Amino fluorescamine reaction

Fluoranthene, >3, 103, 293 Fluorescamine, reaction with amino acids. JHO... 

The ninhydrin reaction (see Basic Protocol 1), the TNBS reaction (see Alternate Protocol 1), the fluorescamine reaction (see Alternate Protocol 2), and formol titration (see Alternate Protocol 3) all evaluate released amino groups by comparing the amounts of free amino groups before and after hydrolysis. The first three methods are spectro-photometric techniques, whereas the fourth is a potentiometric technique. The first and second are chromogenic techniques, whereas the third is fluorometric. These techniques are usually performed as time-course experiments. As the hydrolysis reaction proceeds, aliquots (samples) of the reaction are taken periodically and treated with a test reagent. Products of this reaction are proportional to the amount of free amino groups at each time point. 

Method 3 (TLC). The amino acids and peptides are separated on silica gel plates with butanol-acetic acid-ethyl acetate-water (1 1 1 1). The chromatogram is dried at 110 °C for 10 min and then cooled to room temperature. The plate is sprayed with a 10% solution of triethylamine in methylene chloride and is dried in air for several seconds. A solution of 0.05% fluorescamine in acetone is then sprayed on to the plate. The plate is again dried in air and is then resprayed with the triethylamine solution before observation under UV light. This procedure was found to be superior to the earlier procedure of using aqueous buffers for spraying the plates prior to fluorescamine reaction [89]. The limit of detection for the modified spray method is 0.5 nmole of the amino acid or peptide. 

The main advantage of this fluorescamine method is its simplicity. The reaction is fast (1-2 min) and can be performed in test tubes under mild conditions. As little as 0.1 -1.0 /ig/ml of dipeptides has been routinely reacted with fluorescamine, and useful CD spectra of the reaction mixture have been obtained. It is hoped that the CD spectra of the fluorescamine reaction products with larger peptides and NH2-terminal secondary amino acids will also be investigated. 

Fluorescamine was developed by Weigele et al. in 1972 [8], based on the fact that strongly fluorescent pyrrolinones were formed by the reaction of ninhydrin, phenylacetaldehyde, and primary amines. The reagent, 4-phenylspiro[furan-2(3H),l -phthalan]-3,3 -dione (fluorescamine), is nonfluorescent, and it reacts with primary amines, amino acids, and peptides under aqueous conditions in a few minutes at room temperature to form intensely fluorescent substances (Figure 6.1). On the other hand, nonfluorescent derivatives are formed by the reaction of fluorescamine and secondary amino compoimds. Therefore, fluorescamine can be used for the selective determination of primary amino compounds, and the fluorophore produced by the reaction is the expected pyrrolinone. Because the reaction is sufficiently rapid and the hydrolysis products are nonfluorescent, the fluorescamine reaction is applicable for the postcolumn fluorescence derivatization of primary amino compounds [9]. The amino acids are separated by a cation-exchange column similar to the ninhydrin method, and the column effluent is mixed with an alkaline-buffered solution and fluorescamine reagent. The fluorescent derivatives are detected at 480 nm with excitation at 390 nm. 

The specific detection of aromatic nitro compounds is a second example. These can be converted by reduction to primary amines, which are then diazotized and coupled to yield azo dyes (cf. reagent sequence Titanium(III) chloride — Bratton-Marshall reagent ). Sodium nitrite —naphthol reagent, diazotized sulfanilic acid and other reagents specific for amino groups (e.g. ninhydrin, fluorescamine, DOOB, NBD chloride [9]) can also be used in the second stage of the reaction (Fig. 21). 

For fast reactions (i.e., < 1 min.), open tubular reactors are commonly used. They simply consist of a mixing device and a coiled stainless steel or Teflon capillary tube of narrow bore enclosed in a thermostat. The length of the capillary tube and the flow rate through it control the reaction time. Reagents such as fluorescamine and o-phthalaldehyde are frequently used in this type of system to determine primary amines, amino acids, indoles, hydrazines, etc., in biological and environmental samples. 

All primary amines react with fluorescamine under alkaline conditions (pH 9-11) to form a fluorescent product (Figure 10.12) (excitation maximum, 390 nm emission maximum, 475 nm). The fluorescence is unstable in aqueous solution and the reagent must be prepared in acetone. The secondary amines, proline and hydroxyproline, do not react unless they are first converted to primary amines, which can be done using A-chlorosuccinimide. Although the reagent is of interest because of its fast reaction rate with amino acids at room temperature, it does not offer any greater sensitivity than the ninhydrin reaction. 

Figure 10.12 The reaction of fluorescamine with a primary amino group. The...

Latent fingerprints on paper have been revealed by combining the amino acids present with reagents such as ninhydrin (see 37), dansyl chloride (92), fluorescamine (154), 4-chloro-7-nitrobenzofurazan (127a) and o-phthalaldehyde (see reaction 7). To avoid some problems encountered with these reagents it was proposed to use 1,8-diazafluorenone (155), leading to the formation of highly fluorescent ylides (156)349. 

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. 

Fluorescamine reacts with primary amines to form fluorophores (see Fig. B2.2.4) that are excited at 390 nm and fluoresce at 475 nm. Peptides react with fluorescamine at pH 7.0, giving higher fluorescence than amino acids, which have maximum fluorescence at pH 9. The reaction proceeds rapidly with primary amines at 25°C. The resulting fluorescence is proportional to the amine concentration. The fluorophores are stable for several hours. A negligible interference is produced with ammonia. 

Fluorescamine, or 4-phenylspiro[furan-2(3//),l/-phthalan]-3,3,-dione, is used to introduce a fluorescent label on electroblotted proteins via reaction with free amines. Transferred proteins are visualized on blot transfer membranes with UV light. This stain can be very sensitive and can be used in conjunction with a second detection method such as immunoblotting (also see Basic Protocol 3). However, the protein is irreversibly modified because fluorescamine reacts with available amino groups (i.e., lysines and the protein N terminus if it was not previously blocked). 

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. 

Since then, there have been a number of reversed-phase separations employing precolumn derivatization. Interestingly, fluorescamine (not frequently employed for RP-HPLC of amino acids with precolumn reaction) has been reported for taurine analysis in milk (197) and human plasma (198). Precolumn derivatization with OPA/2-mercaptoethanol has been reported for the analysis of infant formula and human breast milk (199). Although not the principal focus of the study, Carratu et al. (200) report taurine values in parenteral solutions as determined by FMOC. In an excellent article, Woollard and Indyk (201) report the dansylation of taurine for its determination in a wide variety of dairy-related products. Subsequently, the same authors report the results of a large collaborative study (202) for the determination of taurine (again, by dansylation) in milk and infant formula. This study afforded an overall interlaboratory RSD of 7.0% and established a lower limit for determination at 5 mg taurine per 100 g of product. 

The terminal amino and carboxyl groups react in the same way as the corresponding amino acids (acylation, animation, esterification, etc.). Some of the reactions of the amino groups (with ninhydrin, orthophthaldehyde, fluorescamine, etc.) are used for detection purposes, as will be discussed later. Peptides also react in ways that free amino acids do not, like the classic biuret reaction, which consists of the formation of a colored complex with a transition metal (Cu, Ni, etc.)... 

The use of chemiluminescence reactions for the detection of metal ions by liquid chromatography was recently reported [59,60]. The detectors made use of the chemiluminescence produced in the reaction between luminol and hydrogen peroxide which is catalyzed by transition metals. The column effluent was mixed with the reagents in order to yield the chemiluminescence. The reaction was fast and was carried out at room temperature. By varying the pH of the buffer, selectivity towards certain metals was also achieved. For example, at pH 10-11 nickel could be analyzed but lead and aluminium were inactive at pH 13-14, the converse was true [59]. Aminco-Bowman has marketed a liquid chromatographic system in which amino acids and amines are analyzed by means of the fluorescence produced on reaction with the reagent fluorescamine. Fluorescamine does not fluoresce, but it does react with primary amino groups to produce fluorescent derivatives. The reaction is instantaneous and may be carried out at room temperature, usually at pH 9. This detection system promises to be far more sensitive than the ninhydrin detection system and is much more easily adapted to HPLC. 

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. 

Fluorimetric methods for the determination of amino acids are generally more sensitive than colorimetric methods. Fluorescamine (4-phenyl-spiro[furan-2(3H),l -phthalan]-3,3 -dione) and o-phthaldialdehyde (OPA) substances are used for protein analysis. Fluorescamine reacts with amino groups to form fluorophores that excite at 390 nm and emit at 475 nm (Weigele et al., 1972). Applications of fluorescamine include monitoring the hydrolysis of K-casein (Beeby, 1980 Pearce, 1979) and quantification of proteins, peptides, amino acids in extracts (Creamer et al., 1985). OPA produces fluorescence on reaction with 2-mercaptoethanol and primary amines, with strong absorption at 340 nm. Lemieux et al. (1990) claimed that this method was more accurate, convenient, and simple for estimating free amino acids than the TNBS, ninhydrin, or fluorescamine methods. 

The number of each type of amino acid in a protein can be determined by acid hydrolysis and separation of the individual amino acids by ion exchange chromatography The amino acids are detected by colorimetric reaction with, for example, ninhydrin or fluorescamine. 

Fluorescamine (VI) is more advantageous because of its specificity. The reaction can be performed with any compound that has a primary amine group or that can be chemically treated to yield a primary amine this is the case with a pesticide such as fenitrothion, that has a nitro group which can be reduced to an amino group. This property of fenitrothion has been used to advantage and a very reliable quantitative method has been developed (36). 

Fig. 8.3. Coupled assay for amidohydrolase activity. In the enzymatic reaction (A) an amide bond is cleaved releasing carboxylic acid and amine. The free amino group of the amine is then reacted with fluorescamine (B) in a rather fast reaction yielding fluorescent compounds. The excess of fluorescamine hydrolyzes spontaneously within seconds leaving only non-fluorescent compounds.

Fluorescamine (4-phenylspiro(furan-2-(3H),r-phthalan)3,3 -dione) is also a commonly used fluorescence reagent. It reacts almost instantly and selectively with primary amines, while the excess of the reagent is hydrolyzed to a non-fluorescent product. The reagent itself is non-fluorescent. The reaction is carried out in aqueous acetone at a pH of about 8-9 and the derivatives can be chromatographed directly. The excitation and emission wavelengths are 390 nm and 475 nm respectively. Two disadvantages of the reagent are its cost and the fact the products are less stable, cannot be stored and should be injected onto the column immediately after formation. Fluorescamine has been employed in the analysis of polyamines, catecholamines and amino acids. 

Figure 6-16. Reaction of fluorescamine obtained from Hofmann-LaRoche, Inc. with primary amino groups of amino acids.

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