Effects of deficient of the Hoogsteen base-pairs on the G-quadruplex stabilization and binding mode of a cationic porphyrin

BACKGROUND: In stabilization of the G-quadruplex, formation of a Hoogsteen base-pair between the guanine (G) bases is essential. However, the contribution of each Hoogsteen base-pair at different positions to whole stability of the G-quadruplex has not been known. In this study, the effect of a deficiency of the Hoogsteen type hydrogen bond in the G-quadruplex stability was investigated. Spectral properties of meso-tetrakis(1-methylpyridinium-4-yl)porphyrin (TMPyP) associated with various G-quadruplexes were also examined. METHODS: The thermal stability of the thrombin-binding DNA aptamer 5′G(1)G(2)TTG(5)G(6)TG(8)TG(10)G(11)TTG(14)G(15) G-quadruplex, in which the guanine (G) base at 1, 2, 5, 6 and 8th positions was replaced with an inosine (I) base, one at a time, was investigated by circular dichroism (CD). The absorption, CD and fluorescence decay curve for the G-quadruplex associated TMPyP were also measured. RESULTS: The transition from the G-quadruplex to a single stranded form was endothermic and induced by an increase in entropy. The order in stability was 0>8>6>2>5>1, where the numbers denote the position of the replacement and 0 represents no replacements of the G base, suggesting the significant contribution of the G(1) base in the stability of the G-quadruplex. Alteration in the spectral property of TMPyP briefly followed the order in thermal stability. CONCLUSIONS: Replacement of a G base with an I base resulted in destabilization of the G-quadruplex. The missing hydrogen bond at position 1 destabilized the G-quadruplex most efficiently. TMPyP binds near the I base-replaced location namely, the side of the G-quadruplex. GENERAL SIGNIFICANCE: The Hoogsteen base-pairing is confirmed to be essential in stabilization of G-quadruplex. When G is replaced with I, the latter base is mobile to interact with cationic porphyrin.