Effect of phosphate group in switching off electron transfer in different media

We have studied the interactions of individual nucleoside 5′-monophosphates of all the five nucleic acid bases, adenosine 5′-monophosphate (AMP), guanosine 5′-monophosphate (GMP), thymidine 5′-monophosphate (TMP), cytidine 5′-monophosphate (CMP) and uridine 5′-monophosphate (UMP) with two quinones, 9,10-anthraquinone (AQ) and 2-methyl 1,4-naphthoquinone (MQ) commonly known as menadione using laser flash photolysis in two different types of media. Although electron transfer (ET) in excited state is a well-known phenomenon in DNA with quinones, this work reveals a total failure of ET with these isolated 5′-monophosphate molecules in a homogeneous medium (acetonitrile/water), except GMP. Similar experiments in SDS micelles have been repeated but here even GMP failed to transfer electrons. We have attributed the failure of ET in homogeneous medium to a semi-circular conformation adopted by the 5′-monophosphates, which is assumed to pull back the electrons from the electron donor center, the nitrogenous base. In SDS micelles a rapid spin exchange within an abnormally short inter-radical distance has been considered to prevent ET.