Synthesis and evaluation of the sensitivity and vibrational lifetimes of thiocyanate and selenocyanate infrared reporters†
Two novel 2′-deoxyadenosine (dA) analogues, Si2–dA–SCN and Si2–dA–SeCN, and two novel phenylalanine (Phe) analogues, Boc–Me–PheCH2SCN and Boc–Me–PheCH2SeCN, have been synthesized and the thiocyanate (SCN) and selenocyanate (SeCN) functional groups evaluated as vibrational reporters. The syntheses of Si2–dA–SCN and Si2–dA–SeCN were accomplished in three steps in 16% and 32% overall yields, respectively, and the syntheses of Boc–Me–PheCH2SCN and Boc–Me–PheCH2SeCN were completed in four steps in 8.9% and 2.3% overall yields, respectively. The SCN and SeCN stretch vibrational modes were shown to be sensitive to the local environment by frequency shifts and full-width half-maximum (fwhm) changes in response to tetrahydrofuran (THF) and THF/water solvent mixtures. The vibrational lifetimes of the Si2–dA–SeCN (237 ± 12 ps) and Boc–Me–PheCH2SeCN (295 ± 31 ps) in THF solution were determined by ultrafast infrared pump-probe spectroscopy to be 1.5 to 3 times longer than those for Si2–dA–SCN (140 ± 6 ps) and Boc–Me–PheCH2SCN (102 ± 4 ps). The longer lifetimes for the SeCN analogues were attributed to the better insulating effects of the heavier selenium atom compared to the sulfur atom. The solvent sensitivity and longer vibrational lifetimes compared to other vibrational reporters suggest that SCN and SeCN vibrational reporters are well suited to studying several dynamic processes including protein and nucleic acid hydration and conformational changes, however stability issues may require post-synthetic modification methods to incorporate these reporters into biomacromolecules.