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Issue 10, 2016
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C[triple bond, length as m-dash]N stretching vibration of 5-cyanotryptophan as an infrared probe of protein local environment: what determines its frequency?

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Abstract

Recently it has been suggested that the C[triple bond, length as m-dash]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[triple bond, length as m-dash]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[triple bond, length as m-dash]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* + βα. This linear relationship provides a firm indication that both specific interactions, i.e., hydrogen-bonding interactions with the C[triple bond, length as m-dash]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[triple bond, length as m-dash]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.

Graphical abstract: C [[triple bond, length as m-dash]] N stretching vibration of 5-cyanotryptophan as an infrared probe of protein local environment: what determines its frequency?

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Publication details

The article was received on 27 Jul 2015, accepted on 26 Aug 2015 and first published on 26 Aug 2015


Article type: Paper
DOI: 10.1039/C5CP04413H
Citation: Phys. Chem. Chem. Phys., 2016,18, 7027-7034
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    C[triple bond, length as m-dash]N stretching vibration of 5-cyanotryptophan as an infrared probe of protein local environment: what determines its frequency?

    W. Zhang, B. N. Markiewicz, R. S. Doerksen, A. B. Smith, III and F. Gai, Phys. Chem. Chem. Phys., 2016, 18, 7027
    DOI: 10.1039/C5CP04413H

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