Showing papers by "Tamás Beke-Somfai published in 2014"

UV Transition Moments of Tyrosine

Abstract: To assist polarized-light spectroscopy for protein-structure analysis, the UV spectrum of p-cresol, the chromophore of tyrosine, was studied with respect to transition moment directions and perturbation by solvent environment. From linear dichroism (LD) spectra of p-cresol aligned in stretched matrices of poly(vinyl alcohol) and polyethylene, the lowest pi-pi* transition (L-b) is found to have pure polarization over its entire absorption (250-300 nm) with a transition moment perpendicular to the symmetry axis (C-1-C-4), both in polar and nonpolar environments. For the second transition (L-a), polarized parallel with the symmetry axis, a certain admixture of intensity with orthogonal polarization is noticed, depending on the environment. While the L-b spectrum in cydohexane shows a pronounced vibrational structure, it is blurred in methanol, which can be modeled as due to many microscopic polar environments. With the use of quantum mechanical (QM) calculations, the transition moments and solvent effects were analyzed with the B3LYP and omega B97X-D functionals in cyclohexane, water, and methanol using a combination of implicit and explicit solvent models. The blurred L-b band is explained by solvent hydrogen bonds, where both accepting and donating a hydrogen causes energy shifts. The inhomogeneous solvent-shift sensitivity in combination with robust polarization can be exploited for analyzing tyrosine orientation distributions in protein complexes using LD spectroscopy.

Shear-induced membrane fusion in viscous solutions.

Abstract: Large unilamellar lipid vesicles do not normally fuse under fluid shear stress. They might deform and open pores to relax the tension to which they are exposed, but membrane fusion occurring solely due to shear stress has not yet been reported. We present evidence that shear forces in a viscous solution can induce lipid bilayer fusion. The fusion of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes is observed in Couette flow with shear rates above 3000 s(-1) provided that Couette Cell the medium is viscous enough. Liposome samples, prepared at different viscosities using a 0-50 wt % range of sucrose concentration, were studied by dynamic light scattering, lipid fusion assays using Forster resonance energy transfer (FRET), and linear dichroism (LD) spectroscopy. Liposomes in solutions with 40 wt % (or more) sucrose showed lipid fusion under shear forces. These results support the hypothesis that under suitable conditions lipid membranes may fuse in response to mechanical-force-induced stress.

Orientation of aromatic residues in amyloid cores: Structural insights into prion fiber diversity

Abstract: Structural conversion of one given protein sequence into different amyloid states, resulting in distinct phenotypes, is one of the most intriguing phenomena of protein biology. Despite great efforts the structural origin of prion diversity remains elusive, mainly because amyloids are insoluble yet noncrystalline and therefore not easily amenable to traditional structural-biology methods. We investigate two different phenotypic prion strains, weak and strong, of yeast translation termination factor Sup35 with respect to angular orientation of tyrosines using polarized light spectroscopy. By applying a combination of alignment methods the degree of fiber orientation can be assessed, which allows a relatively accurate determination of the aromatic ring angles. Surprisingly, the strains show identical average orientations of the tyrosines, which are evenly spread through the amyloid core. Small variations between the two strains are related to the local environment of a fraction of tyrosines outside the core, potentially reflecting differences in fibril packing.