hexamminecobalt(iii)-induced condensation of calf thymus DNA: circular dichroism and hydration measurements

B.I. Kankiar, V. Buckin and V. A. Bloomfield

Physicochemical and Engineering Aspects, 183-185, pp. 517-526 (2001)

The interaction of hexamminecobalt(lII), Co(NH3)6 3+, with 160 and 3000—8000 bp length calf thymus DNA has been investigated by circular dichroism, acoustic and densimetric techniques. The acoustic titration curves of 160 bp DNA revealed three stages of interaction: (i) Co(NH3)6 3+ binding up to the molar ratio [Co(NH3)6 3+]/[P] = 0.25, prior to DNA condensation; (ii) a condensation process between [Co(NH3)6 3+]/[P] = 0.25 and 0.30; and (iii) precipitation after [Co(NH3)6 3+]/[P] = 0.3. In the case of 3000— 8000 bp DNA only two processes were observed: (i) binding up to [Co(NH33)6 3+]/[P] = 0.3; and (ii) precipitation after this point. In agreement with earlier observations, long DNA aggregates without changes in its B-form circular dichroism spectrum, while short DNA demonstrates a positive B→φtransition after [Co(NH3)6 3+]/[P] = 0.25. From ultrasonic and densimetric measurements the effect of Co(NH3)6 3+ binding volume and compressibility have been obtained. The binding of Co(NH3)6 3+ to both short and long DNA is characterized by similar changes in volume and compressibility calculated per mole Co(NH3)6 3+: ΔV = 9 cm3 mol-1 and ΔK= 33 x 10-4 cm3 mol-1 bar-1. The positive sign of the parameters indicates dehydration, ie water release from Co(NH3)6 3+ and the atomic groups of DNA. This extent of water displacement would be consistent with the formation of two direct, hydrogen bonded contacts between the cation and the phosphates of DNA. DNA in viruses and cells exists  in  a  highly  condensed, tightly packed state.  For instance, the  concentration of DNA in  the  head  of bacteriphages  is  ~800 mg/ml and in metaphase chromosomes ~170 mg/ml (1).  In contrast, DNA concentration in spectroscopic studies is usually ~10-2  mg/ml.  Knowledge of the condensation process  is  essential for understanding the molecular  mechanisms  of  biological  processes such  as  DNA transcription and replication.  Studies  on  DNA condensation have received additional  impetus  in  recent  years  from  an interest  in  gene  therapy,  which  is  based  on delivery of foreign DNA molecules into  cells  (2 and references therein).

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