Tautomerism, acid-base properties and conformation of methylated analogues of the promutagenic N4-hydroxycytosine.
UV and NMR spectroscopy were employed to study the tautomerism, acid-base properties and conformation of the exocyclic N(4)-OH group in 1-methyl-N(4)-hydroxycytosine (1-mOH(4)C), and its methyl derivatives, viz. the fixed imino forms (1,3-m(2)OH(4)C and 1,3,5-m(3)OH(4)C), the fixed amino form (1,N(4)-m(2)OH(4)C), and analogues sterically constrained to the form syn (1,5-m(2)OH(4)C) or anti (1,3-m(2)OH(4)C) with respect to the ring N(3). Relative to 1,N(4)-m(2)OH(4)C, UV spectroscopy showed that the other analogues were predominantly imino and that all analogues formed a structurally common cation in acid medium, with results pointing to approximately 90% population of the imino species for 1-mOH(4)C and 1,5-m(2)OH(4)C, further supported by NMR spectroscopy. Both exhibited two sequential dissociations in alkaline medium, the first due to N(4)-OH, followed by the N(3)-H. (1)H and (13)C NMR spectroscopy showed 1-mOH(4)C in the conformation syn. With 1,3,5-m(3)OH(4)C, an ;overcrowded' planar molecule with steric constraints to both the syn and anti conformations, a syn-anti equilibrium is observed, with a preference of approximately 75% for the anti rotamer, independently of the polarity of the medium. Exchange between the rotamers is slow on the NMR time-scale, with a minimal barrier to exchange exceeding 100 kJ/mol. In low-polar media, the analogues associate as dimers via O(4)-Hcdots, three dots, centeredO(2) or O(4)-Hcdots, three dots, centeredN(4) hydrogen bonds, with association constants at ambient temperature of 4.6 (1,3-m(2)OH(4)C), 12.8 (anti 1,3,5-m(3)OH(4)C), 36 (1,5-m(2)OH(4)C), 109 (syn 1,3,5-m(3)OH(4)C) M(-1). Implications of the overall findings to the promutagenic activities of OH(4)C and OMe(4)C are examined.
Niedźwiecka-Kornaś
A
A
Department of Biophysics, Institute of Experimental Physics, University of Warsaw, Poland.
Kierdaszuk
B
B
Stolarski
R
R
Shugar
D
D