monolayer. The kinetics roughly follow a biexponential time dependence with two very different time constants, corresponding to photochemical quantum yields of 0.22 and 0.0032, respectively.

Optical Detection of Photorelease Kinetics on Gold and Glass Surfaces using Streptavidin‐Coupled Biotinylated Photolabile Protecting Groups for Nucleosides

“…So, the slow rise is probably due to loosely adsorbed molecules that are not part of the SAM and can be washed off. The step of 10 mV is not much larger than the step of 6–8 mV we observed in case of NPPOC‐T‐SH . This small change, in spite of the bulkiness of the added linker with biotin, indicates that the refractive index of the newly added moiety does not significantly exceed the refraction index of EtOH.…”

“…The slow kinetics may be assigned to a minor pathway of the photoreaction that proceeds through primary reduction of the nitro group to a nitroso group . Such biphasic photokinetics were also observed with the unbiotinylated compound . In the case of the biotinylated compounds, however, the weight of the side reaction is significantly smaller.…”

“…in which Δ R a represents the voltage change due to the adsorbed layer, n a and n s are the refraction indices of the adsorbed material and solvent, respectively, and l e is the characteristic penetration depth of the evanescent field. Using the values m= 3400 mV RIU −1 , n a =1.59, n (H 2 O)=1.33, and l e =243 nm, we obtain d a f s ≈2.61 nm. This value is similar to the value of 3 nm estimated by Knoll and co‐workers .…”

“…The details of the SPR cell and setup are described in the Experimental Section and the Supporting Information. Any effect in the adsorbed layer on the gold surface was detected by a change in reflectivity at a fixed angle chosen at the steepest slope of the surface plasmon resonance curve, and the sensitivity amounted to 3400 mV (refraction index unit) −1 (RIU) …”

“…In the present work, we followed a different concept of probing that was based on surface‐sensitive detection methods. In previous work, we showed that the photodeprotection of 2‐(2‐nitrophenyl)propoxycarbonyl (NPPOC)‐caged thymidine with an attached mercaptohexyl succinate linker (see Figure ) if organized as a self‐assembled monolayer (SAM) on gold could be followed by surface plasmon resonance (SPR). Gold is not a suitable substrate to build DNA microarrays, because the reaction cycles involve an oxidative step to convert phosphite into phosphate.…”