Quantitative phosphorescence study of interactions of cytosine and cytidine and its nucleotides in frozen aqueous solution: evidence for anomalous heavy-atom effect.
Phosphorescence vs. pH titration curves of cytosine, cytidine, cytidine-5'-mono-phosphate (CMP), -diphosphate (CDP) and -triphosphate (CTP) were obtained in methanol/water 10/90 v/v and in various aqueous sodium halide solutions frozen at 77 degrees K. As shown by the shape of the titration curve, molecular aggregates or "puddles" of cytosine and cytidine were shown to occur only in relatively concentrated frozen solution (10(-3)M), these aggregates being dissociated in dilute frozen solutions (<or=10(-4)M) or in approximately 1M sodium chloride. No molecular aggregation could be found even in concentrated solutions of CMP, CDP, and CTP. Lower concentrations of sodium bromide or iodide (10(-1)M) were demonstrated to give "reversed" sigmoidal phosphorescence titration curves in the case of cytidine, due to an anomalously large heavy-atom enhancement factor in acidic solution, ranging between 30 and 50.
Aaron
J J
JJ
Department of Chemistry, University of Florida, Gainesville, Florida 32601, U.S.A.
Spann
W J
WJ
Winefordner
J D
JD