Adsorption of (S)-Histidine on Cu(110) and Oxygen-Covered Cu(110), a Combined Fourier Transform Reflection Absorption Infrared Spectroscopy and Force Field Calculation Study

Abstract

Adsorption of (S)-histidine on Cu(110), and Cu(110) modified by adsorbed oxygen, has been studied by reflection absorption infrared spectroscopy (RAIRS). The molecular form and the orientation of the histidine molecule were deduced from experimental RAIRS data; moreover, vibrational spectra were simulated with a molecular mechanics force field (MMFF) calculation to address vibrational modes and the geometry of the adsorbate. The molecule preferentially binds to the surface in its HHis -molecular form. On the metallic Cu(110) surface, the adsorption occurs via the carboxylate group (COO -), with the two oxygen atoms equidistant from the surface, and via the dehydrogenated nitrogen of the imidazole group; the latter adopts an upright position with respect to the copper surface. The amino group (NH 2 ) is likely to be maintained close to the surface, thus facilitating the interaction with the metal. Preadsorption of oxygen (θ ≈ 0.3) on the copper surface induces minor changes in the molecular orientation: the ring tends to be less upright than in the absence of oxygen; the ring now interacts via its NH group as deduced from RAIRS and MMFF calculations. The molecule does not reorient with increasing oxygen coverage (θ ≈ 0.6), and no surface blocking was observed upon oxygen adsorption; both the orientation and the amount of histidine only slightly vary when the oxygen coverage is increased.

Infrared and Raman spectroscopic study of pH-induced structural changes of l-histidine in aqueous environment

“…3), confirm the protonated state of the carboxyl group. The same can be concluded from the strong IR band at 1257 cm À1 , which can be assigned to nC-O in accordance with literature [23,24,26]. In line with the pK a of 1.8 of the carboxyl group, the relative intensity of both bands decreases at pH 1 and the bands disappear at pH >2, illustrating deprotonation to CO 2 À and conversion to H 3 His + .…”

“…The vibration remains visible in IR and slowly increases up to 1416 cm À1 at pH 14. Assignment of the IR band at 1620 cm À1 to n as CO 2 À seems obvious, but contrary to some reports [23,24,27,40], the peak cannot be attributed to n as CO 2 À solely, since a band at this position is also observed in the IR spectrum of the fully protonated form H 4 His 2+ at pH 0. A plausible explanation is, that n as CO 2 À coincides with d as NH 3 + , a vibration which is also expected to exhibit strong IR, but weak Raman activity around 1620 cm À1 .…”

“…Referring to the general literature [9,10], both modes should show medium to high IR, but low Raman activity and indeed, as can be seen from Fig. 2, a band arises in the IR spectrum around 1402 cm À1 at pH 2, which can be assigned straightforward to n s CO 2 À [9,10,23,26,27]. The vibration remains visible in IR and slowly increases up to 1416 cm À1 at pH 14.…”

“…According to literature [16,[23][24][25]31,32,45,46] histidine can be present in five different pH-dependent ionic forms, which are shown in Fig. 1.…”

“…However, this research concerned the conformational dependence of the pK a values www.elsevier.com/locate/vibspec Vibrational Spectroscopy 39 (2005) [114][115][116][117][118][119][120][121][122][123][124][125] and not a vibrational analysis. For that reason, only results of theoretical calculations on the model compounds imidazole [12,13], 4-methyl-imidazole [12,[14][15][16][17] and 4-ethyl-imidazole [18] have been used to support the conclusions from empirical IR and Raman studies of histidine [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] and closely related molecules [37][38][39][40][41]. The majority of this research has been focussed on IR or Raman bands that can be used as a marker to determine the state of protonation of the imidazole nitrogen atoms [15,16,[20][21]…”