Cytosine: N-formylglycine complex crystallizes in the space group Pna21 with cell dimensions of a=8.444(2), b=21.169(5), and c=5.120(1) Å. The structure was solved by the trial-and-error method on the basis of a Patterson map. A full-matrix least-squares refinement led to an R value of 0.035. The difference map revealed the protonation of cytosine by the carboxyl group of N-acylamino acid and the involvement of ionic interaction in complex formation. This mode is the same as proposed for cytosine: N-benzoylglycine complex in spite of different molecular environment. Such arrangement of cytosine and ionized carboxyl group of amino acid may suggest a mutual recognition scheme between nucleic acid and protein.
Crystals of cytosine: N,N-phthaloyl-Dl-glutamic acid complex dihydrate were obtained from an aqueous solution of the two components. The space group is P21/c with unit-cell dimensions of a=5.243(1), b=39.133(6), c=9.192(1) Å, β=93.66(1)°, and Z=4. The structure was solved by the direct method and its parameters were refined by a block-diagonal least-squares method. The complex is formed by the two hydrogen bonds between the oxygen atoms of the ionized α-carboxyl group of glutamic acid and the nitrogen atoms, N(4) and protonated N(3), of cytosine. This binding mode is the same as that found in cytidine: N-benzyloxycarbonyl-l-glutamic acid complex, but quite different from that of 5-bromocytosine: N-acylglutamic acid complexes. The substituted bromine atom at C(5) of cytosine is thought to prescribe the binding mode in complex formation. The binding mode between cytosine and glutamic acid, which might be a possible elementary pattern of protein: nucleic acid interactions, is discussed in terms of dissociation constant.
The complex crystals of 5-bromocytosine and N-tosyl-l-glutamic acid were obtained from their aqueous solution. The space group is P1 with dimensions of a=10.843(1), b=11.690(2), c=10.879(2) Å, α=122.15(1), β=121.38(1), γ=69.93(1)°, and Z=2. The structure was solved by the heavy atom method and refined by a block-diagonal least-squares method. The undissociated γ-carboxyl group of glutamic acid is bound with O(2) and N(4) of 5-bromocytosine through hydrogen bonds. This interaction mode is different from those found in some cytosine : glycine complexes and cytidine : N-benzyloxycarbonyl-l-glutamic acid complex dihydrate in which the dissociated α-carboxyl group plays a role. This is the first example of interaction between cytosine and the acidic side group of amino acid.
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