Abstract
Proteins/peptides, which are involved in various biochemical processes in biological systems, contain infrared (IR) active vibrations. Among all the IR absorption bands of proteins/peptides, the amide I band arises mainly from the stretching vibration of the carbonyls (C=O) in backbone amide bonds and is sensitive to the conformations (such as α– helix, β–sheet, unstructured conformation, and so on) in a protein/peptide. Therefore, the amide I band has been used to monitor the biophysical/biochemical behavior of proteins/peptides in biological samples (e.g., living cells or tissues). However, obtaining reproducible IR spectra of proteins/peptides in H2O solution was challenging by direct transmission measurement using a liquid cell with milli-meter level path length, due to the intensive IR absorption of H2O around 1620 cm-1 which overlaps the amide I band. Thus, lots of the IR spectra of proteins/peptides were accomplished in D2O, which has IR absorption around 1200 cm-1. Since D2O may not be a favorable solvent for biological samples, the position of the amide I band of various conformations was needed as a reference for biological samples. Consequently, various surface FTIR techniques (such as Infrared Reflection-Absorption Spectroscopy or IRRAS, and Attenuated Total Reflection or ATR) have been developed to obtain the IR spectra of proteins/peptides in H2O environment and have been reviewed here.
Highlights
Background of peptides/proteinsProteins are biological macro-molecules which are involved in various biochemical processes
When covalently linking amino acids by amide bonds (O=CNH in which the carbonyl group is from one amino acid and NH is from the adjacent amino acid as shown in Figure 1), the polypeptide chain of peptides or proteins is formed
Lots of Fourier Transform IR (FTIR) spectra of proteins/peptides were obtained in D2O [15,16,17,18,19,20], not H2O, because the intensive IR absorption of H2O between 1500 to 1800 cm-1 overlaps the position of the amide I band [21,22]
Summary
Proteins are biological macro-molecules which are involved in various biochemical processes (e.g., oxidative phosphorylation [1,2], DNA replication [3,4], response to stimuli [5,6] and so on [7,8]). Proteins/peptides contain groups with IR active vibrations such as the amide group (i.e., O=C-NH), αC-H, and residue groups of amino acids These groups result in several intensive bands in the FTIR spectra. Lots of FTIR spectra of proteins/peptides were obtained in D2O [15,16,17,18,19,20], not H2O, because the intensive IR absorption of H2O between 1500 to 1800 cm-1 overlaps the position of the amide I band [21,22]. To maintain the protein molecules in an H2O environment, IR Reflection-Absorption Spectroscopy (IRRAS) which can measure the FTIR spectroscopy of proteins at the air-water interface has been developed Amphiphilic molecules such as stearic acid, which contains both a hydrophilic head group and a hydrophobic alkyl chain, can form a Langmuir monolayer (i.e., one single layer of molecules) at the airwater interface. Β-amyloid (Aβ) can form various types of aggregates, which are in β-sheet conformation and have been shown to be responsible for Incident IR beam
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