The density functional theory (DFT) calculations (B3LYP) with aug-cc-pVDZ basis sets on various substituted neutral and radical cationic guanine-cytosine (GC*+) base pairs suggest that an electron-withdrawing ring deactivating substituent group like -NO2, -COOH attached to a G ring lowers the reorganization/activation energy values whereas an electron releasing ring activating group like -OH increases these values. An electron releasing, but a ring deactivating group -F, exhibits its effect in between the above two extreme cases. A weak electron releasing, ring activating methyl group, as well as substituent groups attached to C(cytosine)-ring show almost no effect. (C) 2009 Elsevier B. V. All rights reserved.