Mixture Of Rotamers Research Articles

Effect of tautomeric shift on mutation: N4-methoxycytidine forms hydrogen bonds with adenosine in polymers.

N4-Methoxycytidine (mo4C), previously found to act only as uridine (U) in transcription [Singer, B., & Spengler, S. (1981) Biochemistry 20, 1127], was tested for its ability to base pair as U in copolymers of (U,mo4C) annealed with poly(A) or transcribed with ATP and DNA-dependent RNA polymerase. Mixing curves have now indicated that the derivative is retained in a poly(U,39% mo4C).poly(A) helix, unlike unmodified C in poly(U,35% C). The presence of 13-39% mo4C in U polymers lowered the melting temperature, Tm, observed in annealed complexes both with poly(A) and after transcription with ATP. However, complexes isolated after transcription had a large hyperchromicity and melted cooperatively, which indicated that they are hydrogen bonded. The decreased Tm for poly(U,mo4C).poly(A) compared to that for poly(U).poly(A) can be attributed to stacking changes and adjacent base-pair disruption by mo4C. The greater cooperative melting of transcribed poly(U,39% mo4C) as compared to the annealed complex may indicate that the methoxy substituent is normally a mixture of rotamers and that the syn rotamer is required for transcription. The interference of the methoxy substituent was also shown by the loss of helix formation by poly(C,mo4C) in acid solution. mo4C decreased the Tm much more than A, which stacks well in acid. U, which neither stacks nor participates in an acid structure, caused more distortion than either of the other bases. It is inferred that mo4C has the base-pairing ability of U but that the planarity of the substituent is lost.

Alpinigenine from Papaver bracteatum Lindl. Restricted rotation in an unusual oxidation product

Alpinigenine (1) was isolated in relatively large amount from a type of Papaver bracteatum Lindl. from Iran. Oxidation of this alkaloid with Jones reagent produced a lactone function and, unexpectedly, converted the NMe into an NCHO, present as a mixture of rotamers. Crystallization afforded one pure component, which equilibrated in solution within 2 h. 13C N.m.r. spectroscopy was used to confirm these results and enabled an insight into the conformation in solution of the seven-membered rings in alpinigenine and its oxidation product. X-Ray analysis determined unambiguously the absolute configuration of alpinigenine.

Conformational Equilibria in Trans 1,2‐Diarylethylenes Manifested In Their Emission Spectra. Part III. (1a). Pyridyl and quinolyl derivatives

AbstractThe emission spectra of four 1,2‐diarylethylenes, in which at least one aryl is 3‐pyridyl or 7 or 8‐quinolyl, vary with the wavelength of excitation, indicating the existence of two or three species of each compound with slightly‐shifted emission (and absorption) peaks, in analogy to our earlier reports (1). At temperatures below ‐70°C in solutions in aliphatic hydrocarbons the normal fluorescence is gradually replaced by a broad and almost structureless emission band in the range 400‐550 nm. The conversion takes time, it is virtually complete at ‐120°C. No similar phenomena were observed in toluene/methylcyclohexane mixtures. The results at temperatures above ‐70°C are ascribed to the existence of an equilibrium mixture of rotamers, while those below ‐70°C are suggested to reflect association phenomena.

Low Temperature Photolysis of o-Benzoquinones. Spectroscopic Detection of Cyclopentadienones and Bis-Ketenes

Abstract The photodecarbonylation of o-benzoquinone to cyclopentadienone at low temperature was monitored by infrared and ultraviolet spectra which showed that the reaction is highly sensitive to substituents. 3,5-Di-t-butyl-o-benzoquinone undergoes clean decarbonylation to give cyclopentadienone under irradiation at −170 °C. Similar irradiation of 4-t-butyl-, 4-methyl-, 3-methyl-, and unsubstituted o-benzoquinones also leads to decarbonylation to cyclopentadienones. However, competitive α-cleavage to bis-ketene becomes important in these systems. Direct spectroscopic observation of the elusive cyclopentadienone was unsuccessful because of the overlapping absorption of the by-product. The infrared spectrum of bis-ketene from 4-t-butyl-o-benzoquinone shows that it exists as a mixture of rotamers, not undergoing thermal interconversion even at −170 °C.

Electronic energy transfer between tyrosine and tryptophan in the peptides Trp-(Pro)n-Tyr.

AbstractA series of peptides, Trp‐(Pro)n‐Tyr, n = 0,…,5 were synthesized. Details of the synthetic method using the Merrifield method are given. The initial objective was to determine the characteristic distance R0 for energy transfer from tyrosine to tryptophan. This objective became submerged in the question of the structure of the chain of prolyl spacers. The efficiency of the energy transfer diminishes as the number of intervening prolines increases. Quantitatively the fit is much better if we assume that the peptides have a polyproline I structure (cis) than if we assume a trans polyproline II structure. Proton nmr spectra on the other hand suggest that the n = 1,3,5 peptides are mixtures of rotamers. Our conclusions are similar to those of previous workers, i.e., that in water there is a transition from a mixture of structures for small n to an all‐trans structure at some n > 5. This means that in water, at least, proline is a dubious spacer for energy‐transfer studies.

Photoelectron spectra of Group 5 compounds. II. Conformational analysis of diphosphine (P2H4)

The photoelectron spectra of certain hydrazines, disulfides, peroxides, and aminophosphines have been assigned to a unique conformer, being present under normal PE spectroscopic conditions. In contrast, different rotamers could be detected in the PE spectra of hexahydropyridazines and tetrasubstituted diphosphines and diarsines and were assumed for polysilanes. The composition of the rotameric mixture (transgauche) obtained for tetramethyldiphosphine from the relative PE peak areas had to be revised.