Zwitterion l -cysteine adsorbed on the Au 20 cluster: enhancement of infrared active normal modes
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ORIGINAL PAPER
Zwitterion L-cysteine adsorbed on the Au20 cluster: enhancement of infrared active normal modes Alfredo Tlahuice-Flores
Received: 10 November 2012 / Accepted: 9 January 2013 / Published online: 24 January 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract The study reported herein addressed the structure, adsorption energy and normal modes of zwitterion L-cysteine (Z-cys) adsorbed on the Au20 cluster by using density functional theory (DFT). It was found that four Z-cys are bound to the Au20 apexes preferentially through S atoms. Regarding normal modes, after adsorption of four Z-cys molecules, a more intense infrared (IR) peak is maintained around 1,631.4 cm−1 corresponding with a C=O stretching mode, but its intensity is enhanced approximately six times. The enhancement in the intensity of modes between 0 to 300 cm−1 is around 4.5 to 5.0 times for normal modes that involve O–C=O and C-S bending modes. Other two normal modes in the range from 300 to 3,500 cm−1 show enhancements of 6.0 and 7.4 times. In general, four peaks show major intensities and they are related with normal modes of carboxyl and NH3 groups of Z-cys. Keywords Gold 20 . Normal mode . Amino acid . Zwitterion L-cysteine
Introduction The study of small gold clusters and their striking properties due to their reduced size and shapes are the focus of intense research nowadays. The interaction between gold clusters and organic molecules constitutes an interesting topic due to its implications in biologic systems and in advancing our understanding of these novel systems in general. The Au20 cluster was first reported in 2003 by the group of L-S Wang [1], in a combined theoretical and photoelectron spectroscopy A. Tlahuice-Flores (*) Department of Physics and Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA e-mail: [email protected]
study determining its high stability in accordance with a high HOMO–LUMO (HL) gap of 1.77 eV, which is higher than HL gap of the C60 molecule. Moreover, a Td symmetry and its bond distances were established in the range 2.16–3.12 Å [1]. In 2008, by means of vibrational spectroscopy, Fielicke’s group showed that the infrared (IR) spectrum of a neutral Au20 cluster has a maximum at 148 cm−1, which corresponds with a normal mode with a T2 irreducible representation [2]. Regarding functionalization of the Au20 cluster, Zhang et al. [3] successfully synthesized in aqueous solution an Au20 cluster coordinate with eight triphenyl phosphine ligands, finding four ligands bound to the apexes, anticipating its catalytic applications. On the other hand, calculations performed on the Au20 gold cluster were reviewed in a paper by Kryachko [4]. In those previous reports, the BP86 functional was established as the best functional with which to estimate a HL gap closely related to the experimental value [5], and the Perdew-Burke-Ernzerhof (PBE) functional with a double basis set including d polarization functions (DSPP) gave the best value of the IR maximum [6]. The high stabil
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