Interaction between TOAC free radical and photoexcited triplet chromophores linked to peptide templates

The intramolecular quenching of photoexcited triplet states by free radicals linked to peptide templates was studied by time-resolved electron paramagnetic resonance (EPR) with pulsed laser excitation. The systems investigated are 3(10)-helix forming peptides, having in the amino acid sequence the free radical 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) and a triplet precursor, such as Bin, Bpa, or Trp, incorporated at different relative positions. Upon interaction with the excited triplet the TOAC radical spin sublevel populations assume values that differ from the Boltzmann equilibrium values. This spin polarization effect produces EPR lines in emission whose time evolution reflects the triplet quenching rate. In particular, in a series of peptides labeled with Bpa and TOAC at successive positions in the 3(10)-helix, radical-triplet interaction was observed in all cases. However, for the peptide where Bpa and TOAC are at positions 2 and 4 the rate of triplet quenching is lower than for the other peptides in the series. In addition, the radical-excited triplet complex in the quartet spin state was observed in a peptide containing fullerene (C(60)) as a triplet precursor and TOAC.

CIDEP effects of intramolecular quenching of singlet and triplet excited states by nitroxide radicals in oligopeptides: a potentially useful new method for investigating peptide secondary structures in solution

Two hexapeptides, each bearing one photoactive alpha-amino acid (Bin or Bpa) and one nitroxide-containing TOAC residue, have been synthesized and fully characterized. FT-IR absorption measurements indicate that a 3(10)-helical conformation is adopted by these peptides in solution. As two amino acid units separate the photoactive residue from TOAC in the peptide sequences, the two moieties face each other at a distance of about 6 A after one complete turn of the ternary helix. Irradiation by a light pulse from an excimer laser populates the excited states localized on the chromophores. An intramolecular interaction between the singlet (Bin) or triplet (Bin and Bpa) excited states and the doublet state of the TOAC nitroxide makes a spin-selective decay pathway possible, that produces transient spin polarization. In addition, in order to determine whether the intramolecular exchange interaction occurs through-bond or through-space, we have prepared linear and cyclic TOAC-Bin dipeptide units. A CIDEP study revealed that a through-space intramolecular interaction is operative. The observation of spin polarization makes the two helical hexapeptides suitable models to test the possibility of application of this novel technique to conformational studies of peptides in solution.

Differential mechanisms for structural and functional alterations of trypsin by heparin, evidence for a specific, radical-generating mechanism at low heparin concentrations

The oxidative mechanism whereby heparin may interact with various proteins was investigated in detail in this work by addressing the role of doses of heparin on the nature and effects of its binding to bovine trypsin, taken as reference protein. Unfractionated heparin was used at concentrations ranging from 6 to 400 microg/ml with a fixed trypsin concentration (250 microg/ml). At concentrations of up to 60 microg/ml, equivalent to trypsin/heparin molar ratios of between 30 and 3, increasing inhibition of amidolytic activity and radical-dependent peptide bond cleavage of the enzyme was observed, with the appearance in the electrophoretic pattern of new bands of trypsin fragments to which heparin was demonstrated to be bound specifically. Structural modifications were also revealed by increases in fluorescence emission spectra. On the whole, however, the alterations induced by these heparin concentrations only involved a limited number of trypsin molecules. At concentrations from 120 to 400 microg/ml (equivalent trypsin/heparin molar ratios of 1.5-0.46), heparin binding to trypsin appeared to cause more profound and generalized alterations of enzyme structure and function, with dose-dependent quenching of fluorescence emission and almost complete loss of amidolytic activity, although evidence of radical production was lacking. Collectively, the results stress the crucial role of heparin dose on both the nature and effects of its binding to trypsin. The change in heparin effects which reflects distinct underlying molecular mechanisms occurs dramatically at a critical concentration threshold. While a specific, radical-generating mechanism operates at low concentrations, less specific ionic linkages, apparently independent of radical production, best explain the effects of high heparin concentrations.

Role of reducing terminals in unfractionated and low-molecular-mass heparins in causing free radical generation and loss of structure and activity of trypsin

The role of both the length of saccharide chain and reducing terminals in the heparin molecule in causing oxidative effects on proteins was investigated by employing unfractionated and low-molecular-mass heparins (LMMH), with both intact and reduced reducing terminals on bovine trypsin. The effects of heparin were found to be dependent on both the concentration and time of incubation. Heparins with intact reducing terminals caused significantly higher structural and functional alterations of trypsin compared with heparins with reduced reducing terminals. LMMH was slightly more effective than unfractionated heparin (UNFH) in reducing structural integrity and inhibiting the amidolytic activity of trypsin when used at the same mass, but not molar concentrations. Neither the length of saccharide chains nor the number of intact reducing terminals on the heparin molecule appeared to influence the characteristics of the initial binding of heparin to trypsin, but both these variables crucially affected linkages which in time mediate the inhibition of catalytic activity and the formation of free radicals, ultimately responsible for peptide bond cleavage in trypsin. The results suggest that both a critical number of saccharide units, preferentially lying on shorter chains, and intact reducing terminals in the heparin molecule are involved in setting up the binding which generates radicals and leads to loss of structure and function of the proteinase.

Spin-trapping agent alpha-phenyl N-tert-butylnitrone binds to trypsin and enhances heparin-induced inhibition of amidolytic activity and structural degradation of the enzyme

The effects of heparin on trypsin have recently been demonstrated to involve inhibition of catalytic activity and degradation of the enzyme by means of an oxidative mechanism. The possibility that alpha-phenyl N-tert-butylnitrone protects heparin-induced radical formation on trypsin was investigated by measuring amidolytic activity and changes in the structure of trypsin in the presence of heparin with and without alpha-phenyl N-tert-butylnitrone. The results show that alpha-phenyl N-tert-butylnitrone does not only prevent, but it even significantly enhances effects of heparin on the enzyme. This is due to the unique property of alpha-phenyl N-tert-butylnitrone, independently of spin-trapping capacity, to modify the trypsin structure by binding irreversibly to the catalytic triad, at sites distinct from those to which heparin binds.

Aluminum (III) induces alterations on the physical state of the erythrocytic membrane: an ESR evaluation

The action of aluminum [Al(III)] as Al(acac)3 on erythrocytes causes biophysical effects such as osmotic fragility and echino-acanthocytes formation. In this paper, we present these effects in terms of variation of membrane fluidity, together with findings regarding conformational modifications of membrane proteins consequent to Al(III) exposure, as well as the effects on the mobility of the membrane protein bound sialic acid. To this end, we utilized ESR measurements of rabbits and humans erythrocytic ghosts after probing or labeling with suitable stable radicals used as spin probes or labels. Our results show that the lipophilic, hydrolytically stable toxicant Al(acac)3 causes a remarkable reduction of membrane fluidity in rabbit erythrocytes, an appreciable structural compacting effect on cytoskeletal and transmembrane proteins, as well as a reduction of rotational mobility of cell-surface sialic acid of human erythrocytes.

Synthesis and conformational studies of peptides containing TOAC, a spin-labelled C alpha, alpha-disubstituted glycine

A variety of host L-alanine homo-peptides (to the pentamer) containing one or two spin-labelled TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) residues were synthesized by solution methods and fully characterized. The conformational features of the terminally blocked, doubly spin-labelled TOAC-(Ala)2-TOAC-Ala-pentapeptide were examined in the crystal state by X-ray diffraction and in solution using a combination of techniques (Fourier transform infrared, circular dichroism, cyclic voltammetry and electron spin resonance) in comparison with singly labelled shorter peptides. The 3(10)-helical structure of the pentapeptide, promoted by the two C alpha, alpha-disubstituted glycines under favourable experimental conditions, allows an interaction to take place between the two nitroxide TOAC side chains spaced by one turn of the helix. Taken together, these results suggest that TOAC is an excellent probe for exploring bends and helices in doubly labelled peptides.

Electron spin resonance studies of dental composites: effects of irradiation time, decay over time, pulverization, and temperature variations

Polymerization induced by UV-VIS light of composite dental materials produces a solid matrix within which terminal radicals of non-polymerized monomers remain trapped. Electron Spin Resonance (ESR) allowed three different types of radicals to be identified. The analysis of ten normally available commercial products gave information on: (1) the propagation of the conversion reaction as a result of exposure to light; (2) the time necessary for the decay of each type of radical; and (3) the variations with temperature and the effects of shattering on the materials under study. The presence of inorganic filling material slowed the process of polymerization, while it accelerated the decay of radicals. It was suggested that the nature of these processes depended on the composition of the base resin materials, whereas it did not depend on the sizes of the filler particles. Moreover, the complete propagation of the conversion reaction needed a period of light exposure greater than that currently suggested by the manufacturers. The structural stability and the resistance of the composites were confirmed by both the long period of decay and the high temperatures needed to overcome the potential barrier for starting the radical decay process. Finally, the composite shattering investigation indicated that particles removed by surface abrasion experience rapid radical decay, thus reducing the possibility of harmful effects on internal organs.