X-ray structural determination

The fact that only 11 structures were reported in (72PMH(5)l) whereas in Table 5, which is non-exhaustive, 36 more have been added shows the great development of X-ray structural determinations in recent times. Some of these structures deserve brief comment. 

Long loop regions are often flexible and can frequently adopt several different conformations, making them "invisible" in x-ray structure determinations and undetermined in NMR studies. Such loops are frequently involved in the function of the protein and can switch from an "open" conformation, which allows access to the active site, to a "closed" conformation, which shields reactive groups in the active site from water. 

Figure 10.2 (a) Amino acid sequence of a fragment of the Zif 268 protein that contains three zinc fingers. Residues forming the p strands and a helices are red and green, respectively, and those involved in the turn between the last p strand and the a helix are blue, (b) The nucleotide sequence of the DNA fragment that was used in the x-ray structure determination of the Zif 268 fragment complexed with DNA. 

The tetramethylammonium salt [Me4N][NSO] is obtained by cation exchange between M[NSO] (M = Rb, Cs) and tetramethylammonium chloride in liquid ammonia. An X-ray structural determination reveals approximately equal bond lengths of 1.43 and 1.44 A for the S-N and S-O bonds, respectively, and a bond angle characteristic bands in the IR spectrum at ca. 1270-1280, 985-1000 and 505-530 cm , corresponding to o(S-N), o(S-O) and (5(NSO), respectively. Ab initio molecular orbital calculations, including a correlation energy correction, indicate that the [NSO] anion is more stable than the isomer [SNO] by at least 9.1 kcal mol . ... 

Fig. 8.11 and was, in fact, the first definitive X-ray structural determination of a fullerene derivative. 

The tautomeric equilibrium between the hydroxy, 154a, and oxo, 154b, forms of l,2-benzisothiazolin-3-one is shifted to the right (Scheme 57) [76AHC(S1), p. 312 81G71 84CHEC-I(6)131] a conclusion supported by X-ray structural determinations of some derivatives of 154 e.g., R = Cl [69AX(B)2349]. 

Evidence in favor of the amino-thionic tautomeric form of 2-amino-l,3,4-thiazoline-5-thione 239 (A = X = S R = H) was obtained from X-ray structural determinations [72AX(B)1584]. A NMR-spectroscopic study (77JOC3725) of compounds 239 (A = Se X = S R = Me) demonstrated their amino-thionic structure. A similar tautomeric form 240 is also dominant for the hydrazine derivative (R = NHNHCOPh) [73JCS(P2)4]. 

Some triazepinones in which one of the nitrogen atoms belongs to a fused azole moiety have been reported. For instance, compound 63 was prepared (74JHC751) and its X-ray structure determined (75CSC317). Similarly, and NMR spectra and X-ray structural determination of 64 revealed that this compound exists as a 3//-tautomer with the 1,2,4-triazepine ring in a distorted boat conformation (88T7185). 

Among the variety of nitrogen-containing fulvalenes emerging from types 7-14, X-ray structural determinations have been performed on about 20 representative examples. Tire first crystal structure determination was carried out by application of the folding-molecule method on 3,3 -diphenyl-l,l -bi-isoindolylidene 64 (R = FI) (71CB3108). Tire dimeric isoindolenine system... 

An X-ray crystal structure of 7-methoxy-5//-dibenz[c,c]azepine reveals that the seven-mem-bered ring is in the boat conformation.7 Likewise, X-ray structural determinations of 2-morpho-lino-5H-d benz.[A/]azepi ne,7 and 5//-dibenz[/>, /]azepi ne.1 " 11 and its 5-acyl derivatives,12 in-... 

The bis(dimethylamino) compound 8 again exhibits a tub conformation, as revealed by X-ray structural determination. 

For the "nonaromatic members, an X-ray structural determination of the carboxamide 1 (R = CONMe2) shows the heavily buckled geometry of a re-localized system.10... 

It is a particular pleasure to thank the institutions which provided financial support to this work. These are the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and the Bund der Freunde der Technischen Universitat Miinchen. I am also indebted to Professor H. Schmidbaur, Garching, for his continued interest in our investigations, to Professor G. Muller, Konstanz, for the x-ray structure determinations and to Professor G. Bowmaker, who was on sabbatical leave from Auckland, New Zealand, for critical reading of the manuscript. 

Hantzsch s proposal is consistent with characteristic infrared vibration frequencies of syn- and a fr-diazoates, suitably labeled with 15N, measured by Kiibler and Liittke (1963) and by an X-ray structure determination of a sy -diazoate by Alcock et al. (1980b). It is appropriate, therefore, to replace the syn/anti by the (Z/E) nomenclature for isomers (Sec. 7.1)... 

Energy minimization methods that exploit information about the second derivative of the potential are quite effective in the structural refinement of proteins. That is, in the process of X-ray structural determination one sometimes obtains bad steric interactions that can easily be relaxed by a small number of energy minimization cycles. The type of relaxation that can be obtained by energy minimization procedures is illustrated in Fig. 4.4. In fact, one can combine the potential U r) with the function which is usually optimized in X-ray structure determination (the R factor ) and minimize the sum of these functions (Ref. 4) by a conjugated gradient method, thus satisfying both the X-ray electron density constraints and steric constraint dictated by the molecular potential surface. 

G. H. Stout and L. H. Jensen (Eds.), X-ray Structure Determination A Practice Guide, John Wiley Sons, New York, 1989. 

---