Amino acids secondary

The advantages of this method are a short reaction time and the nonfluorescence of the OPA reagent. Therefore, excess reagent must not be removed before the chromatography stage. Using this method, it is possible to measure tryptophan, but not secondary amino acids such as proline or hydroxyproline. Cysteine and cystine can be measured, but because of the low fluorescence of their derivatives, they must be detected using an UV system, or alternatively oxidized to cysteic acid before reaction. 

TABLE III. Kinetic Data for Nitrosation of Secondary Amino Acids at pH 2.5... 

Fig. 5.5 The oligopeptide synthesis at cationic micelles using the condensation agent CDI leads to the intermediate (I), which is in equilibrium with an IV-carboxyanhydride (II). A free primary or secondary amino acid reacts with (II) and forms an amide linkage as well as a carbamide terminus. ...

With secondary amino acids, ninhydrin reacts to form a yellow complex with different absorbance characteristics. For this reason, detection occurs at both 570 and 440nm. 

PITC has been used extensively in the sequencing of peptides and proteins and reactions under alkaline conditions with both primary and secondary amino acids. The methods of sample preparation and derivatization follow a stringent procedure which involves many labour-intensive stages. However, the resulting phenylthio-carbamyl-amino acids (PTC-AA s) are very stable, and the timing of the derivatization step is not as critical as when using OPA. 

Derivatization of primary amino acids with o-phthalaldehyde (OPA) is simple and the poor reproducibility due to the instability of the reaction product can be improved by automation and the use of alternative thiols, e.g. ethanthiol in place of the 2-mercaptoethanol originally used. An alternative fluorimetric method using 9-fluoroenylmethylchloroformate (FMOC-CL) requires the removal of excess unreacted reagent prior to column chromatography. This procedure is more difficult to automate fully and results are less reproducible. However, sensitivity is comparable with the OPA method with detection at the low picomole or femtomole level, and it has the added advantage that both primary and secondary amino acids can be determined. 

The first success in sequence-specific peptide cleavage by an antibody was claimed by Iverson (Iverson and Lemer, 1989). He used hapten [43] containing an inert Cora(trien) complexed to the secondary amino acid of a... 

Peter, A., Tdrok, R., and Armstrong, D.W., Direct high-performance liquid chromatographic separation of unusual secondary amino acids and a comparison of the performances of CHIROBIOTIC T and TAG columns, J. Chromatogr. A, 1057, 229, 2004. 

Non-stabilized a, p y, 5-unsaturated azomethine ylides (158), generated by the decarboxylation method from 3,3-diarylpropenals (156) and secondary amino acids (157), have been found to undergo [1,7]-electrocyclization followed by a [1,5]-hydrogen shift, to yield 2,3-dihydro-17/-2-benzazepines (159). 

There are no special requirements in the selection of an N-terminal amino acid residue in a segment with which the carboxy component of a segment is to be coupled, unless a highly hindered amino acid or a secondary amino acid is selected. If a Pro or Hyp residue is located at the N-terminus of the segment, monitoring of the coupling reaction with ninhydrin or fluorescamine is extremely difficult. 

The earliest report on such lactim ether formation was from Sammes [72JCS(P1)2494], who converted piperazine-2,5-dione to 2,5-diethoxy-3,6-dihydropyrazine (173) with an excess of triethyloxonium fluoroborate. Subsequently, Rajappa and Advani (73T1299) converted proline-based piperazine-2,5-diones into the corresponding monolactim ethers. The starting material was a piperazinedione in which one of the amino acid units was the secondary amino acid proline, and the other a primary amino acid. This naturally led to the regiospecific formation of a monolactim ether (169) (on O-alkylation) from the secondary amide, whereas the tertiary amide remained intact. This was later extended to piperazine-2,5-diones in which the secondary amino acid was sarcosine [74JCS(P 1)2122], leading to the monolactim ethers (170). 

Secondary amino acids such as sarcosine have some color development with ninhydrin, but its sensitivity is considerably less than that of the primary amino acids. 

Detection of amino acids is typically by UV absorption after postcolumn reaction with nin-hydrin. Precolumn derivatization with ninhydrin is not possible, because the amino acids do not actually form an adduct with the ninhydrin. Rather, the reaction of all primary amino acids results in the formation of a chromophoric compound named Ruhemann s purple. This chro-mophore has an absorption maximum at 570 nm. The secondary amino acid, proline, is not able to react in the same fashion and results in an intermediate reaction product with an absorption maximum at 440 nm. See Fig. 5. Detection limits afforded by postcolumn reaction with ninhydrin are typically in the range of over 100 picomoles injected. Lower detection limits can be realized with postcolumn reaction with fluorescamine (115) or o-phthalaldehyde (OPA) (116). Detection limits down to 5 picomoles are possible. However, the detection limits afforded by ninhydrin are sufficient for the overwhelming majority of applications in food analysis. 

Dabsyl Chloride (4-dimethyl-aminoazobenzene-4-sulfonyl chloride) Aabs = 420 nm. Derivatives are very stable (days) and can be formed from both primary and secondary amino acids. Detection is by absorption only. Detection limits are in the high picomole range. Reaction time is typically around 10 minutes at 70°C. Completeness of reaction can be adversely affected by the presence of high levels of various salts. Because reaction efficiency is highly matrix dependent and variable for different amino acids, standard amino acid solution should be derivatized under similar conditions/matrix for accurate calibration. Commercial systems available uti-... 

PITC (phenylisothiocyanate) Aabs = 254 nm. Phenylthiocarbamyl amino acid derivatives are moderately stable at room temperature (1 day). PITC reacts well with both primary and secondary amino acids. Reaction time is approximately 5 minutes at room temperature. Excess reagent must subsequently be removed under vacuum. Also, for hydrolyzed samples, hydrochloric acid must be completely removed prior to derivatization. As a result, even though the actual reaction time is reasonably fast, the total time for various sample manipulations can add up to 2 hours. This is partially compensated by the extremely fast separation possible (12 minutes). Detection is by UV absorption only. Detection limits are typically in the high picomole range. Short column life can result due to unreacted PITC getting into the column. Unlike some of the other reagents, PITC quantifies tyrosine and histidine very well. PITC analysis is available as a commercially prepackaged system dubbed Pico-Tag by Waters Corporation. Representative references include 184-188. See Fig. 11 for a typical separation. 

Since fluorescamine reacts only with primary amino groups, secondary amino acids do not give a fluorescent product with this reaction. A method for converting secondary amino acids into primary amines has been described for analysis using fluorescamine [87], and is based on treatment of the amino acid with N-chlorosuccinimide. The reaction involves an oxidative decarboxylation of the amino acids. This method has been incorporated into the automatic analysis of amino acids with fluorescamine [88]. The fluorescence spectra and the sensitivities are similar to those of the derivatives of the primary amino acids. 

Method 4 (secondary amino acids). The procedure for the analysis of secondary amino acids by automatic ion-exchange chromatography is the same as that mentioned earlier [88] except for the following changes. A 10"4Jf solution of N-chlorosuccinimide in 0.05 M... 

The N-chlorosuccinimide is introduced by means of an auxiliary pumping system after the elution of glutamic acid and is removed after the elution of proline. Without this treatment, proline would not appear in the chromatographic results. The decrease in baseline is due to a change in the reagents and the total flow-rate of the cell. The limit of detection for secondary amino acids is ca. 0.25 nmole. 

Fig.4.43. Separation of primary and secondary amino acids with fluorescamine detection. (From ref. 85 with permission of Academic Press, New York.)...

Amino acid analyzers have improved the analysis of free amino acids to a great extent. They offer superior sensitivity, speed, and accuracy to conventional methods. Many such systems are based on IEC. Postcolumn detection is done by ninhydrin derivatization followed by photometric measurement at 570 and 440 nm for primary and secondary amino acids, respectively. Amino acid analyzers are now common and are being manufactured by many companies (e.g., Hitachi, Beckman, PerkinElmer, HP, Pharmacia, etc.). Numerous authors have used amino acid analyzers to monitor proteolysis in several kinds of cheeses (Ardo and Gripon, 1995 Edwards and Kosikowski, 1983 Fenelon et al., 2000 Gardiner et al., 1998 Kaiser et al., 1992 Yvon et al., 1997). A comparison of amino acid analyzers and several other methods for amino acid analysis is available from Biitikofer and Ardo (1999) and Lemieux et al. (1990). 

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