Sensitivity to conformation

A consequence of the orientation of the 11-carbonyl function towards the C-19 methyl group which is retained in the excited state is the exclusive functionalization at C-19. Ring cleavage products of the Norrish II type are not observed but the reaction is rather sensitive to conformational changes in the substrate. In a series of experiments conducted under comparable conditions (24 hr irradiation) the yield of cyclobutanols drops... 

Static (multi-angle laser) light scattering (SLS or MALLs) Molecular weight, radius of gyration, Rg solution conformation and flexibility. Solutions need to be clear of supramolecular aggregates. Rg more sensitive to conformation than s. [3]... 

The most important factor determining the sensitivity of the conformation to the concentration of polyions is the change in ion activity or osmotic pressure with conformation. If the activity coeflScient of the counterions is sensitive to conformation then conformational change resulting from concentration changes of polyions becomes large. 

The similarity in lifetimes of 56 and 99 suggests that these 1,2-H shifts are not particularly sensitive to conformational effects in the 6-membered ring. The lifetimes are not markedly longer in benzene n complexes of 56 and benzene (see Section III.B) do not seem to be kinetically significant. The lifetime of 56 in cyclohexane is similar to that of dipropylcarbene ( 0.3 ns),84 as would be expected from the fact that they are both dialkylcarbenes. 

It is clear that the wide range of protein phosphorescence lifetimes is due to various specific quenching mechanisms kq) or due to flexibility of the tryptophan site, thereby affecting km. It also follows that phosphorescence will be very sensitive to conformational fluctuations since subtle changes in distance or orientation relative to a specific quenching moiety within the protein will affect the lifetimes dramatically. The phosphorescence emission from protein tryptophan remains relatively unexplored in terms of investigation of dynamic structure-function relationships. 

Phosphorescence is readily detectable from most types of proteins at room temperature. Tryptophan phosphorescence lifetimes and yields are very sensitive to environment, and therefore phosphorescence is sensitive to conformational changes in proteins. Fundamental questions concerning exactly what parameters affect lifetime and spectra of tryptophan in proteins remain still to be answered. 

A coirplete understanding of the role of carbohydrates in biological systems requires knowledge of the distribution at equilibrium of the various conformers in aqueous solution. The conformational behavior of carbohydrates in solution can be examined from different vantage points (1,), but the most relevant approach is, no doubt, study of dilute solutions themselves. At present, high resolution NMR spectroscopy is the primary tool for determination of three-dimensional structure of oligosaccharides in solution. Optical rotation is also very sensitive to conformation (2) and there is a new, semi-enqpirical theory of optical rotation of oligosaccharides ( ). 

It is also worth pointing out that some NOE s are more sensitive to conformational flexibility than others. When linkage flexibility results in motions which cause a particular inter-hydrogen distance to fluctuate widely, then the NOE associated with that pair of hydrogens will be very sensitive to the nature of the potential energy surface used to simulate that motion. In contrast when the motions result in very little alteration in an inter-hydrogen distance, then the associated NOE s will be insensitive to the potential functions used. 

Circular dichroism (CD) is a spectroscopic method that is uniquely sensitive to conformation. CD has therefore been widely applied to the conformational analysis of peptidesJ1,2 ... 

Noteworthily. the sign of the 220-200 CD band of enones is not sensitive to conformational changes. Examples of the Colton effects of two configurational types of bicyclic enones are shown below51. 

Moreover, as will be shown below, electronic coupHngs are very sensitive to conformational changes of DNA fragments their magnitude can change... 

This limitation has been overcome with a special NDDO-HT parameterization for calculating hole coupling matrix elements in DNA-related systems [72]. As reference data, coupling matrix elements were calculated for a set of 130 structures of WCP dimers with different step parameters at the HF/6-31G level. As discussed below in more detail, electronic couplings between neighboring pairs are extremely sensitive to conformational fluctuations of the DNA structure. For instance, the matrix element between base pairs in... 

The electronic couplings in DNA r-stacks are very sensitive to conformational changes [31]. CT matrix elements were found to be very responsive to variations in the mutual positions of base pairs. As an example, let us compare the electronic couplings calculated for different conformations of the duplex [(GC),(GC)] (Table 4) [41]. In addition to the reference structure with a base-pair arrangement as in ideal B-DNA, we studied 12 distorted configurations each of them differed from the reference by a single step pa-... 

Absorbance subtraction can be considered as a spectroscopic separation technique for some problems in polymers. An interesting application in FT-IR difference spectroscopy is the spectral separation of a composite spectrum of a heterophase system. One such example is a semicrystalline polymer which may be viewed as a composite system containing an amorphous and crystalline phase53). In general, the infrared spectrum of each of these phases will be different because in the crystalline phase one particular rotational conformation will predominate whereas in the disordered amorphous regions a different rotamer will dominate. Since the infrared spectrum is sensitive to conformations of the backbone, the spectral contributions will be different if they can be isolated. The total absorbance A, at a frequency v of a semicrystalline polymer may be decomposed into the following contributions... 

As the time scale of the Raman scattering event ( 3.3 x 10 14 s for a vibration with a Stokes wave number shift of 1000 cm 1 excited in the visible) is much shorter than that of the fastest conformational fluctuations, an ROA spectrum is a superposition of snapshot spectra from all the distinct conformations present in a sample at equilibrium. Since ROA observables depend on absolute chirality, there is a cancellation of contributions from enantiomeric structures arising as a mobile structure explores the range of accessible conformations. Therefore, ROA exhibits an enhanced sensitivity to the dynamic aspects of biomolecular structure. In contrast, conventional Raman band intensities are blind to chirality and so are generally additive and therefore less sensitive to conformational mobility. Ultraviolet circular dichroism (UVCD) also demonstrates an enhanced sensitivity to the dynamics of chiral structures ... 

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