Conformational Flexibility Controls Proton Transfer between the Methionine Hydroxy Sulfuranyl Radical and the N-Terminal Amino Group in Thr−(X)n−Met Peptides

Comparative Study of Chemical Stability of a PK20 Opioid-Neurotensin Hybrid Peptide and Its Analogue [Ile9]PK20-The Effect of Isomerism of a Single Amino Acid

Chemical stability is one of the main problems during the discovery and development of potent drugs. When ignored, it may lead to unreliable biological and pharmacokinetics data, especially regarding the degradation of products’ possible toxicity. Recently, two biologically active drug candidates were presented that combine both opioid and neurotensin pharmacophores in one entity, thus generating a hybrid compound. Importantly, these chimeras are structurally similar except for an amino acid change at position 9 of the peptide chain. In fact, isoleucine (C₆H₁₃NO₂) was replaced with its isomer tert-leucine. These may further lead to various differences in hybrids’ behavior under specific conditions (temperature, UV, oxidative, acid/base environment). Therefore, the purpose of the study is to assess and compare the chemical stability of two hybrid peptides that differ in nature by way of one amino acid (tert-leucine vs. isoleucine). The obtained results indicate that, opposite to biological activity, the substitution of tert-leucine into isoleucine did not substantially influence the compound’s chemical stability. In fact, neither hydrolysis under alkaline and acidic conditions nor oxidative degradation resulted in spectacular differences between the two compounds—although the number of potential degradation products increased, particularly under acidic pH. However, such a modification significantly reduced the compound’s half-life from 204.4 h (for PK20 exposed to 1M HCl) to 117.7 h for [Ile⁹]PK20.

N-Terminal Decarboxylation as a Probe for Intramolecular Contact Formation in γ-Glu-(Pro)n-Met Peptides

The kinetics of intramolecular-contact formation between remote functional groups in peptides with restricted conformational flexibility were examined using designed peptides with variable-length proline bridges. As probes for this motion, free radicals were produced using the ^•OH-induced oxidation at the C-terminal methionine residue of γ-Glu-(Pro)_n-Met peptides (n = 0-3). The progress of the radicals' motion along the proline bridges was monitored as the radicals underwent reactions along the peptides' backbones. Of particular interest was the reaction between the sulfur atom located in the side chain of the oxidized Met residue and the unprotonated amino group of the glutamic acid moiety. Interactions between them were probed by the radiation-chemical yields (expressed as G values) of the formation of C-centered, α-aminoalkyl radicals (αN) on the Glu residue. These radicals were monitored directly or via their reaction with p-nitroacetophenone (PNAP) to generate the optically detected PNAP^•- radical anions. The yields of these αN radicals were found to be linearly dependent on the number of Pro residues. A constant decrease by 0.09 μM J^-1 per spacing Pro residue of the radiation-chemical yields of G(αN) was observed. Previous reports support the conclusion that the αN radicals in these cases would have to result from (S∴N)⁺-bonded cyclic radical cations that arose as a result from direct contact between the ends of the peptides. Furthermore, by analogy with the rate constants for the formation of intramolecularly (S∴S)⁺-bonded radical cations in Met-(Pro)_n-Met peptides ( J. Phys. Chem. B 2016, 120, 9732), the rate constants for the formation of intramolecularly (S∴N)⁺-bonded radical cations are activated to the same extent for all of the γ-Glu-(Pro)_n-Met peptides. Thus, the continuous decrease of G(αN) with the number of Pro residues (from 0 to 3) suggests that the formation of a contact between the S-atom in the C-terminal Met residue and the N-atom of a deprotonated N-terminal amino group of Glu is controlled in peptides with 0 to 3 Pro residues by the relative diffusion of the S^•+ and unoxidized N-atom. The overall rate constants of cyclization to form the (S∴N)-bonded radical cations were estimated to be 3.8 × 10⁶, 1.8 × 10⁶, and 8.1 × 10⁵ s^-1 for peptides with n = 0, 1, and 2 Pro residues, respectively. If activation is the same for all of the peptides, then these rate constants are a direct indication for the end-to-end dynamics along the chain.

Freezing of Aqueous Solutions and Chemical Stability of Amorphous Pharmaceuticals: Water Clusters Hypothesis

Molecular mobility has been traditionally invoked to explain physical and chemical stability of diverse pharmaceutical systems. Although the molecular mobility concept has been credited with creating a scientific basis for stabilization of amorphous pharmaceuticals and biopharmaceuticals, it has become increasingly clear that this approach represents only a partial description of the underlying fundamental principles. An additional mechanism is proposed herein to address 2 key questions: (1) the existence of unfrozen water (i.e., partial or complete freezing inhibition) in aqueous solutions at subzero temperatures and (2) the role of water in the chemical stability of amorphous pharmaceuticals. These apparently distant phenomena are linked via the concept of water clusters. In particular, freezing inhibition is associated with the confinement of water clusters in a solidified matrix of an amorphous solute, with nanoscaled water clusters being observed in aqueous glasses using wide-angle neutron scattering. The chemical instability is suggested to be directly related to the catalysis of proton transfer by water clusters, considering that proton transfer is the key elementary reaction in many chemical processes, including such common reactions as hydrolysis and deamidation.

Computational insights into the charge relaying properties of β-turn peptides in protein charge transfers

Density functional theory calculations suggest that β-turn peptide segments can act as a novel dual-relay elements to facilitate long-range charge hopping transport in proteins, with the N terminus relaying electron hopping transfer and the C terminus relaying hole hopping migration. The electron- or hole-binding ability of such a β-turn is subject to the conformations of oligopeptides and lengths of its linking strands. On the one hand, strand extension at the C-terminal end of a β-turn considerably enhances the electron-binding of the β-turn N terminus, due to its unique electropositivity in the macro-dipole, but does not enhance hole-forming of the β-turn C terminus because of competition from other sites within the β-strand. On the other hand, strand extension at the N terminal end of the β-turn greatly enhances hole-binding of the β-turn C terminus, due to its distinct electronegativity in the macro-dipole, but does not considerably enhance electron-binding ability of the N terminus because of the shared responsibility of other sites in the β-strand. Thus, in the β-hairpin structures, electron- or hole-binding abilities of both termini of the β-turn motif degenerate compared with those of the two hook structures, due to the decreased macro-dipole polarity caused by the extending the two terminal strands. In general, the high polarity of a macro-dipole always plays a principal role in determining charge-relay properties through modifying the components and energies of the highest occupied and lowest unoccupied molecular orbitals of the β-turn motif, whereas local dipoles with low polarity only play a cooperative assisting role. Further exploration is needed to identify other factors that influence relay properties in these protein motifs.

Stability of protein pharmaceuticals: an update

In 1989, Manning, Patel, and Borchardt wrote a review of protein stability (Manning et al., Pharm. Res. 6:903-918, 1989), which has been widely referenced ever since. At the time, recombinant protein therapy was still in its infancy. This review summarizes the advances that have been made since then regarding protein stabilization and formulation. In addition to a discussion of the current understanding of chemical and physical instability, sections are included on stabilization in aqueous solution and the dried state, the use of chemical modification and mutagenesis to improve stability, and the interrelationship between chemical and physical instability.

Oxidation of methionines by oxochromium(V) cations: A kinetic and spectral study

The oxidation of methionine (Met) plays an important role during biological conditions of oxidative stress as well as for protein stability. By choosing [oxo(salen)chromium(V)] ions, [(salen)Cr(V)=O](+) (where salen = bis(salicylidene)ethylenediamine) as suitable biomimics for the peptide complexes that are formed during the reduction of Cr(VI) with biological reductants, the oxidation of methionine and substituted methionines with five [oxo(salen)chromium(V)] complexes in aqueous acetonitrile has been investigated by spectrophotometric, electron paramagnetic resonance (EPR) spectroscopy and electrospray ionization mass spectrometry (ESI-MS) methods. In aqueous solution [(salen)Cr(V)=O](+) ion is short lived, ligation of H(2)O to the Cr center takes place and [O=Cr(V)(salen)-H(2)O](+) adduct is the active oxidant. The reaction is found to be first order each in the oxidant and the substrate. The presence of water in the reaction system accelerates the reaction rate and an inactive, stable mu-oxo dimer is also formed during the course of the reaction. On the basis of spectral, kinetic and product analysis study a mechanism involving direct oxygen transfer from [O=Cr(V)(salen)-H(2)O](+) to methionine has been proposed as a suitable mechanism for the reaction.

Methionine oxidation by reactive oxygen species: reaction mechanisms and relevance to Alzheimer's disease

The oxidation of methionine plays an important role in vivo, during biological conditions of oxidative stress, as well as for protein stability in vitro. Depending on the nature of the oxidizing species, methionine may undergo a two-electron oxidation to methionine sulfoxide or one-electron oxidation to methionine radical cations. Both reaction mechanisms derive catalytic support from neighboring groups, which stabilize electron-deficient reaction centers. In vivo, methionine sulfoxide is subject to reduction by the methionine sulfoxide reductase (Msr) system, suggesting that some methionine sulfoxide residues may only be transiently involved in the deactivation of proteins through reactive oxygen species (ROS). Other methionine sulfoxide residues may accumulate, depending on the accessibility to Msr. Moreover, methionine sulfoxide levels may increase as a result of a lower abundance of active Msr and/or the required cofactors as a consequence of pathologies and biological aging. On the other hand, methionine radical cations will enter predominantly irreversible reaction channels, which ultimately yield carbon-centered and/or peroxyl radicals. These may become starting points for chain reactions of protein oxidation. This review will provide detailed mechanistic schemes for the reactions of various prominent, biologically relevant ROS with methionine and organic model sulfides. Emphasis will be given on the one-electron oxidation pathway, characterizing the physico-chemical parameters, which control this mechanism, and its physiological relevance, specifically for the oxidation and neurotoxicity of the Alzheimer's disease beta-amyloid peptide (betaAP).

Free Radical Reactions of Methionine in Peptides: Mechanisms Relevant to β-Amyloid Oxidation and Alzheimer's Disease

The pathogenesis of Alzheimer's disease is strongly associated with the formation and deposition of beta-amyloid peptide (beta AP) in the brain. This peptide contains a methionine (Met) residue in the C-terminal domain, which is important for its neurotoxicity and its propensity to reduce transition metals and to form reactive oxygen species. Theoretical studies have proposed the formation of beta AP Met radical cations as intermediates, but no experimental evidence with regard to formation and reactivity of these species in beta AP is available, largely due to the insolubility of the peptide. To define the potential reactions of Met radical cations in beta AP, we have performed time-resolved UV spectroscopic and conductivity studies with small model peptides, which show for the first time that (i) Met radical cations in peptides can be stabilized through bond formation with either the oxygen or the nitrogen atoms of adjacent peptide bonds; (ii) the formation of sulfur-oxygen bonds is kinetically preferred, but on longer time scales, sulfur-oxygen bonds convert into sulfur-nitrogen bonds in a pH-dependent manner; and (iii) ultimately, sulfur-nitrogen bonded radicals may transform intramolecularly into carbon-centered radicals located on the (alpha)C moiety of the peptide backbone.

Methionine residue 35 is critical for the oxidative stress and neurotoxic properties of Alzheimer's amyloid beta-peptide 1-42

Amyloid beta-peptide 1-42 [Abeta(1-42)] is central to the pathogenesis of Alzheimer's disease (AD), and the AD brain is under intense oxidative stress. Our laboratory combined these two aspects of AD into the Abeta-associated free radical oxidative stress model for neurodegeneration in AD brain. Abeta(1-42) caused protein oxidation, lipid peroxidation, reactive oxygen species formation, and cell death in neuronal and synaptosomal systems, all of which could be inhibited by free radical antioxidants. Recent studies have been directed at discerning molecular mechanisms by which Abeta(1-42)-associated free radical oxidative stress and neurotoxicity arise. The single methionine located in residue 35 of Abeta(1-42) is critical for these properties. This review presents the evidence supporting the role of methionine in Abeta(1-42)-associated free radical oxidative stress and neurotoxicity. This work is of obvious relevance to AD and provides a coupling between the centrality of Abeta(1-42) in the pathogenesis of AD and the oxidative stress under which the AD brain exists.

Redox processes of methionine relevant to beta-amyloid oxidation and Alzheimer's disease

This minireview gives an overview over the oxidation mechanisms of methionine (Met) relevant for analogous processes which may lead to the oxidation of beta-amyloid (betaA) peptides. The Cu(II)-catalyzed oxidation of a C-terminal Met(35) residue in betaA peptides may be a key to the known propensities of these peptides to form H2O2 and free radicals. Though the reduction potentials of Cu(II) and Met would seem unfavorable, there are several structural features of betaA, which may promote a one-electron oxidation of Met. The potentially close association of the Met sulfur with the C=O group C-terminal of Ile(31) in the C-terminus of betaA may support the formation of an S-O bonded radical cation intermediate. Evidence for such S-O bond formation has recently been obtained for a model, N-acetylmethionine amide. Additional support for a potential catalytic role of an oxygen-containing functional group comes from numerous studies with organic model sulfides.