X-ray diffraction of histidine-silver(?)
The different X-ray diffraction patterns of the histidine-silver(I) complex and the histidine blank ligand (Figures 1A and 1B, respectively) showed that histidine has reacted with silver(?) to form a new species^[12]. The products were metallic silver and silver oxide. The average crystallite sizes were calculated from the peak broadening of X-ray diffraction patterns by the Scherrer equation (D=57.3×kλ/βcosθ)^[13]. The mean particle size of histidine-silver(I) was 34.6 nm. The Ag colloids aggregated with the histidine solution.

Ultrastructure of histidine-silver(?) complex
The transmission electron microscope image of histidine-silver(I) (Figure 2) showed that histidine-silver(?) nanoparticles were granular and tend to aggregate, and then absorbed on the columnar surface. The size distribution of single particles was found to be 16% < 20 nm, 21% between 20-40 nm, 23% between 40-60 nm, 26% between 60-80 nm, and 14% >80 nm. The mean particle size was 39.2 nm. The size of the aggregated samples was 300-500 nm.

Irfrared spectra of histidine-silver(?) complex
The interaction of silver(I) with histidine was investigated furtherly by infrared. The infrared spectra of blank histidine (Figure 3A) was compared with that of histidine-silver(I) (Figure 3B). Blank histidine displayed a shoulder peak at 3 016.30 cm⁻¹, which corresponded to O-H stretching vibrations. After contacted with silver(I), the interaction of silver(I) with the oxygen of the carboxyl group caused a clear shift and splitting of the δOH+νCH₂, which moved from 3 016.30 cm⁻¹ to 3 012.98 cm⁻¹ and 2 879.28 cm⁻¹. The infrared spectra showed no shift of the amido bond νC-O band at 1 634.12 cm⁻¹ after contacted with silver(I). The band at 1 111.78 cm⁻¹ assigned to -NH₂ twisting or -NH₃⁺ in-plane wagging shifted to lower frequencies, which indicated that the -amino-group coordinates with silver(I).

In the spectra of histidine, five bands between 732.59-852.58 cm⁻¹ were assigned to out of plane deformation vibrations of the imidazole ring CH bonds. In the spectra of histidine-silver(I), these bands decreased in number and intensity with a frequency difference of approximately 20-40 cm⁻¹. In the spectra of histidine in this range, the most intense band was at 830.63 cm⁻¹, while the band closed to 833.29 cm⁻¹ had a quarter intensity of the band at 830.63 cm⁻¹, and there was no band at 852.58 cm⁻¹ in the spectra of histidine-silver(I). A band of moderate intensity was observed at 732.59 cm⁻¹, which was the characteristic of histidine. A narrow band at 778.38 cm⁻¹ was observed for histidine-silver(I), and assigned to the stretching (ring) of imidazole ring. In the spectra of histidine-silver(I), these bands were present at about the same frequency but decreased in number and intensity. The ring band of the imidazole ring was observed at 421.43 cm⁻¹ in histidine spectra and at 472.82 cm⁻¹ in the histidine-silver(I) spectra.

[1] Wigley RA, Brooks RR. Noble Metals and Biological systems. Boca Raton: CRC Press. 1992:277-297. [2] Stillman MJ, Presta A, Gui Z, et al. Spectroscopic studies of copper, silver and gold-metallothioneins. Met Based Drugs. 1994;1(5-6):375-393. [3] Zingel V, Leschke C, Schunack W. Developments in histamine H1-receptor agonists. Prog Drug Res. 1995;44: 49-85. [4] Yamaguchi T, Lindqvist O. The crystal structure of diammine silver nitrate, Ag(NH3)2NO3, at 223K. Acta Chem Scand. 1983;37(8):685-689. [5] Choi SH, Lee SH, Hwang YM, et al. Interaction between the surface of the silver nanoparticles prepared by γ-irradiation and organic molecules containing thiol group. Rad Phys Chem. 2003;67(3-4):517-521. [6] Li L, Zhang CP, Shen DJ. Preparation and characterization of polymeric ultrafine particles on Hb-SDS-Ag hydrogeles. Guang Pu Xue Yu Guang Pu Fen Xi. 2002;22(3):501-503. [7] Zhang CP, Zhang W. IR, Raman and UV/VIS spectroscopic studies on some proteins with silver(I)-copper(II) ion complexes. Huaxue Shiji. 1999;21(1):11-14. [8] Zhang CP, Xia M. The formation and spectra of the proteins-silver-copper complexes. Guang Pu Xue Yu Guang Pu Fen Xi. 1996;16(5):43-49. [9] You YH, Zhang CP. The photochemistry properties on interaction silver with tryptophan. Spectrochim Acta A Mol Biomol Spectrosc. 2008;69(3):939-946. [10] You YH, Zhang CP. Interaction between silver ions and histidine. Zhongguo Zuzhi Gongcheng Yanjiu yu Linchuang Kangfu. 2010;14(29):5498-5504. [11] Xiao ZD, Zhang CP, Shan DJ. The photochemistry properties on reaction of amino acids and silver. Yingxiang Kexue yu Guanghuaxue. 2005;23(3):209-217. [12] Hua JC. Microstructure Analysis of Polycrystalline X-ray Diffraction. Shanghai: Huadong Normal University Press. 1989:155-180. [13] Klug HP, Alexander LE. X-ray Diffraction Procedure for Polycrystalline and Amorphous Materials. New York: Wiley. 1974:652-660. [14] Bellamy LJ. The Infra-red Spectra of Complex Molecules. 3rd ed. London: Chapman and Hall. 1978. [15] Silverstein RM, Bassler GC, Morrill TC. Spectrometric Identification of Organic Compounds. 5th ed. New York: Wiley. 1991. [16] Fu JZ, Liu YY, Gu PY, et al. Spectroscopic charcterization on the biosorption and bioreduction of Ag(I) by Lactobacillus sp.A09*. Acta Phys Chim Sin. 2000;16(9): 779-782. [17] Zhou WJ, Wang Y, Wu JG, Eds. Modern Fourier-Transform Infrared Spectroscopy and Its Applications, Part (in Chinese). Beijing: Literature of Science and Technology Press. 1994:273-293. [18] Castro JL, Lopez Ramírez MR, Lopez Tocón I, et al. Vibrational study of the metal-adsorbate interaction of phenylacetic acid and α-phenylglycine on silver surfaces. J Coll Inter Sci. 2003;263(2):357-363. [19] Lu YQ, Deng ZH. The Practical Infrared Spectrum and Analyze. Beijing: Electron-industry Press. 1989:8.

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The bio-inorganic chemistry of the silver(I) ion is rich and fascinating. The silver(I) ion has long been used as a bactericide in the form of eye drops for newborns^[1]. The metallothioneins, a class of small proteins believed to be responsible for heavy-metal detoxification in mammals, exhibit very high affinity for silver(I)^[2].

L-histidine is an essential amino acid for protein synthesis and various other functions in cells and tissues, such as a precursor for histamine. Simple and complex 2-alkylhistamines have been of interest for more than thirty years as potential histidine agonists and/or antagonists[^3]. Histidine is involved in the allergic response, and this has stimulated interest in the synthesis of numerous antihistamines and histamine derivatives to study histamine receptors. Amino acids such as histidine and tryptophan that contain nitrogen-bearing side-chains have been shown to have higher silver(I) ion binding energies than copper(I) ion binding energies. The more favorable binding of silver(I) is probably due to its larger ionic size^[4] and softer properties compared with copper(I). Vibrational study of amino acids adsorbed on metal surface is quite complex, given that the species giving rise to the SERS records may be the anion, the cation or even the zwitterions. Moreover, the interaction with the metal ion may take place through only one or both functional groups, or even through an additional functional group presented in the side chain. Immobilization of silver ion for metal affinity chromatography^[5], and the interaction between the surface of silver nanoparticles prepared by γ-irradiation and organic molecules containing thiol group^[6] has been studied, but the interaction of silver with histidine in the micro emulsion and formatted nanoparticles have not been studied. Our group have previously investigated the interactions of silver(I) with bovine serum albumin, hemoglobin, tryptophan, and hisditine^[7-11], as well as the photochemistry properties of silver’s reactions with amino acids. The present work is aimed at the preparation and characterization of hisditine-silver(?) nanoparticles.

Design
The interaction of the metal ions with amino acids at the molecular level using spectroscopic technique.
Time and setting
This study was performed in the Department of Chemistry, Guizhou University, China from September 2004 to July 2006.
Materials
The silver nitrate (AgNO₃, 99.9%) was purchased from Shanghai Chemicals Co., Ltd. (China). The histidine was obtained from Sigma-Aldrich Co. The sodium dodecyl sulfate was obtained from Inorganic Chemical Co., Ltd. (Chong-Qing, China). Trihydroxymethyl aminomethane as a buffer was purchased from Chemical Co., Ltd. (Beijing, China). n-Hexyl alcohol (CH₃(CH₂)₄CH₂OH, A.R) as a co-surfactant was purchased from Shanghai Chemicals Co., Ltd. (China). Used chemicals were in analytical-reagent grade.
Methods
Preparation of histidine-silver(I) nanoparticles
Two micro emulsion systems were prepared as indicated in Table 1 by mixing together the sodium dodecyl sulfate, n-heptane, n-hexyl alcohol, and the appropriate aqueous phase. The aqueous phase of micro emulsion II contained formaldehyde as a reducing agent. The two micro emulsion systems were mixed together in a 2:1 volume ratio at room temperature. Histidine-silver(I) nanoparticles formed upon contact of the precursor (micro emulsion I) and formaldehyde (micro emulsion II). After filtering, the sample was washed with acetone and double distilled water, then dried and grounded.

The reduction reaction occurring when mixing can be expressed as:

Morphology of nanoparticles observed with transmission electron microscope
Particle sizes were determined by transmission electron microscope using a JEM-2000FXII (Japanese Electron Company, Japan). The sample was cut into several strips such that there were multiple opportunities to image the feature of interest. The strips were arranged between two glass or silicon strips and create a sandwich structure. The sandwich structure was glued to the transmission electron microscope grid. Appropriate polishing equipment was used to polish the
sample to the thickness of less than 2 μm. Next, ion milling was used to thin the sample to electron transparency.
Particle size of the nanoparticles detected with X-ray diffraction
X-ray diffraction measurements were performed on a Rigaku DLMAX-2200. X-ray diffractometer was used for CuKα radiation (λ=0.154 05 nm). The sample was placed into the sample holder. Point survey software “Execute aging”, start scanning and the data was automatically stored. Compared with Ag₂N₂H₂O₂S (JCPDS:32-1024), it suggested the homophyly of histidine-silver(?).
Structure of nanoparticles detected with infrared spectroscopy
Infrared spectra were measured on a fourier transform infrared spectrometer (VERTEX-70, Brucker Co.). Samples were prepared as KBr disks. The signal-to-noise ratio was 55 000:1, the sample rate was 80 spectra per second, the spectral region was 30 000-10 cm⁻¹, and the resolution was 0.4 cm⁻¹.
Main outcome measures
Infrared spectra of histidine-silver(I) complex and nanosize particles on histidine-silve(I) were investigated using X-ray diffraction and transmission electron microscope.

His-silver(I) nanoparticles synthesized from micro emulsions were determined to have a mean particle size of 34.6 nm by X-ray diffraction, and 39.2 nm from transmission electron microscope images. Fourier transform infrared provided further insight into the interaction of histidine with silver(I). The asymmetrical (OCO) stretching band of the carboxylate anion (COO−) at 1 590.18 cm^−1[14-15] became a shoulder. The COO− symmetrical stretching band at 1 460.53 cm⁻¹ and 1 383.83 cm⁻¹ increased in intensity. These changes are typical complexation of the carboxylate anion with a metal cation^[16-17]. In this case, most of the carboxylate anion must have become bound, complexed or chelated to the silver(I) because the asymmetrical stretching band at 1 590.18 cm⁻¹ moved to a higher frequency^[18]. This band may be overlaid by other higher frequency bands, resulting in deepening of the peak valley between 1 634 and 1 590 cm⁻¹. Furthermore, shifting of the symmetrical (OCO) stretching band at 1 455 cm⁻¹ to a higher frequency also suggests the complexation of silver(I)^[19], and this band may also be concealed by other higher frequency bands that increase the intensity of the band. Three bands between 600-700 cm⁻¹ were assigned to the in-plane bending (δ) or wagging (w) of (OCO)^[18]. Reports have suggested that Ag⁺ may bind to the oxygen atom of the carbonyl group from the amidogen (HNCO) in this system^[16]. However, the carboxylate anion competes against the amidogen for binding, complexation or chelatation of silver(I). Oxygen is more electronegative than nitrogen, which means that the carboxylate anion with two oxygen atoms may more easily bind, complex or chelate with Ag⁺ than the amidogen one oxygen and one nitrogen. The mechanism of binding involves formaldehyde reduction of soluble silver(I) to elemental silver. Previous studies have shown that the coordination of imidazole with silver(I) produces an imidazole ring band at about 1 820 cm⁻¹, but its source is still unknown^{[16, 19]}. However, in our experiments, it shows that a band is observed at 1 797.09 cm⁻¹ in the histidine spectra (Figure 3A), but no band is observed in the histidine-silver(I) spectra (Figure 3B). In summary, the infrared spectra suggested that silver(I) could coordinate with the oxygen atom of the α-carboxy group, and nitrogen atoms of the α-amino-group and the imidazole ring. These coordination possibilities are all consistent with the structure of histidine-silver(I) shown in Figure 4, and the results could also confirm previous studies^[10].

实验在微乳液中制备出组氨酸-银纳米微粒，并用X射线衍射、透射电镜、红外光谱对其表面结构进行表征，发现组氨酸中的咪唑环参与了同银离子的成键作用。

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