J. Phys. Chem. Lett. 4, 2105–2110 (2013)

Infrared (IR) spectroscopy has emerged as an important technique for monitoring the dynamic structure of biomolecules such as nucleic acids and proteins. Unlike fluorescence-based techniques in which relatively large dye molecules are attached to the analyte and monitored, infrared spectroscopy uses probe groups — such as NO, CO and CN — that are much smaller and thus have less of a perturbing effect on the native structure and function of the entity being studied. Infrared probes are very sensitive to many dynamic structural features of interest, such as local electrostatics and hydrogen bonding, and advances in biochemical techniques have enabled their site-specific incorporation into biomolecules.

One such method that allows conformational changes to be monitored in peptides and proteins is to swap a natural amino acid for a synthetic one that contains an IR-active group. The probe must have certain characteristics to be of use — its IR peak must be separated from other regions of intense IR activity, its vibrational frequency should be very sensitive to local electric fields, and if it is to be used to monitor dynamics occurring on long timescales it must have a sufficiently long vibrational lifetime. Now a team led by Minhaeng Cho, Hogyu Han and Sungnam Park, from Korea University, Seoul, have shown that the lifetime of a nitrile probe can be extended by insulating it from the amino acid to which it is attached.

A nitrile group was attached to a proline derivative through either a sulfur atom, to give a −SCN probe group, or a selenium atom, to give a −SeCN probe. Infrared pump–probe spectroscopy was used to monitor the speed at which the vibrationally excited nitrile groups relaxed, and it was observed that the vibrational lifetime was approximately four times longer when attached through a selenium atom rather than a sulfur atom. Cho, Han, Park and colleagues suggest that the heavier selenium atom is better than sulfur at blocking the intramolecular transfer of vibrational energy from the stretching −CN group to the attached pyrrolidine ring, thus extending its vibrational lifetime.