B-TRANSITION TO Z-TRANSITION IN POLY(DG-DC) MODIFIED WITH BENZO(A)PYRENE DIOL EPOXIDES STUDIED WITH POLARIZED-LIGHT SPECTROSCOPY

The NaCl‐induced transition from B to Z conformation in poly (dG‐dC), covalently modified with benzo(a)pyrene diol epoxides (BPDE), has been studied by use of flow linear dichroism (LD), CD, and fluorescence spectroscopy. In contrast to unmodified poly (dG‐dC), the BPDE‐modified polynucleotide displays a B to Z conversion that is incomplete in 2.7 M NaCl. This and a number of other observations are found consistent with a locally retained BPDE perturbed B‐like conformation in the vicinity of the BPDE adduct. In 2.7 M NaCl the polynucleotide orientation in flow is strongly reduced, which could indicate flexibility at the junctions between domains of BPDE perturbed B‐like conformation and Z conformation. A virtually stable high‐salt B form is found in 2.25 M NaCl at 0°C; its CD spectrum resembles that of the transient so‐called B* form, which is observed at an early stage of the B to Z transition. Characteristic B to Z transition times were evaluated by taking this B*‐form CD spectrum as the initial state spectrum. The kinetics of the B to Z transition, monitored by the CD signal at 287 nm, shows an increased transition rate with (+)‐anti‐BPDE modification whereas the (−)‐anti‐ and (±)‐syn‐BPDE adducts have retarding effects. The difference is discussed against the background of the different binding geometries of the BPDE isomer adducts. Copyright © 1990 John Wiley & Sons, Inc.