9- guanine crystal structure

In the SBDD project that produced (86), the design work was initiated through use of the X-ray structure of the PNP apoenzyme, but was more successful when the structures of the PNP-guanine complex and other complexes were available (199). The PNP-guanine crystal structure showed no important interactions with N9, and indicated a potential for hydrophobic interaction in the vicinity of the substrate ribose (Fig. 10.19). To test this, the 9-deaza compound (87) was synthe-... 

The X-ray crystal structure of platinated duplex DNA dodecamer d(CCTCTG G TCTCC) d(GGAGACCAGAGG), where G G represents the binding sites (G-N7) of m-[Pt(NH3)2]2+, is depicted in Figure 12 [67]. The coordination of Pt(II) to adjacent guanine bases distorts the... 

The carbacyclic analog of acyclovir is 9-(4-hydroxybutyl)guanine (HBG), whose crystal structure was determined (87MI7). The side chain is fully extended and almost perpendicular to the guanine base. 

An interesting feature was discovered by Sharma and co-workers494,495 in the crystal structure of isocytosine. Two tautomers of isocytosine (42 and 43) exist in an exact 1 1 ratio in the crystal. The tautomers are hydrogen-bonded to each other in a manner analogous to that proposed by Watson and Crick496,497 for the guanine-cytosine pair in DNA. It is worth underlining that the base pair of isocytosine was not obtained by expedient cocrystallization of the two components. It seems therefore probable that both forms 42 and 43 of isocytosine are of approximately equal stability and are present in comparable amounts in solution. 

In the case of guanine derivatives (80), coordination to the 0-6 atom may also be possible. In fact this has only rarely been observed.146,147,154 Coordination at N-3 is sterically hindered, whereas binding at the NH2 group is rare and binding at N-l can only occur after deprotonation in an alkaline medium.146,147,155 Coordination of guanines and hypoxanthines (80, but with the NH2 replaced by hydrogen) takes place almost universally at the N-7 position, as proved by numerous crystal structures, unless N-7 is blocked.155... 

Established cases of metal binding to the exocyclic amino groups of guanine bases are rare. Amino group mercuration has been reported for 1-methylguanosine (169e), and the X-ray crystal structure of a dianionic 9-methylguanine complex with Pt(II) at N7, and Hg(II) at both N1 and N2 has been determined (237). The compound has been depicted in Fig. 19. The pK values have not been determined. 

The first experimental identification of the formation of guanine tetrad can be dated back to 1962 [1], Since 1992, an increasing number of the X-ray crystal structure determinations of guanine tetrads has been reported [2-13]. The advances... 

G. Laughlan et al., The high-resolution crystal structure of a parallel-stranded guanine tetraplex. Science 265, 520-524 (1994)... 

The problem may and has been considered also in relation to the crystal structure of other purines, although in somewhat less detail.173 Recent findings indicate that although the crystals of guanine, hypoxanthine, and 8-azaguanine contain the N(9)H tautomer of the bases,174 175 the crystal of 6-mercaptopurine monohydrate is made of the N(7)H form.176 177... 

We chose H-RAS as the first target protein for several reasons. RAS proteins are, perhaps, the most intensively studied and best understood signal transduction proteins from the perspective of structure and function [8], The H-RAS crystal structure has been determined to the level of 1.5 A, and the domains involved in guanine nucleotide binding and hydrolysis, effector interactions, interactions with regulating proteins, and processing have been extensively characterized. RAS proteins are initially synthesized in the cytoplasm where they undergo a series of post-translational modifications at their carboxy-terminal sequence, the CaaX-box (where C is cysteine, a an aliphatic amino acid, and X either serine or... 

Fig. 1. Two ribbon representations of the crystal structure of the DNA decamer d(CCTCG -CTCTC/GAGAG CGAGG) containing a unique cisplatin interstrand cross-link at d(GpC)-d(GpC) site (asterisks indicate the chelated bases in the adduct). A front view (A) allows to see the structure with the lesion in the minor groove. A side view (B) shows the chicane of the backbone with the helix-sense reversal. Ptn atom, yellow ammine groups, navy blue sugars, pink guanines, navy blue adenines, red thymines, yellow cytosines, hght blue phosphodiester backbone, green.

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