Udenfriend reagent

X = NH2, Y = H). Oxidation (54) of tetracyclines usiag the Udenfriend reagent has yielded 9-hydroxytetracyclines and disubstituted (C-7 and C-9) products (48) can also be obtained. Substituent assignments are made from nmr spectral iaterpretations. The 7- and 9-methyl tetracyclines have been prepared and reported to retain biological activity (55). 

On the basis of differences in the product distributions in oxidations by the Fenton reagent, and by the Udenfriend reagent a different mechanism for the latter was suggested. In aprotic solvents, however, the Fenton reagent gives results similar to those in enzymatic hydroxylations. It was observed that the hydrogen displaced... 

Udenfriend reagent A reagent formed from a mixture of ferrous ions, oxygen, ascorbic acid, and ethylenetetraaminetetraacetic acid (EDTA) that is used to oxidise aromatic rings to phenol. 

An interesting historical quirk is the fact that the first oxygenase model system, the Udenfriend reagent (Figure 2), was reported in 1954 [10], one year prior to the discovery of the enzymes it emulates. Udenfriend and coworkers found that a mixture of Fe(II), EDTA, ascorbic acid and dioxygen is able to hydroxylate aromatic rings at neutral pH and under mild conditions. It was later found that the ascorbic acid can be replaced by a variety of hydrogen donors... 

It is notable that flavone and its derivatives are substituted predominantly in the phenyl group at positions 3 and 4 whereas reaction with Fenton s reagent gives mainly the S-hydroxy isomer." " Similarly, comparison of the distribution of products from coumarin obtained with Udenfriend s system with that given by Fenton s reagent re-... 

Udenfriend, S., Stein, S., Bohlen, P., Dairman, W., Leimgruber, W., and Weigele, M., Fluorescamine a reagent for assay of amino acids, peptides, proteins, and primary amines in the picomole range, Science, 178, 871, 1972. 

Hydrogen peroxide can be replaced by molecular oxygen in vanadium-catalyzed hydroxylations of arenes, as first reported by Mukaiyama (Eq. 3) [24]. Improved versions were later described by Battistel (VCl3/ascorbate) [25] and Shul pin ((n-Bu)4NV03/pyrazine-2-carboxylic acid/ascorbic acid) [26], It is noteworthy that a molecular oxygen/sacrificial reductant (crotonaldehyde, zinc, or ascorbate) combination was used (cf. Udenfriend s reagent above) and that the mechanism is most likely to involve hydroxyl radicals. 

Much of the work on model systems was stimulated by the observation of Udenfriend and co-workers in 19546S4a,b that a mixture of Fe(II), EDTA, ascorbic acid, and molecular oxygen could hydroxylate arenes to phenols under mild conditions. Udenfriend s reagent also hydroxylates alkanes to alcohols and epoxidizes olefins.670 6 74 The EDTA in Udenfriend s reagent probably reduces the redox potential of the Fe(II)/Fe(III) couple. The ascorbic acid functions as an electron donor, analogous to the cofactor in monooxygenases, and can be replaced by other enediols.672... 

However, since Udenfriend s reagent does not cause the NIH shift,676 hydrox-ylation by it does not proceed by the same mechanism as that involved in the enzymatic oxidation. A radical process seems to be more likely. Norman and Lindsay Smith70 have suggested a mechanism without directly implicating the hydroxyl radical ... 

S. Udenfriend, S. Stein, P. Bohlen, W. Dairman, W. Leimgruber, and M. Weigele, Science, 178 871-872 (1972). Fluorescamine A Reagent for Assay of Amino Acids, Peptides, Proteins and Primary- Amines in the PicoMole Range. 

There has been a recent trend towards the use of high pressure systems and reagents other than ninhydrin to increase sensitivity and speed (e.g. see Moore 1972). Although not yet in general use, such systems hold considerable promise for the future. Expanded scales and one-column systems of analysis have also been used to boost sensitivity. However, in most of these more sensitive systems, the baseline is usually not as straight and free of drift as in the two-column systems, and there are often more problems encountered with interfering substances. At present we would recommend the use of the common commercial analyzers without special modifications (except for the possible inclusion of expanded scale modes) for the greatest accuracy in routine work. However, for situations where samples are obtained in limited amounts or where many samples need to be analyzed quickly, some of the newer systems may well be worthy of consideration. The application of fluorescamine to the quantitative fluorimetric determination of picomole quantities of amino acids, peptides and proteins is of particular interest in this context (Udenfriend et al. 1972). 

Fluorescamine (Fluram). Fluorescamine was developed from the study of ninhydrin chemistry as a reagent for detecting amino groups (Udenfriend et al., 1972) and has been successfully used in the reversed phase chromatographic analysis of proteins and peptides using an on-line post-column detection system. This reagent requires two separate pumps for the delivery of the mobile phase buffer and the reagent and this method may therefore be considered to be less... 

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