Base unstacking of DNA result in:

Hyperchromicity refers to a propey in which there is an increase in the optical absorbance of UV light by pyrimidine and purine bases due to unstacking of the DNA which occur secondary to denaturation of the native DNA. Normally due to resonance all of the bases in the nucleic acid absorb ultraviolet light. All nucleic acids are characterized by a maximal absorption of UV light at wavelengths near 260nm. EFFECTS OF DNA DENATURATION ARE: Increase in absorption of UV light Decrease in specific optical rotation Decrease in viscosity Ref: Principles of Biochemistry and Biophysics By Dr. B.S. Chauhan page 216.