Analysis of DNA−Protamine Interactions by Optical Detection of Magnetic Resonance

Optically detected magnetic resonance (ODMR) has been used to identify the binding site of a synthetic protamine subdomain to the major groove of DNA. A 14 amino acid peptide (R6WGR6) analog of the central DNA binding domain of bull protamine was synthesized with phenylalanine replaced by tryptophan (Trp). The peptide was bound to double-stranded poly(dABrdU) and to calf thymus DNA (CT DNA) and the complexes characterized as “wet” solids using ODMR techniques. The appearance of the D + E transition in the slow passage ODMR and of short-lived components in the phosphorescence decay of the complex of R6WGR6 with poly(dABrdU) is diagnostic of a heavy atom effect. This can only occur if the peptide binds in the major groove of poly(dABrdU). The microenvironment of Trp in the nucleoprotein complex was characterized by phosphorescence, radiative decay lifetimes, and low-temperature ODMR measurements before and after binding to DNA. Bathochromic shifts in the phosphorescence emission upon exciting to the red in CT DNA−peptide suggest that the Trp is in a polar environment, while the red-shifted position of the 0,0-band emission points to a more polarizable environment. The heavy atom effect strongly suggests a Trp location within the major groove of DNA. A partial stacking of Trp with the polarizable nucleobases and simultaneous interactions with the phosphate-guanidinium ion pairs and/or water molecules in the major groove of DNA which might not be totally displaced upon binding of the peptide could explain this conflicting evidence. Extrapolation of results from the system studied to protamine binding in sperm chromatin strongly suggests that the predominant binding site of protamine is the major groove of DNA.