Racemization of Amino Acids in Dipeptides Shows COOH > NH2 for Non-Sterically Hindered Residues

In-peptide amino acid racemization via inter-residue oxazoline intermediates during acidic hydrolysis

Isopedopeptins are antibiotic cyclic lipodepsipeptides containing the subsequence L-Thr—L-2,3-diaminopropanoic acid—D-Phe—L-Val/L-3-hydroxyvaline. Acidic hydrolysis of isopedopeptins in D₂O showed the D-Phe residues to racemize extensively in peptides with L-3-hydroxyvaline but not in peptides with L-Val. Similarly, one Leu residue in pedopeptins, which are related peptides containing the subsequence Leu—2,3-diaminopropanoic acid—Leu—L-Val/L-3-hydroxyvaline, was found to racemize in peptides with L-3-hydroxyvaline. Model tetrapeptides, L-Ala—L-Phe—L-Val/3-hydroxyvaline—L-Ala, gave the corresponding results, i.e. racemization of L-Phe only when linked to a L-3-hydroxyvaline. We propose the racemization to proceed via an oxazoline intermediate involving Phe/Leu and the L-3-hydroxyvaline residues. The 3-hydroxyvaline residue may form a stable tertiary carbocation by loss of the sidechain hydroxyl group as water after protonation. Elimination of the Phe/Leu H-2 and ring-closure from the carbonyl oxygen onto the carbocation results in the suggested oxazoline intermediate. The reversed reaction leads to either retained or inversed configuration of Phe/Leu. Such racemization during acidic hydrolysis may occur whenever a 3-hydroxyvaline residue or any amino acid that can form a stable carbocation on the C-3, is present in a peptide. The proposed mechanism for racemization was supported by incorporation of ¹⁸O in the 3-hydroxyvaline sidechain when the acidic hydrolysis was performed in H₂O/H₂¹⁸O (1:1). The 2,3-diaminopropanoic residues of isopedopeptins and pedopeptins were also found to racemize during acidic hydrolysis, as previously described. Based on the results, the configuration of the Leu and 2,3-diaminopropanoic acid residues of the pedopeptins were reassigned to be L-Leu and D-Leu, and 2 × L-2,3-diaminopropanoic acid.

Compactness of Protein Folds Alters Disulfide-Bond Reducibility by Three Orders of Magnitude: A Comprehensive Kinetic Case Study on the Reduction of Differently Sized Tryptophan Cage Model Proteins

A new approach to monitor disulfide-bond reduction in the vicinity of aromatic cluster(s) has been derived by using the near-UV range (λ=266-293 nm) of electronic circular dichroism (ECD) spectra. By combining the results from NMR and ECD spectroscopy, the 3D fold characteristics and associated reduction rate constants (k) of E19_SS, which is a highly thermostable, disulfide-bond reinforced 39-amino acid long exenatide mimetic, and its N-terminally truncated derivatives have been determined under different experimental conditions. Single disulfide bond reduction of the E19_SS model (with an 18-fold excess of tris(2-carboxyethyl)phosphine, pH 7, 37 °C) takes hours, which is 20-30 times longer than that expected, and thus, would not reach completion by applying commonly used reduction protocols. It is found that structural, steric, and electrostatic factors influence the reduction rate, resulting in orders of magnitude differences in reduction half-lives (900>t_1/2 >1 min) even for structurally similar, well-folded derivatives of a small model protein.

Reaction pathways and kinetics for tetra-alanine in hot, compressed liquid water

pH alters the prevalence of the dissociation states of peptides, through acid–base equilibrium, and influences reaction selectivity under hydrothermal conditions.

Thermal Synthesis of Polypeptides from N-Butyloxycarbonyl Oligopeptides Containing Aspartyl Residue at C-Terminus

The thermal reactions of amino acids have been investigated for pure organic synthesis, materials preparation in industry, and prebiotic chemistry. N-t-Butyloxycarbonyl aspartic acid (Boc-Asp) releases 2-butene and carbon dioxide upon heating without solvents. The resulting mixture of the free molten aspartic acid was dehydrated to give peptide bonds. This study describes the thermal reactions of N-t-butyloxycarbonyl peptides (Boc-Gly-L-Asp, Boc-L-Ala-L-Asp, Boc-L-Val-L-Asp, and Boc-Gly-Gly-L-Asp) having an aspartic residue at the carboxyl terminus. The peptides were deprotected upon heating at a constant temperature between 110 and 170°C for 1 to 24 h to afford polypeptides in which the average molecular weight reached 7800.

Diketopiperazine formation during investigations of amino Acid racemization in dipeptides

The formation of diketopiperazines from the dipeptides isoleucylglycine and glycylisoleucine was investigated at 132 degrees C over the pH range approximately 2 to 10. At pH 6.2, approximately 50 percent of the original dipeptides were converted to the diketopiperazines during the heating experiments. Hydrolysis of the diketopiperazines can yield either the original dipetide or an inverted dipeptide product. The isoleucine in the diketopiperazines was the most highly epimerized component in the system. Previous racemization and epimerization studies with dipeptides have not taken into account the formation of diketopiperazines and, as a result, the cortclusions about the mechanism and geochemical implications of amino acid racemization in dipeptides will require revision.

Simultaneous formation and detection of the reaction product of solid-state aspartame sweetener by FT-IR/DSC microscopic system

The solid-state stability of aspartame hemihydrate (APM) sweetener during thermal treatment is important information for the food industry. The present study uses the novel technique of Fourier transform infrared microspectroscopy equipped with differential scanning calorimetry (FT-IR/DSC microscopic system) to accelerate and determine simultaneously the thermal-dependent impurity formation of solid-state APM. The results indicate a dramatic change in IR spectra from 50, 110 or 153 degrees C, which was respectively attributed to the onset temperature of water evaporation, dehydration and cyclization processes. It is suggested that the processes of dehydration and intramolecular cyclization occurred in the solid-state APM during the heating process. As an impurity, 3-carboxymethyl-6-benzyl-2,5-diketopiperazine (DKP) degraded from solid state APM via intramolecular cyclization and liberation of methanol. This was evidenced by this novel FT-IR/DSC microscopic system in a one-step procedure.

Oxidation of recombinant methionyl human granulocyte colony stimulating factor

The oxidation of methionine residues in recombinant methionyl human granulocyte colony stimulating factor with hydrogen peroxide has been investigated. Kinetic data of the oxidation were obtained by using reversed phase-high performance liquid chromatography. The stability-indicating capability of this system was confirmed with micellar electrokinetic capillary chromatography. In the pH range 1.9-7.5, the kobs value for the oxidation process is constant. Above pH 7.5, kobs tends to increase with increasing pH. In the pH range 1.9-11.8, four oxidation products were detected in RP-HPLC. Mass spectrometric analysis revealed that one mono-, one di- and two trioxidation products were formed. Using the cyanogen bromide cleavage method the nature of the oxidation products was determined. The mono-oxidation product is the protein with Met121 oxidized, while the dioxidation product has oxidized Met121 and Met126 residues. The trioxidation products are the proteins with Met121, Met126 and Met137 or Met0, Met121 and Met126 oxidized.

Resolution of acylated dipeptide stereoisomers by capillary electrophoresis using sulfobutylether derivatized beta-cyclodextrin

The separation of enantiomerically and diastereomerically related stereoisomers of acylated Asp-Phe dipeptides was explored using capillary electrophoresis (CE). This series of dipeptides included the alpha-L,L parent compound and the three other potential Asp containing stereoisomers (alpha-D,D, alpha-L,D, and alpha-D,L), as well the four possible isoAsp containing stereoisomers (beta-L,L, beta-D,D, beta-L,D and beta-D,L). The separation of these substances was explored using both neutral and charged cyclodextrins as the stereoisomer selector added to the running electrolyte. The major experimental parameters investigated included pH, the cyclodextrin type, and the cyclodextrin concentration. Due to differences in the pKa values of the carboxylic acid groups, adjustment of the separation buffer to between pH 3.0 and 4.0 provided for sufficient electrophoretic mobility differences to result in excellent separations of the diastereomerically related peptides in this pH region. The resolution of the enantiomerically related peptide stereoisomers was accomplished using low concentrations (1 mM) of the anionic cyclodextrin derivative, sulfobutylether-beta-cyclodextrin (SBE-beta-CD). This negatively charged cyclodextrin was found to be superior for the resolution of the enantiomerically related peptides as compared to native beta-cyclodextrin or the neutral derivatives, dimethyl beta-cyclodextrin and hydroxypropyl beta-cyclodextrin. An alternative approach using anionic or neutral surfactants in conjunction with the SBE-beta-CDs was also explored and found to be successful but problematic.

Dipeptides in parenteral nutrition: from basic science to clinical applications

The use of intravenous dipeptides shows great promise as an avenue for the provision of amino acids that may otherwise be difficult to deliver via nutrient infusions. The physical/chemical properties and metabolism of numerous dipeptides have now been explored in experimental and human studies. It has been found that these agents have the capacity to spare nitrogen and support serum protein levels in a fashion equivalent to that of intravenous free amino acids. An additional benefit is the ability to deliver certain amino acids that are relatively unstable or poorly soluble in aqueous solutions. These various aspects of intravenous dipeptides are considered in this review.

Stability of protein pharmaceuticals

Recombinant DNA technology has now made it possible to produce proteins for pharmaceutical applications. Consequently, proteins produced via biotechnology now comprise a significant portion of the drugs currently under development. Isolation, purification, formulation, and delivery of proteins represent significant challenges to pharmaceutical scientists, as proteins possess unique chemical and physical properties. These properties pose difficult stability problems. A summary of both chemical and physical decomposition pathways for proteins is given. Chemical instability can include proteolysis, deamidation, oxidation, racemization, and beta-elimination. Physical instability refers to processes such as aggregation, precipitation, denaturation, and adsorption to surfaces. Current methodology to stabilize proteins is presented, including additives, excipients, chemical modification, and the use of site-directed mutagenesis to produce a more stable protein species.

Resolution of stereoisomers of dipeptides by gas chromatography on Chirasil-Val

Several dipeptides have been separated into stereoisomers (enantiomers, diastereomers, sequential isomers) by gas chromatography on Chirasil-Val. Sufficiently volatile N-trifluoroacetyl dipeptide methyl esters were formed at ambient temperature, thus avoiding racemization (or epimerization, respectively, and cleavage of the peptide bond. This method is suitable for ascertaining the stereoisomeric composition of dipeptides in biological samples.

Hydrolytic stability of biomolecules at high temperatures and its implication for life at 250 degrees C

The upper temperature at which a living system can exist is limited by the hydrolytic breakdown rate of its chemical constituents. The peptide bonds of proteins, the phosphodiester and N-glycosyl bonds in RNA and DNA, and the pyrophosphate and N-glycosyl bonds in nucleotides such as ATP and NAD are among the more important bonds that will undergo hydrolysis. The decomposition of biomolecules via non-hydrolytic pathways such as decarboxylations and dehydrations may also be critical factors in determining this upper temperature limit. Baross and Deming recently reported 'black smoker' bacteria, which they isolated from deep-sea hydrothermal vents, growing at 250 degrees C. Here I have attempted to establish the rates for the hydrolysis and/or decomposition of critical biomolecules to determine their ability to exist at this temperature. My results clearly indicate that if these organisms exist, and if their metabolic reactions occur in an aqueous environment, they could not survive at this temperature if they were composed of biomolecules such as proteins and nucleic acids, due to the very rapid rate of decomposition of such molecules.

Comparison of mutagenicity and chemical properties of N-methyl-N'alkyl-N-nitrosoureas

The mutagenic activities of 11 N-methyl-N'-alkyl-N-nitrosoureas were tested on Salmonella typhimurium TA1535 and compared with chemical properties (alkylating activity and decomposition rate). In their relative mutagenicities the N-nitrosoureas that had a cyclic N'-alkyl groups showed far more mutagenic activity than those having a chain N;-alkyl group. M(1-A) NU and M(2-A)NU, which had the most bulky N'-alkyl group in this series, exhibited lethal effects at high concentrations. The mutagenicity showed a small positive correlation with decomposition rates but not with alkylating activities on 4-(p-nitrobenzyl)pyridine. The highest mutagenicity in this series was observed in N-methyl-N'cyclobutyl-N-nitrosourea. These results suggest that, in this series of N-methyl-N'-alkyl-N-nitrosoureas, structural differences in the N'-alkyl groups had great significance in mutagenicity.

4-Norleucine, 7-D-phenylalanine-alpha-melanocyte-stimulating hormone: a highly potent alpha-melanotropin with ultralong biological activity

alpha-Melanocyte-stimulating hormone (alpha-MSH) reversibly darkens frog skins by stimulating melanosome movement (dispersion) within melanophores. Heat-alkali treatment of alpha-MSH results in prolonged biological activity of the hormone. Quantitative gas chromatographic analysis of the hydrolyzed heat-alkali-treated peptide revealed partial racemization particularly at the 4(methionine) and 7(phenylalanine) positions. [Nle4]-alpha-MSH, a synthetic analogue of alpha-MSH, reversibly darkens frog skins and also exhibits prolonged activity after heat-alkali treatment. Synthesis of [Nle4, D-Phe7]-alpha-MSH provided an analogue with prolonged biological activity identical to that observed with heat-alkali-treated alpha-MSH or [Nle4]-alpha-MSH. [Nle4, D-Phe7]-alpha-MSH was resistant to enzymatic degradation by serum enzymes. In addition, this peptide exhibited dramatically increased biological activity as determined by frog skin bioassay, activation of mouse melanoma adenylate cyclase, and stimulation of mouse melanoma cell tyrosinase activity. This Nle4, D-Phe7 synthetic analogue of alpha-MSH is a very porent melanotropin, 26 times as potent as alpha-MSH in the adenylate cyclase assay. The resistance of the peptide to enzymatic degradation and its extraordinarily potent and prolonged biological activity should make this analogue of alpha-MSH an important molecular probe for studying the melanotropin receptors of both normal and abnormal (melanoma) melanocytes.