CN stretching vibration of 5-cyanotryptophan as an infrared probe of protein local environment: what determines its frequency?

Abstract

Recently it has been suggested that the C≡N stretching vibration of a tryptophan analog, 5-cyanotryptophan, could be used as an infrared probe of the local environment, especially the hydration status, of tryptophan residues in proteins. However, the factors that influence the frequency of this vibrational mode are not understood. To determine these factors, herein we carried out linear and nonlinear infrared measurements on the C≡N stretching vibration of the sidechain of 5-cyanotryptophan, 3-methyl-5-cyanoindole, in a series of protic and aprotic solvents. We found that while the C≡N stretching frequencies obtained in these solvents do not correlate well with any individual Kamlet-Taft solvent parameter, i.e., π* (polarizability), β (hydrogen bond accepting ability), and α (hydrogen bond donating ability), they do however, collapse on a straight line when plotted against σ = π* + β − α. This linear relationship provides a firm indication that both specific interactions, i.e., hydrogen-bonding interactions with the C≡N (through α) and indole N-H (through β) groups, and non-specific interactions with the molecule (through π*) work together to determine the C≡N stretching frequency, thus laying a quantitative framework for applying 5-cyanotryptophan to investigate the microscopic environment of proteins in a site-specific manner. Furthermore, two-dimensional and pump-probe infrared measurements revealed that a significant portion (~31%) of the ground state bleach signal has a decay time constant of ~12.3 ps, due to an additional vibrational relaxation channel, making it possible to use 5-cyanotryptophan to probe dynamics occurring on a timescale on the order of tens of picoseconds.

Solvent dependence of cyanoindole fluorescence lifetime

“…Recently, Zhang et al . [36] have shown that the nitrile stretching vibrational frequency of 5-CNI exhibits a linear dependence on the solvent parameter σ = π * + β − α , where π *, β , and α are the Kamlet-Taft parameters that characterize its polarizability ( π* ), its H-bond accepting ability ( β ) and its H-bond donating ability ( α ), respectively. Specifically, the π * parameter manifests the nonspecific electrostatic interactions between the solute (5-CNI) and solvent molecules, whereas the β and α parameters signify the specific H-bonding interactions between the solvent and the nitrile ( β ) and NH ( α ) groups respectively.…”

Do guanidinium and tetrapropylammonium ions specifically interact with aromatic amino acid side chains?

“…In comparison, these linear IR results indicate that TPA + has a larger effect on the local environment of the C≡N probe than Gdm + . For both Phe CN and Trp CN , upon changing the solvent from water to tetrahydrofuran (THF) the C≡N stretching vibrational band is shifted to a lower frequency and also becomes narrower (28,36). Therefore, these linear IR results further suggest that (i) both TPA + and Gdm + (to a lesser extent) can affect the local hydration status of aromatic side chains and (ii) TPA + , but not Gdm + , can specifically accumulate around those aromatic moieties because the latter does not lead to any significant increase in the IR bandwidth and hence the inhomogeneous part of the C≡N vibrational linewidth.…”

“…To achieve this goal, herein we use multiple spectroscopic methods and two unnatural amino acids, p-cyano-phenylalanine (Phe CN ) and 5-cyano-tryptophan (Trp CN ), to directly assess whether protein aromatic side chains can preferentially interact with Gdm + and TPA + ions. Both the C≡N stretching vibration (27,28) and fluorescence quantum yield (29,30) of Phe CN and Trp CN have been shown to be sensitive to their local environment. Thus, the premise of our study is that any specific ion-side chain Significance Ever since the discovery by Franz Hofmeister in 1888 that a series of salts (now commonly referred to as the Hofmeister series) can have different but consistent effects on the solubility and hence stability of proteins, many studies have been devoted to understanding how such effects are achieved.…”

On the positional and orientational order of water and methanol around indole: a study on the microscopic origin of solubility

“…This is consistent with previous neutron diffraction measurements on indole/methanol/water solutions, 9 as well as with infra-red measurements on indole derivatives in solution which have been shown to be sensitive to -OH bonding to the benzene ring motifs. 31,32 From the excess entropy calculations here, this appears to be enthalpically driven rather than entropically driven given that methanol is more ordered compared with water around indole in solution and indole has a much higher solubility in methanol compared with water.…”