Conformation-Specific Spectroscopy and Populations of Diastereomers of a Model Monolignol Derivative: Chiral Effects in a Triol Chain

Abstract

Single-conformation spectroscopy of two diastereomers of 1-(4-hydroxy-3-methoxyphenyl)propane-1,2,3-triol (HMPPT) has been carried out under isolated, jet-cooled conditions. HMPPT is a close analog of coniferyl alcohol, one of the three monomers that make up lignin, the aromatic biopolymer that gives structural integrity to plants. In HMPPT, the double bond of coniferyl alcohol has been oxidized to produce an alkyl triol chain with chiral centers at C(α) and C(β), thereby incorporating key aspects of the β-O-4 linkage between monomer subunits that occurs commonly in lignin. Both (R,S)- and (R,R)-HMPPT diastereomers have been synthesized in pure form for study. Resonant two-photon ionization (R2PI), UV hole-burning (UVHB)/IR-UV hole-burning (IR-UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been carried out, providing single-conformation UV spectra in the S(0)-S(1) region (35200-35800 cm(-1)) and IR spectra in the hydride stretch region. Five conformers of (R,S)- and four conformers of (R,R)-HMPPT are observed and characterized, leading to assignments for all nine conformers. Spectroscopic signatures for α-β-γ, γ-β-α, and α-γ-β-π chains and two cyclic forms [(αβγ) and (αγβ)] of the glycerol side chain are determined. Infrared ion-gain (IRIG) spectroscopy is used to determine fractional abundances for the (R,S) diastereomer and constrain the populations present in (R,R). The two diastereomers have very different conformational preferences. More than 95% of the population of (R,R) configures the glycerol side chain in a γ-β-α triol chain, while in (R,S)-HMPPT, 51% of the population is in α-β-γ chains that point in the opposite direction, with an additional 21% of the population in H-bonded cycles. The experimental results are compared with calculations to provide a consistent explanation of the diastereomer-specific effects observed.

Vibrational circular dichroism of a 2,5‐diketopiperazine (DKP) peptide: Evidence for dimer formation in cyclo LL or LD diphenylalanine in the solid state

“…5 The effects of modifying the handedness of one monomer in a biopolymer have been studied at the molecular level in model systems, both neutral molecules as well as protonated or sodiated species. [6][7][8][9][10] Although often weak in short neutral or protonated peptides, the effects are more pronounced in long protonated polyalanine strands, whose helical shape is disrupted when reverting the chirality of one residue, 11 in sodiated peptides, 10,12,13 or in cyclically constrained β-peptides with a chirality-dependent intramolecular hydrogen bonding pattern. 9 Solution-phase studies of the way diastereomerism impinges on the structure of peptides rely on various structure-sensitive techniques such as nuclear magnetic resonance (NMR) 14 and vibrational spectroscopy.…”

Binding water to a PEG-linked flexible bichromophore: IR spectra of diphenoxyethane-(H2O)n clusters, n = 2-4

“…24,[26][27][28][29][30][31][32][33] UV light was generated by frequency doubling the output of a Nd:YAG pumped dye laser (Radiant Dyes NarrowScan or Lambda Physik ScanMate) into the appropriate frequency range. IR light was generated via a seeded Nd:YAG pumped optical parametric converter (LaserVision).…”

“…31 Spectra were recorded over the entire S 0 -S 1 /S 2 origin regions, ∼36 200-36 800 cm −1 . The wavelength used for the ionization step (∼277-279 nm) was chosen to maximize two-color enhancement while minimizing production of broad background from fragmentation of higher mass ions.…”

“…Conformation specific UV spectra were recorded using both UV-UV and IR-UV holeburning which are described thoroughly in the literature. [28][29][30][31] Single-conformation IR spectra were recorded for each of the observed conformers through the use of resonant iondip infrared (RIDIR) spectroscopy. [27][28][29] Ground state RIDIR spectra were recorded in a configuration in which the IR light counter-propagates the molecular beam.…”

Chirality effects in gas-phase spectroscopy and photophysics of molecular and ionic complexes: contribution of low and room temperature studies

“…Changing the relative chirality of α and β strongly modifies the conformational space. The linear OHγ…OHβ…OHα hydrogen-bonded geometry of the side chain is always the major conformation, but much more structural diversity is observed when C α and C β are of opposite chirality, including OH…OH…OH rings [197]. Interesting influence of chirality is also observed in polypeptides showing a γ turn, a frequently adopted secondary structure [198].…”