TAUTOMERISM, ACID-BASE PROPERTIES AND CONFORMATION OF METHYLATED ANALOGS OF THE PROMUTAGENIC N-4-HYDROXYCYTOSINE

UV and NMR spectroscopy were employed to study the tautomerism, acid-b ase properties and conformation of the exocyclic N-4-OH group in 1-met hyl-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 t o 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 analog ues formed a structurally common cation in acid medium, with results p ointing to similar to 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 exhi bited two sequential dissociations in alkaline medium, the first due t o N-4-OH, followed by the N(3)-H. H-1 and C-13 NMR spectroscopy showed 1-mOH(4)C in the conformation syn. With 1,3,5-m(3)OH(4)C, an 'overcro wded' planar molecule with steric constraints to both the syn and anti conformations, a syn-anti equilibrium is observed, with a preference of similar to 75% for the anti rotamer, independently of the polarity of the medium. Exchange between the rotamers is slow on the NMR time-s cale, with a minimal barrier to exchange exceeding 100 kJ/mol. In low- polar media, the analogues associate as dimers via O-4-H ... O-2 or O- 4-H ... N-4 hydrogen bonds, with association constants at ambient temp erature 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 over all findings to the promutagenic activities of (OHC)-C-4 and (OMeC)-C- 4 are examined. (C) 1998 Elsevier Science B.V. All rights reserved.