Amino acids, characteristic frequencies

The aliphatic members of the amino acids exhibit no absorption in the ultraviolet region above 220 nm, while the aromatic amino acids, HIS, PHE, TRY and TYR, show characteristic maxima above 250 nm. The zwitteronic character of the a-amino acids shows up clearly in their infrared spectra. No absorption due to the normal NH stretching frequency at 3300-3500 cm" is observed, indicating the absence of an NH2 group. Instead, a few of the peaks near 3070 cm", 1600 cm", 1500 cm" due to the NHJ are seen (except in PRO, the NHJ group absorbing at 2900 cm" ). The carbonyl absorption of the unionized carboxyl group at 1700-1750 cm" is likewise replaced by carboxylate ion absorption at 1560-1600 cm" [6]. 

In the solid state, amino acids exist in the zwitterionic form, E (Fig. 4.5). The primary ammonium group, NH3+, exhibits a broad band 3040 cm-1 and two bands common to amino acids at 2500 and 2100 cm-1. Strong bands at 1665 and 1590 cm-1 for the COO- asymmetric and symmetric stretches, respectively, and NH3+ deformations at 1550 cm-1 are also typical of amino acids. A full assignment for glycine is found in Nakamoto.10 The metal-bound COO- and NH2 groups will show characteristic shifts in these stretching frequencies. 

Amino acid side chains, particularly those with aromatic groups, exhibit characteristic frequencies that often are very useful in probing the local environment of the group in the protein (Spiro and Gaber, 1977). From our present point of view, however, we are interested in characterizing spectral features of backbone chain conformation. It is therefore important to know the locations of such bands so that their contribution to the spectrum is not confused with amide and backbone vibrations. We discuss below some features (in the nonstretching region) of such side-chain modes these are summarized in Table XL. 

In addition, the infrared contributions of the side chains of the amino acids which constitute the protein must be considered. Amino acid side chains exhibit infrared modes that are often useful for investigating the local group in a protein. It is also important to be aware of the location of such modes as they may be confused with amide vibrations. Fortunately, these contributions have been found to be small in D2O when compared to the contributions made by the amide I band. The characteristic side-chain infrared frequencies of amino acids are summarised in Table 6.2b. 

Table 6.2b Characteristic frequencies of amino acid side chains...

Recently the SERS spectra of on silver hydrosols adsorbed aromatic amino acids have also been studied The Raman spectra are enhanced from 1(K) to 2(K) times in the presence of silver colloids with primary sol particles 14 nm in size. The hydrosol-Phe interaction shows the strongest SERS spectrum. The frequency shifts between the NSRS- and SERS spectra are small (1-15 cm" ). The following SERS bands are characteristic for the aromatic amino acids Phe, 1005, 1034 and 1049cm" Trp, 762,1340 and 1377 cm - Tyr, 831,988,1169 and 1299 cm - His, 670 and 1310 cm -. ... 

The VOA of a-amino acids has also been examined in some detail as an aid to the use of VCD measurements in the understanding of peptide confonnation . The VCD spectra of L-alanine (l-163) , several other L-a-amino acids and dipeptides wrae measured in water and deuterium oxide between 900 and 1700 cm . In both solvents, a characteristic VCD pattern near 1325 (negative at higher frequency) was... 

Chapman and Morrison (1966) have found NMR evidence favoring a dipolar ionic form for the phosphatidyl ethanolamines. Also, their infrared spectra of chloroform solutions favor a dipolar ionic structure. The evidence was as follows if dioleoyl-phosphatidyl ethanolamines exist in chloroform in a nonionic form, then intense bands in the 3300 cm region should occur because of NH stretching frequencies. Bands were found at 3058, 2710, 2538, and a probable band at 3021 cm , which they correlated with vibrations of an NHj group. A comparison of the spectra of dioleoyl-phosphatidyl ethanolamine and a dipolar ionic amino acid, such as alanine, showed almost identical spectra in the 4000 to 2000 cm region. The spectrum of the non-ionized compound, OL-a-alanine methyl ester in chloroform shows intense absorption in the 3300 cm region characteristic of a free primary amino group. 

Edsall et al., 1950). Because of the dipolar ionic structure many amino acids have a characteristic band at 1587cm which is related to the —COO group, as well as a rather weak absorption at 2128cm which comes from NH frequencies in the —NHj ion (Thompson et al, 1950 Klotz and Gruen, 1948). 

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