They usually become sharper if the polarity of the solvent increases e.g. These peaks change significantly with the polarity of the solvent. The change in peak shape is a result of the different degree of hydrogen bonds in alcohol and carboxylic acids. The O-H-stretching modes from alcohols and phenols ( example 5) are mostly broad and very strong (3200-3650 cm -1 ) The O-H-peaks due to carboxylic acids ( example 6) show a very broad and less intense peak between 25 cm -1. The differences in wavenumbers are mainly due to different hybridization (=bond strength, the s-character in the bond, the stronger the bond is) and number of ligands on the carbon atom. The CH-function on a C-C-triple bond (alkynes) will appear as a sharp, strong peak around 3300 cm -1. Latter ones are usually weak or medium in intensity. The range from 2850-3000 cm -1 belongs to saturated systems (alkanes, sp 3, example 1), while the peaks from 3000-3100 cm -1 indicate an unsaturated system (alkenes, sp 2, example 2 aromatic ring, example 3,4). The C-H-stretching modes can be found between 28 cm -1, depending on the hydrization. In this range typically E-H-stretching modes are observed. The IR-spectrum can be divided into five ranges major ranges of interest for an organic chemist:Ī. The actual number of your vibration changes quite at bit, especially for highly polar compounds (Why ?). Also, be cautious when you compare spectra which were obtain with different techniques (solution, Nujol mull, KBr pellet). However, it is not necessary to interpret every little peak in the IR spectrum. C-H, C-C, C=C, etc., will usually show weak or medium sized peaks in the IR spectrum.Īn interpretation of an IR spectrum should include a detailed assignment of the peaks: exact wavenumber from the spectrum (integer), the intensity (w/m/s/br) and which functional group it represents, and maybe in addition the corresponding literature value.
Groups with a small difference in electronegativity e.g. This is very likely for groups which already possess a significant dipole momentum to start with e.g. The more complicated the molecule is (the more atoms it possesses and the lower the symmetry), the more peaks can be observed in the IR spectrum (see example 3 and 4).Ī signal is only observed in the IR spectrum, if the dipole momentum of the molecule changes during the interaction with the electromagnetic radiation. If there is no symmetry in the molecule most of them will be observed the IR spectrum the remaining modes will be observed in a Raman spectrum. CO 2 one expect to find 3N-5 (3*3-5=4) modes. For non-linear molecules 3N-6 (2N-5 bending, N-1 stretching) vibrations are observed (e.g. The number of basic stretching and bending modes expected for a molecule increases with the number of atoms in the molecule. The required energy for this process is much smaller (1-5 cm -1 ) and causes together with some other effects the broading of the 'lines'. Unfortunately, the change in vibration modes is always accompanied with change in rotational mode (Stokes and Anti-Stokes). H/D-exchange in labeling experiments although the bond strength remains the same.īased on the equation, one would always expect a sharp lines at a well defined wavenumber.
If the masses of the involved atoms increase, the peak will shift to lower wavenumbers e.g. If the force constant F (= bond strength) increases, the stretching frequency will increase as well (in cm -1 )ī. Using the 50-protein database built beforehand to contain as little fold redundancy as possible, the standard error of prediction in cross-validation is 5.5% for the α-helix, 6.6% for the β-sheet, and 3.4% for the β-turn.From this equation, one can deduce some basic trends can be deducted:Ī. The ascending stepwise method proposed here identifies the relevance of each wavenumber of the infrared spectrum for the prediction of a given secondary structure and yields a particularly simple method for computing the secondary structure content. It appears that, at most, the absorbance at three distinct frequencies of the spectra contain all the nonredundant information that can be related to one secondary structure content. We address here the question of the amount of independent information present in the infrared spectra of proteins for the prediction of the different secondary structure contents. The high quality of the current Fourier-transform infrared spectrometers makes the absorbance at every single wavenumber a valid and almost noiseless type of information. A critical parameter that has not been taken into account systematically is the selection of the wavenumbers used for building the mathematical models used for structure prediction. Numerous approaches have been developed during the past 15 years.
Fourier-transform infrared spectroscopy is a method of choice for the experimental determination of protein secondary structure.
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