Conditions for the stability of the B, C, and Z structural forms of poly(dG-dC) in the presence of lithium, potassium, magnesium, calcium, and zinc cations

• Published in: Biochemistry (Easton) 1993-04, Vol.32 (15), p.4077-4082
• Main Authors: Loprete, D. M, Hartman, K. A
• Format: Article
• Language: English
• Subjects: Analytical, structural and metabolic biochemistry; Biological and medical sciences; Calcium; Cations; Drug Stability; Fundamental and applied biological sciences. Psychology; Lithium; Magnesium; Nucleic acids; Nucleic bases, nucleotides; Polydeoxyribonucleotides - chemistry; Sodium; Spectrophotometry, Infrared; Zinc
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi00066a032

The occurrence of alternative structures for the lithium, sodium, and potassium salts of poly(dG-dC) was determined as a function of hydration using IR spectra of nonoriented gels. Poly(dG-dC).K with added KCl (r = 0.56 where r is the moles of KCl per mole of nucleotide residue) gave results essentially identical to the much studied poly(dG-dC).Na with added NaCl (r = 0.56). Both gave a sharp transition from a unique B structure (hereafter designated B*) to the Z structure upon dehydration. Poly(dG-dC).Li with added LiCl (r = 0.36) assumed the B* structure at high hydration but made a broad transition to the C structures as hydration was lowered. We believe this is the first clear evidence of the C structure for poly(dG-dC). No other structures (A, D, or Z) were observed at any hydration in nonoriented gels. Poly(dG-dC).Na with added ZnCl2 (r = 0.2) existed as a mixture of the B* and Z structures in maximally hydrated gels. A broad, incomplete transition to a higher mole fraction of Z structure occurred upon dehydration. Zn2+ promotes the Z structure for poly(dG-dC) and appears to bind to guanine residues. Poly(dG-dC).Na with added MgCl2 or CaCl2 (r = 0.2) assumed the normal B* structure at maximum hydration with no hint of Z structure. Slight dehydration produced a very sharp transition to the Z structure. Both Mg2+ and Ca2+ are strong promoters of the Z structure but do not bind to cytosine or guanine residues.