Kinetic Isotope Effect Provides Insight into the Vibrational Relaxation Mechanism of Aromatic Molecules: Application to Cyano-phenylalanine

Abstract

Varying the reduced mass of an oscillator via isotopic substitution provides a convenient means to alter its vibrational frequency and hence has found wide applications. Herein, we show that this method can also help delineate the vibrational relaxation mechanism, using four isotopomers of the unnatural amino acid p-cyano-phenylalanine (Phe-CN) as models. In water, the nitrile stretching frequencies of these isotopomers, Phe-12C14N (1), Phe-12C15N (2), Phe-13C14N (3), and Phe-13C15N (4), are found to be equally separated by ~27 cm−1, whereas their vibrational lifetimes are determined to be 4.0 ± 0.2 (1), 2.2 ± 0.1 (2), 3.4 ± 0.2 (3), and 7.9 ± 0.5 ps (4), respectively. We find that an empirical relationship that considers the effective reduced mass of CN can accurately account for the observed frequency gaps, while the vibrational lifetime distribution, which suggests an intramolecular relaxation mechanism, can be rationalized by the order-specific density of states near the CN stretching frequency.

Achieving highly efficient aggregation-induced emission, reversible and irreversible photochromism by heavy halogen-regulated photophysics and D–A molecular pattern-controlled photochemistry of through-space conjugated luminogens

“…In addition, this specic heavy halogen-induced uorescence enhancement may be partly attributed to the effective restriction of intramolecular vibrations and rotations due to the large mass effect of heavy halogens. 53,54 We compared the vibration intensities of TPAN and TPAN-Br using the same vibration mode, as shown in Fig. 3e.…”

Evaluation of p-(13C,15N-Cyano)phenylalanine as an Extended Time Scale 2D IR Probe of Proteins

“…The Gai group more recently reported pump− probe spectroscopy studies that found a longer vibrational lifetime for the isotopically labeled amino acid 13 C 15 NPhe than CNPhe in aqueous solution. 25 To explore the potential for 13 C 15 NPhe as a 2D IR probe for the measurement of dynamics in proteins, we report its incorporation into plastocyanin (Pc) and evaluation of its utility for characterizing the dynamics via 2D IR spectroscopy.…”

Extended timescale 2D IR probes of proteins: p-cyanoselenophenylalanine

“…40,41 In H 2 O, the CN lifetime for 13 C 15 NPhe (8.7–10.5 ps) is ~2.0–2.5 fold longer than CNPhe (4.0–4.6 ps), whereas those for 13 CNPhe(3.4–5.0 ps) and C 15 NPhe (0.90–2.2 ps) are roughly the same or even shorter. These are fairly small changes in lifetime and do not correlate with the small changes in mass, which indicates that the increase in mass from isotopic substitution does not substantially decouple the intramolecular vibrational relaxation pathways.…”

“…Rather, the differences in lifetimes have been attributed to the effect of isotopic substitution in tuning the CN frequency among the density spectrum of intramolecular acceptor modes. 40 This mechanism could also contribute to the difference in the CN lifetime for CNSePhe, which has a similar CN frequency as 13 C 15 NPhe but different spectral density of acceptor modes (calculated spectra shown in ESI†). Resonance tuning however is not likely the dominant cause of the extremely long CN lifetime for CNSePhe.…”