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

Abstract

Aqueous solutions of L-histidine have been analysed in parallel by infrared (IR) and Raman spectroscopy over the pH range 0-14 with increments of one pH unit. The vibrational spectra in the region 2000-500 cm À1 have been interpreted and band positions have been assigned tentatively, taking into account assignments from literature after critical evaluation. As a result, a complete and complementary set of vibrational data has been obtained that can be used to determine all possible states of protonation of histidine, i.e. H 4 His 2+ , H 3 His + , H 2 His 0 , HHis À and His 2À . In addition, IR and Raman bands have been proposed as markers for the presence of the imidazole N p -or N t -protonated tautomeric forms of H 2 His 0 and HHis À .

Host–Guest Chemistry of Copper(II)–Histidine Complexes Encaged in Zeolite Y

“…Moreover, as was the case for IR spectra, the Raman spectrum largely resembles that of His in aqueous solution recorded at pH 5. [14] Discussion Effects of the zeolite host: As already mentioned, the IR and Raman spectra of the Cu 2 + /His/NaY samples c and d are very similar to the ones of the reference compound in which only free His was ion exchanged onto the support. This is not surprising regarding the excess of His in the solutions that was used to prepare the samples.…”

“…Therefore, we compared the IR and Raman spectra of samples c and d with the data of His in aqueous solutions at different pH's, although it should be noted that the spectra are not fully comparable, as the environment in the zeolite is not fully aqueous. Referring to our recent paper on this subject, [14] the strong IR band at 1517 cm À1 (Figure 11) proves that the primary amino group (N am ) is protonated, while the absence of a C=O stretching band around 1735 cm À1 shows that the carboxyl group is the only site that is deprotonated. The pK a values of the two groups were found to be 9.1 and 1.8, respectively, and it follows that the pH must be between these two values.…”

“…A better signal to noise ratio was obtained particularly for sample d with the highest Cu 2 + /His concentration, but this spectrum appeared to be virtually identical to that of the reference compound His/Y. To illustrate this, the spectrum of sample d is presented in Figure 11, together with the spectra of the reference compound His/NaY and His in aqueous solution recorded at pH 5 (for the assignments of the bands we refer to earlier work [14,15] ). Because of interfering bands of the zeolite support only the range 1800-1200 cm À1 is shown.…”

“…[6] Consequently, the IR and Raman spectra represent mainly these non-complexed His species. However, as we reported recently, [14,15] IR and Raman can be used to determine the state of protonation of the His molecules. This is dependent on the "pH" of the local environment and thus, the vibrational spectra of c and d can be used as an indirect tool to establish the "pH" within the zeo- Table 1).…”

“…To obtain insight into the host-guest chemistry of the Cu 2 + /His/zeolite system, we decided to carry out a model study on a series of these solid samples as a logical continuation of the research on the Cu 2 + /His complexes in aqueous solution as described in two recent papers. [14,15] It has been shown that X-ray absorption (XAFS), infrared (IR), and Raman spectroscopy can be valuable tools to obtain complementary information not only on the local structure around the central copper ion, but also on the state of protonation of the potential sites for coordination of the His ligand. For that reason we decided to use these techniques in addition to UV/Vis/NIR and ESR spectroscopy to study the structural behavior of Cu 2 + /His complexes on immobilization in the zeolite pores.…”