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Leucine enkephalin—A mass spectrometry standard

Authors

  • Judit Sztáray,

    1. Institute of Structural Chemistry, Chemical Research Center Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri ut 59-67, Hungary
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  • Antony Memboeuf,

    1. Institute of Structural Chemistry, Chemical Research Center Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri ut 59-67, Hungary
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  • László Drahos,

    1. Institute of Structural Chemistry, Chemical Research Center Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri ut 59-67, Hungary
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  • Károly Vékey

    Corresponding author
    1. Institute of Structural Chemistry, Chemical Research Center Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri ut 59-67, Hungary
    • Institute of Structural Chemistry, Chemical Research Center Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri ut 59-67, Hungary.
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Abstract

The present article reviews the mass spectrometric fragmentation processes and fragmentation energetics of leucine enkephalin, a commonly used peptide, which has been studied in detail and has often been used as a standard or reference compound to test novel instrumentation, new methodologies, or to tune instruments. The main purpose of the article is to facilitate its use as a reference material; therefore, all available mass spectrometry-related information on leucine enkephalin has been critically reviewed and summarized. The fragmentation mechanism of leucine enkephalin is typical for a small peptide; but is understood far better than that of most other compounds. Because ion ratios in the MS/MS spectra indicate the degree of excitation, leucine enkephalin is often used as a thermometer molecule in electrospray or matrix-assisted laser desorption ionization (ESI or MALDI). Other parameters described for leucine enkephalin include collisional cross-section and energy transfer; proton affinity and gas-phase basicity; radiative cooling rate; and vibrational frequencies. The lowest-energy fragmentation channel of leucine enkephalin is the MH+ → b4 process. All available data for this process have been re-evaluated. It was found that, although the published Ea values were significantly different, the corresponding Gibbs free energy change showed good agreement (1.32 ± 0.07 eV) in various studies. Temperature- and energy-dependent rate constants were re-evaluated with an Arrhenius plot. The plot showed good linear correlation among all data (R2 = 0.97), spanned over a 9 orders of magnitude range in the rate constants and yielded 1.14 eV activation energy and 1011.0 sec−1 pre-exponential factor. Accuracy (including random and systematic errors, with a 95% confidence interval) is ±0.05 eV and 10±0.5 sec−1, respectively. The activation entropy at 470 K that corresponds to this reaction is −38.1 ± 9.6 J mol−1 K−1. We believe that these re-evaluated values are by far the most accurate activation parameters available at present for a protonated peptide and can be considered as “consensus” values; results on other processes might be compared to this reference value. © 2010 Wiley Periodicals, Inc., Mass Spec Rev 30:298–320, 2011

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