I guess this band corresponds to a CO stretch motion. CO stretch motions can also occur between 1700 and 1800 cm-1 dependent on the interaction with other atoms. I observed bands in this region in gas phase IRMPD spectra of dipeptides and their interaction with alkali metal ions. Flour should also contain Na+ and K+ ions. Maybe the observed band is a hint for protein/metal interactions. Do you do DFT calculations on some of the compounds of your mixture?

No, I don't. This is actually not really my expertise - this is the first time I have been using this technology. I have been using it as a great tool to look for modifications in the flour and go from there. I interestingly see increased absorbance/additional peaks in this area in modified flour, so it must be some sort of structural changes in the proteins due to with what I modified it...you say that between 1700 and 1800 cm-1 CO stretch motions can also occur...is this region then also very sensitive for modifications in secondary protein structures, Alpha-Helices, beta-sheets etc. like the Amide I region is?

This is the region of the C=O; most probably from your fat (esters) in the simple.

1750 cm-1 is greater than the bands from fatty acids below 1700 cm-1 normally

Haifeng, thanks, do you mind rewording your answer?

The frequency of C=O vibration in fatty acids is less than 1700 cm-1,so I doubt the band could be assigned to determine the secondary protein structures.

I agree with the comment by posted by Haifeng Yang. In our work on wheat proteins or wheat flours, we also associated this C=O peak to fatty acid and I am sure there is a plethora of papers supporting this. However having said that could this peak shift due to interactions between proteins and fatty acids? That is a point worth being looked at.

Most probably the carbonyl (C=O) stretching from fatty groups.

Our group thought so too, that the peak at ~1750 cm-1 is most likely coming from fatty groups of the flour. Dominique, I don't think this peak really shifted when I tested modified flour, rather I see an increase in intensity of this peak plus an additional peak at ~1725 cm-1 (probably due to interactions of proteins with chemical groups of my treatment) that varies in intensity depending on the modification. Thanks much for all of your input on this. I really appreciate it. Maybe we are able to look closer into this in the future, as I am really curious about specific modifications/bondings that occured...

Thanks much Jean-François. This is supporting what I just found in several publications based on all of the answers here. There is indeed a band at 1742 to 1746 cm-1 that is assigned to triglyceride ester linkages (c=o stretch).

Most likely it is the C=O stretching mode of esters in phospholipids and triglycerides. Esterified polysaccharides would also contribute to this band. In the case of phospholipids the presence of a doublet (at about 1745 cm-1 and 1725 cm-1) has been interpreted with the presence of two types of C=O groups with different degree of hydration.

Thanks much Luca, I appreciate it. All of this should give me enough information for now.

These could be of carboxylic acids. The C=O stretch bands of the protonated carboxylic acids, Asp & Glu, INSIDE proteins could be observed at as high as 1760 cm-1. The frequency is ~ 1715 cm-1 for a cycle dimer (the strong hydrogen bond in in concentrated solution) and all the way up to ~1760 cm-1 in hydrophobic environment, lacking the hydrogen bonding. A well-studied example is found in retinal proteins like bacteriorhodopsin (for review see Dioumaev, A.K. Biochemistry (Moscow) 66(11)1269-1276 (2001)(English version). A detailed study how the hydrogen bonding affects the frequency could be found, for instance, in Nie, B.N. et al., Biophys. J. 88(4):2833-2847 (2005), and references therein.

Dear Jean-François,

Your arguments are absolutely true for solutions. However, in proteins several conditions that are true in solution do not hold. First, amino acids in proteins cannot be in zwitterionic form because the primary "amine" and the primary "carboxyl" are bound to form an amide bond. Second, unlike in solution, the aminoacids side chains (of Asp, Glu, Lys, Arg, Tyr, His, etc.). could be either protonated or deprotonated, but their correspondent pKa's has very little to do with the values measured in solution (and listed in data-books). Strange as it sounds, but the effective pKa of the Asp's carboxylic acid might be extremely high (~10) both statically and transiently. Therefore, they stay protonated under physiological conditions in the former case, and act as proton acceptor in the later. In retinal proteins this is not just a well-documented fact but is also the main mechanism - reversible aceeptor/donor function of specific Asp/Glu residues - in the light-induced proton transport, the physiological function of these proteins in nature.

Dear Jean-François,

I joined UCI 10 years after you left, in 1996, and I am in the Medical School, doing biophysical (essentially in physical chemistry) research on microbal rhodopsins that act as light-driven ion pumps, pumping protons, chloride, sodium and litium ions. Some of the retinal proteins function as sensors as well, but I have not done anything on later, preferring to to stay close to "physical chemistry" side of the research rather than venturing in "biology/microbiology" topics.

In your case, I suggest peak at around 1750 cm-1 is mainly due to carbony (CO) stretching vibration mode. If your mixture only contain, protein and fat, this carbonyl group is belongs to saturated aliphatic ester (1750 to 1735 cm-1). For carbonyl stretching, it vary from 1760 to 1670 cm-1due to molecular structure, intra-moleculer interaction and other facts.

saturated aliphatic aldehydes -1740-1720

saturated aliphatic ketones - 1715

amides -1680-1630

anhydrides -1830-1800 and 1775-1740

acid chlorides -1810-1775

esters -1750-1735

carboxylic acids -1730-1700

You can refer spectral data base.

Thanks much!

I have come across the question rather late but thought it worth making the following observations. Ketones start at 1710 the addition of an oxygen shortens the double bond shortening the wavelength increasing the wave number. In an aliphatic ester the band moves to about 1740 so what brings it to 1750 - physical environment or strain. i.e. crystalline, isolated or in a ring. A zwitterion is by definition not longer an acid but a salt so the "carbonyl" band moves to lower not higher wavenumber. The phrase carboxylic acid dimers exist even in the gaseous state I have often quoted myself but I have recently seen one separate into the monomer in solution.

Without knowing the exact composition of your sample and not having seen your FTIR spectrum, I guess it is attributed to carbonyl (C=O) stretching. The exact wavenumber for the carbonyl absorbance peak may vary with neighbouring chemical bonds in your compound. For instance, I’ve measured oxidation of a polymer with a growing absorbance carbonyl peak around 1732 cm-1.