Optical studies on complexes between DNA and pseudoisocyanine

Journal article, 1976

Linear dichroism (LD) results when pseudoisocyanine = PIC (1, 1-diethyl-2,2'-cyanine iodide) binds to flow-oriented DNA. LD may be used to follow the complexation both stoichiometrically and structurally, since when specified to unit complex concentration LD provides a measure of the average orientation of the absorbing transition dipole. Two different types of complexes can be distinguished: I. One strong, ionic-strength insensitive complex with monomeric PIC with an orientation indicating intercalation. II. Several weaker complexes of electrostatic nature (only observed at I < 0.2 M NaCl) among which those with dimeric (IIa) and with polymeric (IIb) PIC are concluded both from LD and from circular dichroism (CD). The dimer is probably formed by employing one intercalated PIC as a second site. The polymer complex is characterized by a very sharp absorption band at 553 nm polarized parallel to the DNA-axis (with positive LD and positive CD). Its structure, a right-handed helical array of PIC molecules, is discussed in terms of exciton theory in relation to that of polymeric free PIC (“Scheibe polymer”) which is also shown to interact with DNA (IIc) yielding a large aggregate which is degraded at a distinct flow force field. For I a stability constant K1 = (9 ± 3) × 105 M−1 and n1 = 0.26 t 0-03 Sites per nuclcotide residue (formally one site at every second base pair) was obtained; for IIb KII=(2 ± 1) × 1 0−1 M−1, nII = 0.1 ± 0.5.