Elsevier

Chemical Physics

Volume 445, 5 December 2014, Pages 21-30
Chemical Physics

UV–UV hole burning and IR dip spectroscopy of homophenylalanine by laser desorption supersonic jet technique

https://doi.org/10.1016/j.chemphys.2014.10.012Get rights and content

Highlights

  • •

    Homophenylalanine was measured by laser desorption supersonic jet spectroscopy.

  • •

    Ten conformers were found by UV–UV hole burning spectroscopy.

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    The observed conformers were assigned by the combination of UV and IR spectra.

  • •

    Franck–Condon simulations were employed to assign the flexible molecule.

Abstract

Conformer selected electronic and vibrational spectra of homophenylalanine, phenylalanine analogue molecule, were measured by UV–UV hole burning and IR dip spectroscopy combined with laser desorption technique. 10 conformers were found by UV–UV hole burning spectroscopy and their structures were assigned by IR dip and UV absorption spectra with aid of quantum chemical calculations in both S0 and S1. This study shows that the combination of simulated IR and UV spectra is powerful to assign flexible molecules.

Introduction

Many researchers are exploring the conformational landscapes of amino acids to understand the fundamental mechanisms of the secondary structure of proteins [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Particularly the aromatic amino acids, phenylalanine (Phe) [2], [9], [22], [23], tyrosine (Tyr) [1], [3], [24] and tryptophan (Trp) [7], [8] have been investigated by many groups because of their near UV absorption and unique inter/intra-molecular interactions due to the aromatic ring. According to the former reports, Phe, Tyr and Trp have 6, 12 and 6 conformers in the gas phase and their structures were assigned successfully by the combination of IR–UV double resonance spectroscopy and quantum chemical calculations [3], [6], [7]. On the other hand, any spectroscopic information of their derivatives has not been reported in spite of their importance. For example, homophenylalanine (HPhe), in which one more methylene group is included in the side chain (see Fig. 1), is used as intermediates of drug synthesis and is included in groups of angiotensin-converting-enzyme (ACE)-inhibitors, [25] which can play an important role in regulating blood pressure by blocking switch from angiotensin 1 to angiotensin 2.

In this paper, we report the electronic and vibrational spectra of HPhe in a supersonic jet. Its non-volatility was overcome by the graphite-matrix laser desorption method. Conformer selective UV and IR spectroscopy were measured by the UV–UV hole burning (HB) and IR dip spectroscopic techniques. The number of observed conformers and their structures are discussed from the comparison with the quantum chemical calculations. The effect of the chain length upon the conformations will also be discussed.

Section snippets

Experimental and computational methods

The experimental setup was already described elsewhere [26], [27]. Briefly, HPhe (Tokyo Chemical Industry without further purification) was mixed with carbon black powder about one to one mass ratio and applied to a graphite disk. In order to vaporize the sample on the disc, the fundamental output of a Nd3+: YAG laser (1064 nm) was irradiated to the disc which is placed in front of a pulsed nozzle (Even-Lavie high pressure pulsed valve [28]). Ar carrier gas at 40 bar was used for a supersonic

REMPI and HB spectra

The REMPI spectrum of HPhe measured in a supersonic molecular beam is shown in Fig. 2b. Vibronic bands appear in the energy region above 37,200 cm−1, and all the vibronic structures are well-resolved due to efficient jet-cooling by the high pressure carrier gas. The vibronic bands in the region lower than 37,550 cm−1 are weak, and the most intense band is observed at 37,577 cm−1. This spectral feature is significantly different from Phe, in which vibronic transitions are concentrated around 37,600 

Summary

The conformer selected electronic and vibrational spectra of jet-cooled HPhe were measured by the UV–UV hole burning and IR dip spectroscopy combined with laser desorption technique. It is found that 10 conformers co-exist by the UV–UV hole burning spectroscopy. Molecular mechanics searches and dispersion corrected DFT calculations were used to obtain the optimized structures and theoretical IR spectra in S0. The geometries in S1 were calculated by the quantum chemical calculations at

Conflict of interest

There is no conflict of interest on this work.

Acknowledgments

This work was supported in part by the Core-to-Core Program 22003 from the Japan Society for the Promotion of Science (JSPS), KAKENHI on innovative area (2503), the Cooperative Research Program of “Network Joint Research Center for Materials and Devices” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and “Molecular Systems Research” project of RIKEN.

The experiments performed in Orsay were conducted at the Centre Laser de l’Université Paris Sud (CLUPS).

References (39)

  • S.J. McGlone et al.

    J. Mol. Struct.

    (1999)
  • L.C. Snoek et al.

    Chem. Phys. Lett.

    (2000)
  • K.T. Lee et al.

    Chem. Phys. Lett.

    (2003)
  • T. Hashimoto et al.

    Chem. Phys. Lett.

    (2006)
  • A.L. Ahmad et al.

    Biotechnol. Adv.

    (2009)
  • R. Ahlrichs et al.

    Chem. Phys. Lett.

    (1989)
  • I. Alata et al.

    Chem. Phys.

    (2012)
  • I. Alata et al.

    Chem. Phys.

    (2012)
  • A. Abo-Riziq et al.

    J. Phys. Chem. A

    (2011)
  • K.T. Lee et al.

    J. Chem. Phys.

    (2002)
  • Y. Shimozono et al.

    Phys. Chem. Chem. Phys.

    (2013)
  • C.M. Nunes et al.

    J. Chem. Phys.

    (2013)
  • L.C. Snoek et al.

    Phys. Chem. Chem. Phys.

    (2001)
  • T.R. Rizzo et al.

    J. Chem. Phys.

    (1986)
  • H.S. Biswal et al.

    J. Am. Chem. Soc.

    (2011)
  • T.D. Vaden et al.

    Phys. Chem. Chem. Phys.

    (2007)
  • C. Hu et al.

    J. Am. Chem. Soc.

    (1993)
  • D.J. Tobias et al.

    J. Phys. Chem.

    (1992)
  • W.Y. Sohn et al.

    J. Phys. Chem. A

    (2010)
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