USE OF PHOSPHOIMIDAZOLIDE-ACTIVATED GUANOSINE TO INVESTIGATE THE NUCLEOPHILICITY OF SPERMINE AND SPERMIDINE

Guanosine 5'-phosphate 2-methylimidazolide (2-MeImpG), a labile phosph oimidazolide analog of guanosine triphosphate, was used to test the re activity of the natural polyamines (PAs), spermine (spm) and spermidin e(spd); The products are the guanosine 5'-phosphate-polyamine derivati ves (PA-pG: spd-pG and spm-pG) which are quite stable in the range 4 < pH < 11. Our study is the first of which we are aware that reports on the nucleophilicity of these amines. The main findings are as follows . (i) HPLC analysis of the products indicates the formation of only tw o of the three possible spd products and only one of the two possible spm products. These results can be explained if only the primary amino groups of the two polyamines are reactive, while the secondary amino groups are rendered unreactive by a steric effect. The reactions of 2- MeImpG and other phosphoimidazolide derivatives of nucleosides (ImpNs) with primary and secondary monoamines support this interpretation (Ka navarioti et al. J. Org. Chem. 1995, 60, 632). (ii) The product ratio of the two spd-pG adducts derived from the primary amino groups varies between 2.40, and 0.71 in the range 6.1 less than or equal to pH less than or equal to 11.9. Such small variation in the product ratio can only be rationalized by the similar, but not identical, basicity of th e two primary amino groups and provides strong support for a previousl y reported model for polyamine ionization (Onasch et. al. Biophys. Che m. 1984, 19, 245). (iii) On the basis of our kinetic determinations co nditions at which the nucleophilicity of these amines is at a minimum and at which other interactions with ImpNs could be tested can be chos en.