Stereochemical constraints

Biological fibers, such as can be formed by DNA and fibrous proteins, may contain crystallites of highly ordered molecules whose structure can in principle be solved to atomic resolution by x-ray crystallography. In practice, however, these crystallites are rarely as ordered as true crystals, and in order to locate individual atoms it is necessary to introduce stereochemical constraints in the x-ray analysis so that the structure can be refined by molecular modeling. 

Stereochemical constraints in cyclic sulfones and sulfoxides impart increased weight to strain and conformational factors in the generation of carbanions and their stability, causing distinct differences between the behavior of cyclic and open-chain systems233, due primarily to the prevention of extensive rotation about the C —S bond, which is the major way that achiral carbanions racemize. Study of the a-H/D exchange rate fce and the racemization rate ka may provide information concerning the acidity-stereochemical relationships in optically active cyclic sulfone and sulfoxide systems. 

Several typical reactions of cyclic sulfoxides or sulfones are not observed in the acyclic and large-ring sulfoxide and sulfone analogues, or if they are, they take a different path. In such cases the effect of the cyclic sulfoxide or sulfone function is at least partially a consequence of the particular stereochemical constraints of the cyclic array. 

The reaction of cycloalkene oxides with EH should be a priori mechanistically similar to that of acyclic analogues, except that the presence of a cycle may impose characteristic stereochemical constraints on the products. A look at Table 10.1, however, indicates that cycloalkene oxides are poorer substrates for EH than their acyclic analogues. 

Using the pitch, symmetry, monomer geometries and other stereochemical constraints, a number of types of molecular model can be constructed. Typical dilemmas are whether the molecular helix is left- or right-handed, whether the molecule is a single helix or coaxial double-helix (and in the later case whether the two chains in the duplex are parallel or antiparallel), or whether, if there are two or more molecules in the unit cell, the molecules are parallel or antiparallel. Solution of a structure therefore involves refinement and adjudication All candidate models are refined until the fit with the measured x-ray amplitudes or steric factors allows one model to be declared significantly superior to the others by some standard statistical test. 

Tetrahydrothiopyran and its simply substituted derivatives are to be regarded as cyclic sulfides, and in consequence their chemistry is analogous to that of their acyclic counterparts, which is extensively covered in the standard texts (B-79MI22500). Oxidation, alkylation, halogenation, etc. reactions differ from the corresponding phenomena in the noncyclic species only in so far as the stereochemical constraints imposed by a more rigid framework control the orientation of reactants, transition states and products. 

To answer these questions, we need to represent conformation space. In our approach it is modelled by using a polyprotein that is constructed from individual proteins by linking them together, ensuring that the linker regions do not violate stereochemical constraints (like and I angles in allowed range, no atom overlaps, and so forth). 

In the amino acids, the chelated three-center bonds have the - NH3 group as donor, and they are almost invariably unsymmetrical. The primary and strong interaction is to one of the carboxylate (or sulfate) oxygen atoms, the secondary (and weaker) interaction to the other oxygen atom resulting from stereochemical constraints imposed by packing of the molecules in the crystal lattice. 

The arrangement of coordinating cysteine residues is highly conserved in all Rds including the Rd-type centers in nonheme peroxidases, and involves Cys-X2-Cys motifs near the C- and N-termini. Desulforedoxin and the Dx-type centers in 2Fe SORs provide an interesting variant with Cys-X2-Cys and Cys-Cys motifs for the coordinating cysteine residues the stereochemical constraint imposed by the adjacent cysteines... 

To determine the stereoselectivity of diastereotopic proton abstraction from the Pro-R methylene group of ACPC (9) in the fragmentation (occurring between Pro-S p-C and a-C), 2-ethyl-[3-Di]-ACPC (8a) was prepared with the ethyl side chain and deuterium substituent in cis relationship. Incubation of this compound followed by in situ reductive enzymatic trapping with (25)-lactate dehydrogenase yielded 2-hydroxy-[3-D]-hexanoate where the R,5-placement of D was analyzed by NMR and the D-content by mass spectrometry. These results had defined the stereoselectivity for )5-H-abstraction from the Pro-R methylene of 2-ethyl-ACPC (8) as the proton removal in the overall fragmentation process and by analogy the same in ACPC (9). These results place stereochemical constraints on the ACPC deaminase process and were accommodated in Scheme 10. 

The course of reactions with mercury(II) chloride is subject to control by experimental conditions, and by substitutive and stereochemical constraints within the particular dithioacetal undergoing hydrolysis. Much of the synthetic versatility of dithioacetals derives from this control, and various aspects have been reviewed in detail in this Series6-8 and elsewhere.261... 

The geometric shape of the fixed positions occupied by the ligating atoms defines the coordination polyhedron. Irregular polyhedra may occur when the ligands are not identical, or impose additional stereochemical constraints. 

Stereochemical Constraints and Lattice Relaxation in Crystal Photochemistry. 

Consider in your answer such factors as pK stereochemical constraints, and nucleophilicity. 

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