Cyclobutane pyrimidine dimer (CPD) photolyases are structure specific DNA-repair enzymes that specialize in the repair of CPDs, the major photoproducts that are formed upon irradiation of DNA with ultraviolet light. The purified enzyme binds a flavin adenine dinucleotide (FAD), which is in the neutral radical semiquinone (FADH(center dot)) form. The CPDs are repaired by a light-driven, electron transfer from the anionic hydroquinone (FADH(-)) singlet excited state to the CPD, which is followed by reductive cleavage of the cyclobutane ring and subsequent monomerization of the pyrimidine bases. CPDs formed between two adjacent thymidine bases (T < > T) are repaired with greater efficiency than those formed between two adjacent cytidine bases (C< >C). In this paper, we investigate the changes in Escherichia coli photolyase that are induced upon binding to DNA containing C < > C lesions using resonance Raman, UV-vis absorption, and transient absorption spectroscopies, spectroelectrochemistry, and computational chemistry. The binding of photolyase to a C < > C lesion modifies the energy levels of FADH(center dot), the rate of charge recombination between FADH(-) and Trp(306)(center dot), and protein-FADH(center dot) interactions differently than binding to a T< >T lesion. However, the reduction potential of the FADH(-)/FADH(center dot) couple is modified in the same way with both substrates. Our calculations show that the permanent electric dipole moment of C < > C is stronger (12.1 D) and oriented differently than that of T-T (8.7 D). The possible role of the electric dipole moment of the CPD in modifying the physicochemical properties of photolyase as well as in affecting CPD repair will be discussed.