Subject conformational studies

Most interesting applications of intramolecular energy transfer between nonconjugated chromophores are found in the conformational studies of biomolecules like nucleic acids and proteins. The experiments on rotational depolarization of emission from intrinsic fluorescent groups on externally attached fluorescent probes, have resulted in a vast store of knowledge which has helped to enrich the subject of photobiology. 

The exo-anomeric effect enforces a value of cp around 90° for anomeric axial linkages. This ensures a proclivity towards helical structures (as distinct from the ribbons of 1,4-diequatorially linked polysaccharides), as in starch and glycogen. p-(1 4)-Linked galactans, 1,4 axial-equatorial in the other sense, are known as hemicellulose components of compression and tension wood, the storage polysaccharides of lupins and as arabinogalactans attached to the rhamnogalacturonan I component of pectin, but have not been subject to conformational studies they appear to be biosynthesised from UDP-Gal. " ... 

How the lateral alkyl chains are organized in the liquid crystal state of matter was already a relevant subject of studies starting in the early thirties initiating discussions in which way alkyl chains are conformed in it (e.g., static- or liquid-like all-trans) [24,25]. 

Several 1,3-dialkylimidazolium halides have been intensely structurally investigated, not only as a consequence of polymorphism but also as subjects of studies of the dynamics of processes occurring during melting and crystallisation and associated butyl group conformer lability. These aspects have been introduced in Section 11.2. 

Several conformational studies have been reported. " The trans,cis conformer (35) of 5-methyl dithiocarbazate was isolated by low-temperature crystallization and was subjected to X-ray crystal analysis. The known solid conformer (36) is the ciSytrans one. 

Several papers have used IR and Raman spectra, together with normal coordinate analyses and ab initio calculations, to study the conformational behaviour of substituted vinylsilanes, e.g. CH2=CHSi(H)Cl2. " ° Substituted alkylsilanes were also subjected to studies of the same type, e.g. CH3CH2SiH2F, ... 

Several C-glycosides related to palytoxin, for example the glucose derivatives (18), were the subjects of extensive conformational studies by molecular mechanics. ... 

The eight diastereomeric 2-Q-ethoxycarbonyl-hexofuranosidurono-6,3-lactones (4) fi.e.. the a- and g-D-aluco-. D-manno-. L-j -, and L-aulo-derivatives) were the subject of a conformational study by both H- and n.m.r. spectroscopy. 

The reactivity of dinuclear rhodium compounds toward Mel has been a subject of study since the late 1970s. " " Unfortunately, no kinetic data are available, but the corresponding mono- and bis-acetyl derivatives have been detected. The fact that the oxidative addition/migration on the second metal center becomes more difficult after the first one has taken place is a common observation, especially when the open-book conformation of the complexes is forced by the (bridging) ligands. 

The conformational characteristics of PVF are the subject of several studies (53,65). The rotational isomeric state (RIS) model has been used to calculate mean square end-to-end distance, dipole moments, and conformational entropies. C-nmr chemical shifts are in agreement with these predictions (66). The stiffness parameter (5) has been calculated (67) using the relationship between chain stiffness and cross-sectional area (68). In comparison to polyethylene, PVF has greater chain stiffness which decreases melting entropy, ie, (AS ) = 8.58 J/(molK) [2.05 cal/(molK)] versus... 

Compounds in which conformational, rather than configurational, equilibria are influenced by the anomeric effect are depicted in entries 4—6. Single-crystal X-ray dilfiaction studies have unambiguously established that all the chlorine atoms of trans, cis, ira j-2,3,5,6-tetrachloro-l,4-dioxane occupy axial sites in the crystal. Each chlorine in die molecule is bonded to an anomeric carbon and is subject to the anomeric effect. Equally striking is the observation that all the substituents of the tri-0-acetyl-/ -D-xylopyranosyl chloride shown in entry 5 are in the axial orientation in solution. Here, no special crystal packing forces can be invoked to rationalize the preferred conformation. The anomeric effect of a single chlorine is sufficient to drive the equilibrium in favor of the conformation that puts the three acetoxy groups in axial positions. 

During studies on ditryptophan derivatives, an interesting acid-induced cy-cHzation has been discovered. The 10-membered ring 37 was thus subjected to acidic conditions to produce the indolocarbazole derivative 38 (Scheme 6). Interestingly, calculations performed on the precursor 37 indicated that the lowest energy conformation resembled that of the diastereomer of 38, which was never observed. An additional experiment furnished the parent system 1 on treatment of 38 with a catalytic amount of acid. A TFA-induced formation of an indolo[2,3-<3]carbazole was also observed from a related acyclic 2,2 -connected tryptophan dimer (99JOC8537). 

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