In my experiment, a kind of protein displays a band at 2931 cm-1 and I assign it to
Vas(CH2), but the corresponding Vs(CH2) band at ~2854cm-1 didn't appear. So I
am a little confused about the bands assignment.
May be the sym band is merged in the strong O-H background if any. Have you looked at the corresponding second derivative ?
If you want, send me a typical IR spectrum in an ASCII format; I'll have a look ([email protected]). What kind of sample is it ?
Olivier
Thank you for Olivier Sire's answer.I have analyzed the data in detail and refered to some papers. The peak position in second derivative is at 2934cm-1 and no band is observed at ~2854 cm-1. Most reported band for CH2 asym is around 2916~2932 cm-1, so I think there is no absorption of CH2 for the protein, maybe due to the unfavorable orientation. The band at 2934 cm-1 is probably originated from Fermi resonance between sym CH3 and the overtone of bend vibration band of CH3
I think it could be due to the orientation of the protein. Did you also check the range ~1450 -1380 cm-1，which is another spectral range of CH2 or CH3 groups.
The previous comment could be relevant if the protein is immobilized in a solid layer by example. Is it the case ? What is the protein environment (chemical) and its physical state : solid ? gel ? liquid ?
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