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Progressive stabilization of zwitterionic structures in [H(Ser)(2-8)]+ studied by infrared photodissociation spectroscopy.
| Content Provider | Semantic Scholar |
|---|---|
| Author | Kong, Xianglei Tsai, I-An Sabu, Sahadevan Han, Chau-Chung Lee, Yuan T. Chang, Huan-Cheng Tu, Shih-Yu Kung, A. H. Wu, Chih-Che |
| Copyright Year | 2006 |
| Abstract | Although amino acids are known to exist as zwitterions in aqueous solution and crystals, they should exist in their neutral form in the gas phase because zwitterionic charge separation is not energetically favored in the absence of a solvent. However, it is also known that zwitterionic amino acids can be stabilized by noncovalent interactions with proximal molecules or ions. For example, monomeric glycine is non-zwitterionic in the gas phase, but clustering with five water molecules transforms glycine into its zwitterion. Recently, the clusters of serine have attracted much attention from both experimentalists and theorists. The protonated serine octamer, [H(Ser)8] , first appeared as a “magic number” cluster in electrospray ionization (ESI) mass spectra and showed a pronounced preference for homochirality. 9] Several structures have been proposed for this cluster ion based on tandem mass spectrometry, ion mobility, and H/ D exchange experiments. Although most of the proposed structures contain all zwitterionic serine constituents, an H/Dexchange study indicated that isomers with both neutral and zwitterionic structures can form in the ESI source. Infrared spectroscopy in combination with mass spectrometry and ab initio calculations has been proven to be an effective approach To deduce the structural information of gas-phase cluster ions. McLafferty and co-workers demonstrated that it is possible to obtain infrared photodissociation spectra of protonated amino acid complexes with a pulsed infrared laser coupled to a Fourier-transform ion-cyclotron resonance (FTICR) mass spectrometer; the spectra of [H(Ser)n] + can be similarly obtained. Herein, we present infrared spectra of these cluster ions (n= 2–8) and address the questions of whether the serine dimer has a neutral structure and how the zwitterionic structure evolves as n increases. Results of ab initio calculations based on density functional theory (DFT) for the isomeric structures of [H(Ser)2] + are also reported. By closely examining the infrared spectra of [H(Ser)2–8] + in the frequency range 3400–3750 cm 1 for the free carboxy and hydroxymethyl OH stretches, a size effect on the structural transition from neutral to zwitterionic forms of serine subunits has been identified. Figure 1a shows a typical FTICR mass spectrum of homochiral serine clusters produced by ESI. The spectrum is dominated by two peaks that correspond to [H(Ser)2] + and [H(Ser)8] . The abundance of cluster ions of intermediate size (n= 3–7) is clearly too low to acquire their infrared photodissociation spectra. In a first attempt to overcome this difficulty, [H(Ser)8] + was isolated in the ICR cell by radiofrequency (RF) sweeps and then fragmented by collision with pulsed argon gas (2 > 10 7 Torr) to produce smaller cluster ions of n= 3–6 (Figure 1b). The seemingly less stable heptamer was later produced by in-source collision-induced dissociation (CID) by increasing the ion-transmission energy in the region of the ESI source (Figure 1c). Mass selection was performed in the ICR cell to isolate the desired fragment ions for infrared spectroscopic measurements with an optical parametric oscillator (OPO) laser. A typical infrared spectrum of [H(Ser)2] + at room temperature is shown in Figure 2b. The spectrum was obtained by recording the dissociation fractions of all fragment ions versus total ions against laser frequency. The general feature of the spectrum (normalized with laser power) is in good agreement with the result of Oh et al., who used a similar setup but with a low-repetition-rate pulsed [*] Dr. X. Kong, S. Sabu, Dr. C.-C. Han, Dr. Y. T. Lee, Dr. H.-C. Chang, S.-Y. Tu, Dr. A. H. Kung Institute of Atomic and Molecular Sciences Academia Sinica Taipei 106 (Taiwan) Fax: (+886)2362-0200 E-mail: hcchang@po.iams.sinica.edu.tw |
| File Format | PDF HTM / HTML |
| DOI | 10.1002/anie.200600597 |
| PubMed reference number | 16718712 |
| Journal | Medline |
| Volume Number | 45 |
| Issue Number | 25 |
| Alternate Webpage(s) | https://ir.nctu.edu.tw/bitstream/11536/12792/1/000238718500011.pdf |
| Alternate Webpage(s) | http://ntur.lib.ntu.edu.tw//bitstream/246246/172155/1/12.pdf |
| Alternate Webpage(s) | https://doi.org/10.1002/anie.200600597 |
| Journal | Angewandte Chemie |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
