45 research outputs found
Exceptionally enhanced Raman optical activity (ROA) of amyloid fibrils and their prefibrillar states
Further monoterpene chromane esters from Peperomia obtusifolia: VCD determination of the absolute configuration of a new diastereomeric mixture
A reinvestigation of the monoterpene chromane ester enriched fraction from Peperomia obtusifolia using chiral chromatography led to the identification of a minor peak, which was elucidated by NMR and HRMS as fenchyl-3,4-dihydro-5- hydroxy-2,7-dimethyl-8-(3″-methyl-2″-butenyl)-2-(4′-methyl- 1′,3′-pentadienyl)-2H-1-benzopyran-6-carboxylate, the same structure assigned to two other fenchyl esters described previously, pointing out a stereoisomeric relationship among them. Further NMR analysis revealed that it was actually a mixture of two compounds, whose absolute configurations were determined by VCD measurements. Although, almost no vibrational transitions could be assigned to the chiral chromane, the experimental VCD spectrum was largely opposite to that obtained for the average experimental VCD [(2S,1‴R,2‴R,4‴S + 2R,1‴R,2‴R,4‴S)/2] for fenchol derivatives. These results allowed us to assign the putative compounds as a racemic mixture of the chiral chromane esterified with the monoterpene (1S,2S,4R)-fenchol, which had not been identified in our early work.Fil: Batista Junior, João Marcos. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Batista, Andrea N. L.. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: Kato, Massuo J.. Universidade de Sao Paulo; BrasilFil: Bolzani, Vanderlan S.. Universidade Estadual Paulista Julio de Mesquita Filho; BrasilFil: López, Silvia Noelí. Universidad Nacional de Rosario. Facultad de Ciencias Bioquímicas y Farmacéuticas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario; ArgentinaFil: Nafie, Laurence A.. Syracuse University; Estados UnidosFil: Furlan, Maysa. Universidade Estadual Paulista Julio de Mesquita Filho; Brasi
Electron Transition Current Density in Molecules. 1. Non-Born−Oppenheimer Theory of Vibronic and Vibrational Transitions
Vibrational circular dichroism in amino acids and peptides. 3. Solution- and solid-phase spectra of alanine and serine
Vibrational circular dichroism (VCD) in solution and solid phases is reported for alanine and serine in vibrational
modes involving hydrogen and deuterium stretching motions. The quality of the solution-phase VCD spectra was significantly
enhanced by on-line computer averaging techniques. Solid-phase spectra were obtained as mulls in halocarbon oil and this
method promises to be a useful new sampling technique in VCD spectroscopy. VCD signals in the solid phase are considerablq
larger than the solution spectra for the same molecules. The increased magnitude is attributed to such effects as conformational
uniformity, local ordering, and crqstal lattice interactions between molecules. A neb vibrational assignment of solutionphase
alanine in the carbon-hydrogen stretching region is presented as determined from the solid-phase Ranian spectra of alaninc-do
and alanine-C*-dl. The basic features of the observed VCD spectra of alanine and serine are discussed in terms of their
constituent normal modes and the chiral perturbation of locally symmetric groups
Simultaneous Resonance Raman Optical Activity Involving Two Electronic States
In the present work, the first observation of strong
resonance
Raman optical activity (RROA) involving more that one resonant electronic
state is reported. The chiral transition metal complex bis-(trifluoroacetylcamphorato)
copper(II), abbreviated Cu(tfc)<sub>2</sub>, exhibits both resonance
Raman (RR) and RROA spectra with laser excitation at 532 nm. Vibrational
assignments for this complex were carried out by comparing the non-RR
spectra of Cu(tfc)<sub>2</sub> excited at 1024 nm to density functional
theory (DFT) calculations. The theory of the single-electronic-state
(SES) RROA is extended to the next simplest level of theory involving
two resonant electronic states (TES) without interstate vibronic coupling
as an aide to the interpretation of the observed TES-RROA spectra.
Based on measured UV–vis electronic absorbance spectra and
corresponding TD-DFT calculations, the most likely two states associated
with the RROA spectra are identified