Stepwise Solvation of an Amino Acid: The Appearance of Zwitterionic Structures

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• 作者：Martine N. Blom ; Isabelle Compagnon ; Nick C. Polfer ; Gert von Helden ; Gerard Meijer ; Sá ; ndor Suhai ; Bé ; la Paizs ; Jos Oomens
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：August 9, 2007
• 年：2007
• 卷：111
• 期：31
• 页码：7309 - 7316
• 全文大小：556K
• 年卷期：v.111,no.31(August 9, 2007)
• ISSN：1520-5215

文摘

How many solvent molecules are required to solvate an amino acid? This apparently simple question, whichrelates to the number of solvent molecules necessary to change the amino acid from its gas-phase neutralstructure to the zwitterionic solvated structure, remains unanswered to date. Here we present experimentaland theoretical (density functional theory: B3LYP/6-31+G**) infrared spectra for tryptophan-water_ncomplexes where n = 1-6, which suggest that the zwitterionic structure becomes competitive in energy atthe high end of the series. Compelling evidence for a gradual transition to zwitterionic structures comes fromtryptophan-methanol complexes up to n = 9. Starting from n = 5, the infrared spectra show increasingintensity in the diagnostic asymmetric COO^- stretch and in the weaker NH₃⁺ bending modes as the clustersize increases. Moreover, convergence toward the Fourier transform infrared spectrum of a solution oftryptophan in methanol is clearly observed. For small solvent complexes (n = 1-4), the microsolvation bymethanol and water is shown to behave very similarly. A detailed comparison of the experimental and thetheoretical spectra allows us to determine both the preferred solvent binding sites on the amino acid and theevolution of conformational structures of tryptophan as the number of attached solvent molecules increases.