Abstract
Hydrogen-bonded complexes of uric acid/9-methyluric acid (UA/9MUA) with melamine (MEL) are prepared by the combined technique of laser desorption and supersonic-jet expansion, and their stable structures are investigated by infrared spectroscopy and theoretical calculations. It is shown that the 1:1 complex formed between UA and MEL is of nonplanar type, in which the two chromophore planes are significantly folded and thus allow for triple hydrogen-bonding interactions. An anomalously broad IR band is observed in the low-frequency range 2500-2800 cm(-1), which is taken as evidence for the formation of a strong hydrogen bond between one of the NH sites of UA and MEL. In the case of 9MUA, in which hydrogen bonding to the N9H site of UA is blocked by the methyl group, two planar pairs formed of 9MUA and MEL are found to coexist. The nature of the multiple hydrogen-bonding interactions in these complexes is discussed based on the natural bond orbital analysis and compared with those of the guanine-cytosine base pair. The results are expected to provide important information on the structural characterization of urinary stones developed in infants after ingesting MEL-contaminated formula.
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