Use of a fluorescence microplate reader for the detection and characterization of metal-assisted peptide hydrolysis

Improved speciation of dissolved organic nitrogen in natural waters: amide hydrolysis with fluorescence derivatization

The objective of this study was to improve primary-amine nitrogen (1 degree-N) quantification in dissolved organic matter (DOM) originating from natural waters where inorganic forms of N, which may cause analytical interference, are commonly encountered. Efforts were targeted at elucidating organic-N structural criteria influencing the response of organic amines to known colorimetric and fluorescent reagents and exploring the use of divalent metal-assisted amide hydrolysis in combination with fluorescence analyses. We found that reaction of o-phthaldialdehyde (OPA) with primary amines is significantly influenced by steric factors, whereas fluorescamine (FLU) lacks sensitivity to steric factors and allows for the detection of a larger suite of organic amines, including di- and tri-peptides and sterically hindered 1degree-N. Due to the near quantitative recovery of dissolved peptides with the FLU reagent and lack of analytical response to inorganic nitrogen, we proposed that FLU be utilized for the quantification of primary amine nitrogen. In exploring the application of divalent metal promoted peptide hydrolysis to the analysis of organic forms of nitrogen in DOM, we found that Zn(II) reaction increased the total fraction of organic-N detectable by both OPA and FLU reagents. Zn-hydrolysis improved recovery of organic-N in natural waters from < 5% to 35%. The above method, coupled with standard inorganic-N analyses, allows for enhanced resolution of dissolved organic nitrogen (DON) speciation in natural waters.

Kinetics of Amide and Peptide Cleavage by Alkaline Hydrogen Peroxide

The hydroperoxide anion cleaves unactivated amides and peptides although it is completely unreactive toward ethyl esters. The cleavage by HO2(-) proceeds faster than by OH(-) and involves additional routes with general acid assistance by H2O2 and general base assistance by OH(-) and HO2(-). Cleavage of polypeptides occurs at the N-terminal peptide bond. [reaction: see text]

A comparison of fluorescamine and naphthalene-2,3-dicarboxaldehyde fluorogenic reagents for microplate-based detection of amino acids

The use of appropriate fluorometric derivatization procedures is of considerable importance for accurate determination of amino acids in biological samples and in metal-assisted peptide hydrolysis reactions. It is especially critical for the relative fluorescence intensities (RFI) of equal amounts of amino acids to be as similar as possible. While fluorescamine and naphthalene-2,3-dicarboxaldehyde (NDA) have proven to be excellent fluorogenic reagents for amino acid detection, the effects of various factors such as organic solvent, buffer, and pH have never been rigorously evaluated with respect to normalizing the relative fluorescence intensities of individual amino acids. To this end, here we describe optimized fluorescamine and NDA derivatization reactions that enhance the accuracy of microplate-based detection of amino acids. For both fluorescamine and NDA, we have shown that the RFI values of 16 of 19 amino acids are greater than 70%. Although determination of tryptophan is problematic, this difficulty is overcome by the addition of beta-cyclodextrin to the NDA reaction. In principle, the optimized fluorescamine and NDA microplate procedures reported here can be utilized as complementary techniques for the detection of 19 of 20 naturally occurring amino acids.