RAMAN SPECTRAL STUDIES OF NUCLEIC-ACIDS .44. RAMAN-SPECTROSCOPY OF DNA-METAL COMPLEXES .1. INTERACTIONS AND CONFORMATIONAL EFFECTS OF THE DIVALENT-CATIONS - MG, CA, SR, BA, MN, CO, NI, CU, PD, AND CD

Interactions of divalent metal cations (Mg2+, Ca2+, Ba2+, Sr2+, Mn2+, Co2+, Ni2+, Cu2+, Pd2+ and Cd2+) with DNA have been investigated by la ser Raman spectroscopy. Both genomic calf-thymus DNA (>23 kilobase pai rs) and mono-nucleosomal fragments (160 base pairs) were employed as t argets of metal interaction in solutions containing 5 weight-% DNA and metal:phosphate molar ratios of 0.6:1. Raman difference spectra revea l that transition metal cations (Mn2+, Co2+, Ni2+, Cu2+, Pd2+, and Cd2 +) induce the greatest structural changes in B-DNA. The Raman (vibrati onal) band differences are extensive and indicate partial disordering of the B-form backbone, reduction in base stacking, reduction in base pairing, and specific metal interaction with acceptor sites on the pur ine (N7) and pyrimidine (N3) rings. Many of the observed spectral chan ges parallel those accompanying thermal denaturation of B-DNA and sugg est that the metals link, the bases of denatured DNA. While exocyclic carbonyls of dT, dG, and dC may stabilize metal ligation, correlation plots show that perturbations of the carbonyls are mainly a consequenc e of metal-induced denaturation of the double helix. Transition metal interactions with the DNA phosphates are weak in comparison to interac tions with the bases, except in the case of Cu2+, which strongly pertu rbs both base and phosphate group vibrations. On the other hand, the R aman signature of B-DNA is largely unperturbed by Mg2+, Ca2+, Sr2+, an d Ba2+, suggesting much weaker interactions of the alkaline earth meta ls with both base and phosphate sites. A notable exception is a modera te perturbation by alkaline earths of purine N7 sites in 160-base pair DNA, with Ca2+ causing the greatest effect. Correlation plots demonst rate a strong interrelationship between perturbations of Raman bands a ssigned to ring vibrations of the bases and those of bands assigned to exocyclic carbonyls and backbone phosphodiester groups. However, stro ng correlations do not occur between the Raman phosphodioxy band (cent ered near 1092 cm-1) and other Raman bands, suggesting that the former is not highly sensitive to the structural changes induced by divalent metal cations. The structural perturbations induced by divalent catio ns are much greater for >23-kilobase pair DNA than for 160-base pair D NA, as evidenced by both the Raman difference spectra and the tendency toward the formation of insoluble aggregates. In the presence of tran sition metals, aggregation of high-molecular-weight DNA is evident at temperatures as low as 11-degrees-C. A relationship between DNA meltin g and aggregation is proposed in which initial metal binding at major groove sites locally destabilizes the B-DNA double helix, causing disp lacement of the bases away from one another and exposing additional me tal binding sites. Metal cation linkage of two displaced bases would a llow separate DNA strands to crosslink. Aggregation is proposed to res ult from the formation of an extended net work of these crosslinks.