Phenylisothiocyanate method

If the phenylisothiocyanate method is used, the cyclization and release of the N-terminal derivative occurs under mild conditions that leave the rest of the chain intact. It is therefore possible to take the protein chain, now without its original N-terminal residue, and repeat the procedure to determine the second residue in the sequence, and so on. Unfortunately, at each step, there is a finite chance of additional peptide hydrolysis or incomplete reaction, and uncertainty tends to accumulate after 10 to 20 cycles. 

Edman degradation (Section 26.6) A method for N-terminal sequencing of peptide chains by treatment with Af-phenylisothiocyanate. 

Figure 7.5 The Edman degradation method, by which the sequence of a peptide/polypeptide may be elucidated. The peptide is incubated with phenylisothiocyanate, which reacts specifically with the N-terminal amino acid of the peptide. Addition of 6 mol l-1 HCl results in liberation of a phenylthiohydantoin-amino acid derivative and a shorter peptide, as shown. The phenylthiohydantoin derivative can then be isolated and its constituent amino acid identified by comparison to phenylthiohydantoin derivatives of standard amino acid solutions. The shorter peptide is then subjected to a second round of treatment, such that its new amino terminus may be identified. This procedure is repeated until the entire amino acid sequence of the peptide has been established...

Most HPLC instruments monitor sample elution via ultraviolet (UV) light absorption, so the technique is most useful for molecules that absorb UV. Pure amino acids generally do not absorb UV therefore, they normally must be chemically derivatized (structurally altered) before HPLC analysis is possible. The need to derivatize increases the complexity of the methods. Examples of derivatizing agents include o-phthaldehyde, dansyl chloride, and phenylisothiocyanate. Peptides, proteins, amino acids cleaved from polypeptide chains, nucleotides, and nucleic acid fragments all absorb UV, so derivatization is not required for these molecules. 

Reaction with phenylisothiocyanate (PITC) in alkaline conditions produces stable phenylthiocarbamyl (PTC) adducts which can be detected either in the ultraviolet below 250 nm or electrochemically. However, this method involves a complex derivatization procedure and offers poorer sensitivity than the alternatives available for individual amino acids. It is useful, however, in conjunction with the automated analysis of peptides when single derivatized residues can be cleaved and analysed after conversion in acidic conditions to phenylthiohydantoins. 

Naulet et al. [223] compared three chromatographic techniques lEC on an automatic amino acid analyzer, HPLC (with phenylisothiocyanate derivatization) and GC for the determination of the free amino acid content in 64 orange juices from different countries. The consistency of the different methods was estimated by considering the mean standard deviation for the set of amino acids observed. The best consistency was observed between lEC and HPLC. 

To sequence an entire polypeptide, a chemical method devised by Pehr Edman is usually employed. The Edman degradation procedure labels and removes only the amino-terminal residue from a peptide, leaving all other peptide bonds intact (Fig. 3-25b). The peptide is reacted with phenylisothiocyanate under mildly alkaline conditions, which converts the amino-terminal amino acid to a phenylthiocarbamoyl (PTC) adduct. The peptide bond next to the PTC adduct is then cleaved in a step carried out in anhydrous trifluo-roacetic acid, with removal of the amino-terminal amino acid as an anilinothiazolinone derivative. The deriva-tized amino acid is extracted with organic solvents, converted to the more stable phenylthiohydantoin derivative by treatment with aqueous acid, and then identified. The use of sequential reactions carried out under first basic and then acidic conditions provides control over... 

In modem sequencing methods vinylpyridine, which reacts in a similar way, is often used. It can be detected during amino acid analysis or sequencing after derivatization with phenylisothiocyanate (Eq. 3-30). 

An indirect method makes possible the general use of lithium as well as magnesium organometallic reagents for the synthesis of dithioesters their addition to phenylisothiocyanate followed by alkylation gives A-phenylimidothioesters (themselves valuable intermediates), which are readily sulfhydrolysed [151,152],... 

Even more versatile than the dansyl method is the Edman method (Figure E2.4). The NH2-terminal amino acid is removed as its phenylthiohydan-toin (PTH) derivative under anhydrous acid conditions, while all other amide bonds in the peptide remain intact. The derivatized amino acid is then extracted from the reaction mixture and identified by paper, thin-layer, gas, or high-performance liquid chromatography. The intact peptide (minus the original NH2-terminal amino acid) may be isolated and recycled by reaction with phenylisothiocyanate. Since this method is nondestructive to the remaining peptide (aqueous acid hydrolysis is not required) and results in good yield, it can be used for stepwise sequential analysis of peptides. The method is now automated. 

Gas chromatography (GC) for amino acid analysis is the alternative to HPLC that has found the greatest acceptance. It requires the preseparation derivatization of the amino acids to render them volatile. For this purpose, amino acids are frequently converted into acylated esters. N-Trifluoroacetyl-n-butyl esters and /V-heptafluorobutyrylisobutyl esters are most commonly employed. There have been comparative studies (3,4) that document similar (if not equivalent) analytical results for GC and the classic ion-exchange chromatographic method applied to a variety of food samples. Comparison (5) of GC to the reversed-phase HPLC determination of amino acids (phenylisothiocyanate derivatized) also shows excellent agreement. 

The Edman degradation method for polypeptide sequence determination. The sequence is determined one amino acid at a time, starting from the amino-terminal end of the polypeptide. First the polypeptide is reacted with phenylisothiocyanate to form a polypeptidyl phenylthiocarbamyl derivative. Gentle hydrolysis releases the amino-terminal amino acid as a phenylthiohydantoin (PTH), which can be separated and detected spectrophoto-metrically. The remaining intact polypeptide, shortened by one amino acid, is then ready for further cycles of this procedure. A more sensitive reagent, dimethylaminoazobenzene isothiocyanate, can be used in place of phenylisothiocyanate. The chemistry is the same. 

Another widely used method for qualitative and quantitative analysis of amino acid mixtures is high-performance liquid chromatography (HPLC) (see Experiments 2 and 6). The mixture of amino acids is first subjected to reaction with phenylisothiocyanate (PITC) to convert them to the phenylthiocarbamyl-amino acid derivatives, which are then subjected to chromatographic separation. The derivativatization of the amino acids serves two purposes it attaches a UV-absorbing tag, which makes their quantitative determination easy, and it converts them to a more hydrophobic form, which is necessary for good separation on the reverse-phase system commonly used with this technique. This method of amino acid analysis will be used in Experiment 6. 

Several techniques have been developed to analyse amino acids. The following procedure describes one of the most frequently used methods for the determination of amino acid concentration, the PITC method. Free amino acids in medium samples are derivatized with phenylisothiocyanate (PITC) to produce phenyl-thiocarbamyl (PTC) amino acids. After separation by reverse-phase HPLC, the PTC amino acids are detected using a UV spectrophotometer at 254 nm. 

Each fragment is sequenced through repeated cycles of a procedure called the Edman degradation. In this method phenylisothiocyanate (PITC), often referred to as Edman s reagent, reacts with the N-terminal residue of each fragment. 

A number of other pteridinones and pteridinethiones have been obtained by similar methods, for example the use of phenylisothiocyanate as the one-carbon unit with 3-amino-5,6-diphenyl-pyrazine-2-thiocarboxamide gives 2-anilino-6,7-diphenylpteridine-4(3/f)-thione (2).139... 

The precolumn technique that is most frequently employed today was developed during the early 1980s [32,33]. For this method, The classical Edman reagent phenylisothiocyanate (PITC) is used for amino acid derivatization after hydrolysis. Separation of the PTC amino acids i then accomplished by HPLG, with detection at 254 nm. Although standard Cig columns available Irom a variety of vendors are suitable for separation of the PTC-derivatized amino acids, there are specific columns that have bqen optimized for this purpose (e.g.. Waters). Approximately 0.5 /ug of peptMe should be hydrolyzed for analyses using precolumn derivatization. ... 

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