Triple-Bond Vibrations: Emerging Applications in Energy and Biological Sciences

Unnatural Amino Acids for Biological Spectroscopy and Microscopy

“…The nitrile (CN) group has long been used not only as a functional group, but also as a vibrational reporter in chemistry. − However, it was only until Getahun et al demonstrated that the CN stretching frequency (ν CN ) of a nitrile-containing UAA, p -cyanophenylalanine (4CN-Phe) and cyanoalanine (CN-Ala) could be used to site-specifically probe protein local electrostatic and hydration environment that this vibrational mode began to be widely used in biological studies. Ever since, as shown (Table ), a large number of CN-containing UAAs, including those that contain a −SCN, −SeCN, or −OCN group, have been shown to be useful as site-specific vibrational probes of protein structure, dynamics, and/or function. − In comparison to carbonyl-based vibrational probes, CN-based vibrational probes offer a distinctive advantage because the CN stretching frequency is between 2100 and 2400 cm –1 , which is not only in the transparent window of the IR spectra of biological molecules but is also located between two major water IR peaks .…”

Nitrile Vibrational Lifetimes as Probes of Local Electric Fields

“…In pioneering work, Boxer and co-workers explored the vibrational Stark effect in the liquid phase, leading to the development of the field of vibrational Stark spectroscopy (VSS) . Using linear infrared (IR) absorption spectroscopy, Boxer and co-workers demonstrated that several vibrational modes exhibit sensitivity to applied fields, , with carbonyls and nitriles exhibiting some of the strongest linear responses. , Because IR-induced vibrations are electric dipole-allowed transitions, the shift of a vibrational frequency relative to zero field (Δν 0 ) is expressed as a coupling between the transition dipole moment vector and the external field false( normalΔ ν 0 = prefix− 1 h normalΔ μ ⃗ · E⃗ false) . , In vibrational bioimaging, nitriles are of special interest because they absorb IR light in a frequency region where there is minimal background absorption from native biomolecules (known as the “cell-silent region”). , Thus, nitriles have significant potential for quantitative, intracellular sensing of local electric fields. − …”