Hole transfer in crystals of cytosine monohydrate: an EPR study

Abstract

Single crystals of cytosine monohydrate were used as a model for studying hole transfer in systems related to nucleic acids. Holes (and excess electrons) were induced by ionizing radiation at 15 K. Thiocytosine, embedded in the cytosine lattice in variable amounts, served as a hole trap. The holes which are stabilized at thiocytosine give rise to an EPR signal well separated from the signals of other radicals present. The ratio of the amount of thiocytosine cations (i.e. trapped holes) to the amount of the remaining radicals is at least 10 times larger than the stochiometric thiocytosine/cytosine ratio, which displays significant hole transfer from cytosine to thiocytosine and subsequent trapping by the latter. The analysis of the hole transfer range in the lattice of cytosine monohydrate, composed of parallel sheets of cytosine bases, is rather complex. Unlike DNA, where each base in a π-stack is flanked by one base on each side, each cytosine base in the present system is surrounded by four other bases in a π-stacked structure. This results in a higher probability of finding a thiocytosine base in a closer proximity to the initially formed hole. Consequently, it seems that the net hole transfer is not larger than about 4 inter-base distances (∼13 Å) in the present system.