Purple Fibrils: A New Type of Protein Chromophore

Abstract

A purple color is formed during the fibrillation of lysozyme, a well-studied protein lacking a prosthetic group. The application of Raman spectroscopy, electron paramagnetic resonance and UV-vis absorption spectroscopy indicates the formation of a sulfur∴π-bonded radical cation due to the methionine-phenylalanine interaction, which is consistent with a small molecule model reported in the literature. A purple chromophore with characteristic 550 nm absorption is formed due to a specific orientation of the sulfur-centered radical cation and a phenyl ring stabilized by the fibril framework. A specific fibril conformation and the resulting formation of the chromophore are controlled reversibly by varying the pH. This is the first known example of a side chain self-assembled chromophore formed due to protein aggregation.

The S∴π hemibond and its competition with the S∴S hemibond in the simplest model system: infrared spectroscopy of the [benzene-(H2S)n]+ (n = 1–4) radical cation clusters

“…Then, in the present work, the radical cation clusters of benzene (Bz) and hydrogen sulfide, [Bz-(H 2 S) n ] + , ( n = 1–4), are studied by infrared (IR) spectroscopy in the SH and CH stretch regions. Since IR spectroscopy is sensitive to molecular structures and intermolecular interactions, IR spectroscopy of S∴π hemibonded systems would provide us rich information on the nature of the S∴π hemibond, which is complementary to the previous electronic and photoelectron studies 9,10. The n = 1 cluster can be the simplest prototype of the S∴π hemibond in the gas phase.…”

“…The S∴π hemibond (S–π interaction or two-center three-electron, 2c-3e, bond) is an attractive interaction between a singly occupied lone pair orbital of sulfur and a doubly occupied π orbital, or vice versa (here, we should note that the whole benzene molecule with the delocalized π-orbital is regarded as one “center” in the 2c-3e bond while another center is the sulfur atom). The S∴π hemibond has attracted great interest because it plays crucial roles in chemistry and biochemistry of sulfur radical cations 1–10. For example, it has been pointed out that increased strength of interactions of thioethers and arenes, e.g.…”

“…Experimental confirmation of the S∴π hemibond has been pioneered by Werst with EPR spectroscopy 8. Broad electronic transitions due to the excitation to the hole in the anti-bonding orbital have also been used to confirm the formation of the S∴π hemibond in the condensed phase,9,10 as well as many reports on S∴S hemibonds 11. Bally, Glass, and coworkers have performed photoelectron spectroscopy of thioether compounds carefully designed to explore the S∴π hemibond, and the comparison of the spectra of the compounds with and without the phenyl group, as well as with and without the sulfide group, has clearly evidenced the orbital interaction between the sulfur and phenyl ring 9…”

An interesting possibility of forming special hole stepping stones with high-stacking aromatic rings in proteins: three-π five-electron and four-π seven-electron resonance bindings

“…43 Later, Lednev and coworkers have discovered that the Srp threeelectron hemibond is produced during the brillation of lysozyme using Raman spectroscopy, electron paramagnetic resonance and UV-vis absorption spectroscopy. 44 In addition, our recent DFT calculations have revealed that the associative interactions among two nearly parallel aromatic rings and a sulfur-containing group or two sulfur-containing groups and an inserted aromatic ring can signicantly decrease the localized ionization potential by instantly forming special prp:S, prS:p or Srp:S three-p ve-electron resonance structures. 45,46 More recently, Fujii group have conrmed the formations of SrS and Srp two-center hemibonds, Srp:S multicenter hemibond in the [benzene-(H 2 S) n ] + cluster by the IR spectroscopy and DFT calculations.…”

Chemical Mapping of Essential Oils, Flavonoids and Carotenoids in Citrus Peels by Raman Microscopy

“…Raman spectroscopy is based on Raman scattering effect in which information about molecular vibration, rotation, and other low‐frequency modes can be obtained nondestructively as fingerprint (Zhang and others ). This nondestructive detection along with another large advantage of less interference from water molecules, makes Raman a useful tool with various application, such as detection of protein chromophore (Quinones‐Ruiz and others ), identification of breast cancer (Depciuch and others ), detection of water quality (Depciuch and others ), and so on. Coupled with a confocal microscope and mapping technique, Raman microscopy can be used for semi‐automatic data collection where hundreds of spectra can be collected automatically at every pixel of the defined area, and then integrated to generate artificial color images based on the intensity of a designate peak (Liu and others ).…”