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The hydrogen bonding of cytosine with guanine: Calorimetric and 1 H‐NMR analysis of the molecular interactions of nucleic acid bases
Author(s) -
Williams Loren Dean,
Chawla B.,
Shaw Barbara Ramsay
Publication year - 1987
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360260411
Subject(s) - chemistry , hydrogen bond , chloroform , solvent , dichlorobenzene , enthalpy , cytosine , guanine , crystallography , solvent effects , molecule , organic chemistry , nucleotide , dna , biochemistry , physics , quantum mechanics , gene
Abstract The enthalpy of hydrogen‐bond formation between guanine (G) and cytosine (C) in o ‐dichlorobenzene and in chloroform at 25°C has been determined by direct calorimetric measurement. We derivatized 2′‐deoxyguanosine and 2′‐deoxycytidine at the 5′‐ and 3′‐hydroxyls with triisopropylsilyl groups; these groups increase the solubility of the nucleic acid bases in nonaqueous solvents. Such derivatization also prevents the ribose hydroxyls from forming hydrogen bonds. Consequently, hydrogen‐bond formation in our system is primarily between the bases, and to a lesser extent, between base and solvent, and can be measured directly with calorimetry. To obtain the data on base‐pair formation, we first took into account the contributions from self‐association of each base, and where possible, have determined the Δ H of self‐association. From isoperibolic titration calorimetry, our measured Δ H of C 2 formation in chloroform is −1.7 kcal/mol of C. Our measured Δ H of C:G base‐pair formation in o ‐dichlorobenzene is −6.65 ± 0.32 kcal/mol. Since o ‐dichlorobenzene does not form hydrogen bonds, the Δ H of C:G base‐pair formation in this solvent represents the Δ H of the hydrogen‐bonding interaction of C with G in a nonassociating solvent. In contrast, our measured Δ H of C:G base‐pair formation in chloroform is −5.77 ± 0.20 kcal/mol; thus, the absolute value of the enthalpy of hydrogen bonding in the C:G base pair is greater in o ‐dichlorobenzene than in chloroform. Since chloroform is a solvent known to form hydrogen bonds, the decrease in enthalpic contribution to C:G base pairing in chloroform is due to the formation of hydrogen bonds between the bases and the solvent. The Δ H of hydrogen bonding of G with C reported here differs from previous indirect estimates: Our measurements indicate the Δ H is 50% less in magnitude than the Δ H based on spectroscopic measurements of the extent of interaction. We have also observed that the enthalpy of hydrogen bonding of C with G in chloroform is greater when G is in excess than when C is in excess. This increased heat is due to the formation of C:G n > 1 complexes that we have observed using 1 H‐nmr. Although C:G 2 structures have previously been observed in triple‐stranded polymeric nucleic acids, higher order structures have not been observed between C and G monomers in nonaqueous solvents until now. By using monomers as a model system to investigate hydrogen‐bonding interactions in DNA and RNA, we have obtained the following results: A direct measurement of the Δ H of hydrogen bonding in the C:G complex in two nonaqueous solvents, and the first observation of C:G n > 1 complexes between monomers. These results reinforce the importance of hydrogen bonding in the stabilization of various nucleic acid secondary and tertiary structures.