Raman spectra of amino acids and their aqueous solutions

doi:10.1016/j.saa.2010.12.079

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Volume 78, Issue 3, March 2011, Pages 1187-1195

https://doi.org/10.1016/j.saa.2010.12.079 Get rights and content

Abstract

Amino acids are the basic “building blocks” that combine to form proteins and play an important physiological role in all life-forms. Amino acids can be used as models for the examination of the importance of intermolecular bonding in life processes. Raman spectra serve to obtain information regarding molecular conformation, giving valuable insights into the topology of more complex molecules (peptides and proteins). In this paper, amino acids and their aqueous solution have been studied by Raman spectroscopy. Comparisons of certain values for these frequencies in amino acids and their aqueous solutions are given. Spectra of solids when compared to those of the solute in solution are invariably much more complex and almost always sharper. We present a collection of Raman spectra of 18 kinds of amino acids (l-alanine, l-arginine, l-aspartic acid, cystine, l-glutamic acid, l-glycine, l-histidine, l-isoluecine, l-leucine, l-lysine, l-phenylalanine, l-methionone, l-proline, l-serine, l-threonine, l-tryptophan, l-tyrosine, l-valine) and their aqueous solutions that can serve as references for the interpretation of Raman spectra of proteins and biological materials.

Graphical abstract

Raman spectra of amino acid and its aqueous solution are given, which can serve as reference spectra for the interpretation of Raman spectra of proteins and biological materials.

Research highlights

▶ Raman spectra of 18 kinds of amino acids and collagen were studied. ▶ Raman bands were assigned. ▶ Spectra of solids are more complex and sharper than that of solution. ▶ They can serve as references for the interpretation of Raman spectra.

Introduction

Raman spectroscopy is based on the effect of radiation being scattered with a change of frequency. It was in 1928 that Indian scientists Raman and Krishnan [1] discovered the scattering effect that is named after Raman and that earned him a Nobel Prize in 1930. Raman spectroscopy is nowadays steadily gaining on importance for online monitoring of chemical reactions, analysis of food, pharmaceuticals, and chemicals, and increasingly for many other real-world applications. Raman spectroscopy yields detailed information about molecular vibrations. As molecular vibrations are very sensitive to strength and types of chemical bonds, Raman spectroscopy is useful not only in identifying molecules but also in shedding light on molecular structures. In addition, Raman spectra also reflect changes in the surroundings of the molecules and are thus helpful in studying intra- and intermolecular interactions.

Amino acids are the basic “building blocks” that combine to form proteins. In every species, proteins are constructed from the same set of twenty amino acids. As well as forming proteins, enzymes and other body tissue they are also found throughout the body participating in a wide variety of chemical reactions, and are vital in basic energy production cycles, energy transfer and muscle activity. Amino acids play an important physiological role in all life-forms. Amino acids are also important for several medical, cosmetic, and other industrial applications. Besides the direct chemical synthesis, the fermentation by microorganisms, the production by means of protein hydrolyzate extraction, and enzymatic methods are established techniques for the production of amino acids. Sub- and supercritical water have been gaining increasing attention as both an environmentally friendly solvent and attractive reaction medium for a variety of applications. Sub-critical water hydrolysis is employed as an effective method for producing amino acids from biomass wastes [2], [3], [4], [5], [6], [7].

The behavior of amino acids in aqueous solutions is of a major interest because water is the natural medium for biological molecules. A detailed knowledge of amino acid interactions with water is a primary step in understanding the solvation process of larger systems, such as peptides and proteins.

Raman spectroscopy has been applied extensively as a structural probe and is one of the important methods to deeply study reaction mechanism in sub-critical water [8]. Yet, only a fraction of the potentially informative Raman bands has so far been exploited for structural conclusions. A major reason is the prerequisite for unambiguous Raman band assignments. The number of definitive assignments can be advanced by comparisons with model compounds of known Raman signature.

In this paper, we give an overview of Raman spectra of amino acids and their aqueous solutions. These spectra can serve as reference spectra for the interpretation of Raman spectra of proteins and biological materials. They provide the basic information necessary to track changes induced by the protein and amino acids reactions in sub-critical water, to assist in understanding the reaction mechanism, to check the presence of amino acids in biological materials. Finally, this approach is illustrated by the assignment of several bands in Raman spectra of collagen protein.

Section snippets

Experimental

18 kinds of pure l-amino acid standard samples (from Shanghai Kangda amino acid factory) and collagen (from Jiangxi Cosen biochemical Co.) are available and used without further purification. The stated purity of the chemicals used was more than 99%. Table 1 lists the amino acids along with their technical information [9]. The amino acids were divided into six groups: hydrocarbon, alcohol, sulfur, basic, aromatic and acidic. All sample solutions were prepared with 18.22 MΩ ultra-pure water.

Results and discussion

Similar to infrared absorption spectra, the rather crowed area from 500 to 1700 cm⁻¹. Raman shift is known as the fingerprint region and contains the majority of the Raman bands used to uniquely identify a particular material. Like human fingerprints, these regions are very complex and in general, they cannot be used directly for structural determination but are valuable for structural confirmation. There is a lacuna from 1700 to 2850 cm⁻¹. This region contains only one main band of interest in

Conclusions

Raman spectroscopy was used to study 18 kinds of amino acids and collagen. Some structure assignments were discussed in terms of the observed spectra. These spectra can be easily distinguished from each other. Comparisons of certain values for these frequencies in amino acids and their aqueous solutions are given. Spectra of solids when compared to those of the solute in solution are invariably much more complex and almost always sharper. The frequencies of the bands in the solid are shifted

Acknowledgements

The authors thank the National Natural Science Fund of China (50578091) and Shanghai LeadingAcademic Disciplines(S30109) for financial support.

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Raman spectra of amino acids and their aqueous solutions

Abstract

Graphical abstract

Research highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Chin. J. Chem. Eng.

J. Anal. Appl. Pyrol.

Spectrochim. Acta A

J. Mol. Struct.

Vib. Spectrosc.

Nature

Amino Acids

J. Chem. Eng. Chin. Univ.

Acta Sci. Circumstant.

Chem. Eng. (China)

Prog. Chem.

Fundamental Organic Chemistry

J. Phys. Chem. B

J. Phys. Chem. B

J. Raman Spectrosc.