Add time:07/18/2019         Source:sciencedirect.com

Metal complexes of the following uracil series of pyrimidine nitrogen bases were studied to assist in elucidating the effect of metal ions on the double-stranded helical structure of nucleic acids: 5-bromo-uracil-6-carboxylic acid (5-bromoörotic acid), 5-nitroörotic acid, 5-aminoörotic acid and 5-iodoörotic acid. Formation constants, calculated for divalent metal-ion complexes, indicated that stable complexes were formed with these pyrimidine acids and that the order of complex stability was found to be, Cu(II) > Ni(II) > Co(II) > Zn(II) > Cd(II) > Mn(II). It was also observed that these metal ions were capable of removing the proton on the N1 pyrimidine ring-nitrogen, which is believed to be partly responsible for hydrogen bonding between the two helical ribose phosphate chains. The trend in complex stability for the ligands was found to be similar to that observed for the basicity of the N1 pyrimidine ring-nitrogen, that is, 5-aminoörotic acid > 5-iodoörotic acid > 5-bromoörotic acid > 5-nitroörotic acid. The possible formation of protonic complexes was either negligible or completely absent in the pH region where one would normally expect to find these complexes. Only mononuclear complexation was observed.An equation was derived to determine formation constants for 1:1 complexes of these pyrimidine acids from their ultra-violet light absorption spectra. These spectra, which were determined at various pH values in the absence or presence of metal ions, are briefly discussed.

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