Synthesis and Peptidyl Group Migration in Isopropylidene Acetals Derived from the Glucose-substituted Imidazolidinone Related to Opioid Peptide Leucine-enkephalin

The reaction of the glucose-substituted imidazolidinone 1, structurally related to endogenous opioid pentapeptide leucine-enkephalin (Tyr-Gly-Gly-Phe-Leu), with acetone in the presence of acid catalyst gives 2,3-O-isopropylidene derivative 2 along with the isomeric monoacetal 3. Evidence is presented by using RP HPLC and NMR spectroscopy that, under the conditions of ketal formation, the peptidyl group migrated from the primary (2) to the adjacent secondary hydroxy group of the sugar moiety to give ester 3. The product distribution in the equilibrated reaction mixture (2 : 3 ≈ 1.5 : 1) indicated that 5-O-peptidyl derivative 2 is slightly more stable than ester 3. Peptidyl group migration in aqueous solution at pH = 5 shows slow 2 → 3 rearrangement and markedly faster 3 → 2 conversion

1H and 13C Nuclear Magnetic Resonance Studies of Leucine-Enkephalin Glucoconjugates

Synthetic leucine-enkephalin glucoconjugates (1-3) were studied by several one- and two-dimensional \u27H and 13C nuclear magnetic resonance (NMR) methods. Although all the values of the temperature coefficients of amide protons were larger than expected for protons involved in hydrogen bonds, using ROESY spectra, it was possible to detect several inter- and intraresidual nuclear Overhauser effects. The results do not support the existence of a single folded conformation, indicating that the structures of glucoconjugates 1-3 in DMSO-đe solution are not completely random. The interresidual ROESY cross-peaks 1 point to the existence of at least one con- former with two turns in 1, involving the Tyr1 and Gly2, and the Leu5 and glucose residues, while, in the glucoconjugates 2 and 3, inter-residual ROESY cross-peaks indicate the presence of a conformer with a different compact structure

CD and FTIR Spectroscopic Studies of Carbohydrate-modified Opioid Peptides

The variety of vital functions played by glycoproteins in many physiological and pathological processes inspired the design of glycopeptides and the study of the effect of glycosylation on conformation. This work reports comparative circular dichroism (CD) and Fourier transform infrared (FTIR) spectroscopic studies on linear and cyclic Amadori and imidazolidinone-type glycopeptides 3-8 in comparison with spectroscopic data of the non-modified flexible parent peptides, Leu-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH. 1) and its amide (H-Tyr-Gly-Gly-Phe-Leu-NH, 2). The CD and FTIR measurements were performed in different solvents in order to expose the structural and conformational differences caused by a keto-sugar. a rigid 5-membered imidazolidinone ring and/or cyclization. The combined application of CD and MR spectroscopy allowed to evaluate the relative amount of the extended and folded conformers of Amadori as well as imidazolidinone compounds and proved to be an effective tool for characterization of their structural features leading to a deeper understanding of their conformational behaviour

Synthesis of Trehalose-centered Dipeptide Esters

Chemical esterification of α,α-trehalose with two different dipeptide acyl donors, Boc-Phe-Met-OH and Boc-Gly-Gly-OH, gave a set of products differing in the number and site of substitution. With both dipeptides 6,6\u27-diester was isolated as the major product. In the Boc-Phe-Met-OH line, the dipeptide units in all higher substituted esters were asymmetrically distributed between the two trehalose rings. In contrast, the first higher substituted Boc-Gly-Gly-OH derived ester found was the symmetric 2,2\u27,6,6\u27-tetraester. These four sites remained occupied in all other isolated products, two asymmetric pentaesters and one symmetric hexaester. The data presented here show that in addition to sugar hydroxyl group reactivity, the structural properties of the acylating reagent, in particular its sterical demands, are of the utmost importance for the distribution and arrangement of acyl moieties around the non-reducing disaccharide core molecule

CD and FTIR Spectroscopic Studies of Carbohydrate-modified Opioid Peptides

The variety of vital functions played by glycoproteins in many physiological and pathological processes inspired the design of glycopeptides and the study of the effect of glycosylation on conformation. This work reports comparative circular dichroism (CD) and Fourier transform infrared (FTIR) spectroscopic studies on linear and cyclic Amadori and imidazolidinone-type glycopeptides 3–8 in comparison with spectroscopic data of the non-modified flexible parent peptides, Leu-enkephalin (H-Tyr-Gly-Gly-Phe-Leu-OH, 1) and its amide (H-Tyr-Gly-Gly-Phe- -Leu-NH2, 2). The CD and FTIR measurements were performed in different solvents in order to expose the structural and conformational differences caused by a keto-sugar, a rigid 5-membered imidazolidinone ring and/or cyclization. The combined application of CD and FTIR spectroscopy allowed to evaluate the relative amount of the extended and folded conformers of Amadori as well as imidazolidinone compounds and proved to be an effective tool for characterization of their structural features leading to a deeper understanding of their conformational behaviour

In Vitro Enzymatic Stabilities of Methionine-enkephalin Analogues Containing an Adamantane-type Amino Acid

The enzymatic stability of synthetic methionine-enkephalin peptide analogues containing an unnatural amino acid of the adamantane-type 3-5 was examined in human serum, at 37 °C, and compared with the results of the degradation of the parent endogenous pentapeptide 1 and the tripeptide, Tyr-Gly-Gly (2). Methionine-enkephalin (Tyr-Gly-Gly-Phe-Met, 1) and tripeptide 2 are rapidly degraded in 80 % human serum with half-lives of 12.2 and 23.0 minutes, respectively, preferably by aminopeptidase cleavage of the N-terminal Tyr-Gly peptide bond. Incorporation of the rigid and sterically hindered 1-adamantylglycine moiety into the peptide sequence resulted in increased stability of compound 3, while compounds 4a and 5a were not at all susceptible to the enzymes present in human serum. Strong binding of peptides 3-5 to human serum proteins was demonstrated

Metal-Binding Ability of Leu-Enkephalin, Related Glycoconjugates and Peptidomimetics

Both the chemistry and consequences of the nonenzymatic reaction between reducing sugars and reactive amino groups of amino acids, peptides and proteins (known as the Maillard reaction), have received considerable attention in food and health science fields. This initial reaction results in Amadori and similar products formation, followed by degradation to advanced glycation end products (AGEs). It is well established that AGEs are associated with color and odor of thermally processed or stored food, as well as with pathogen products in a number of diseases. The model systems of early stage Maillard reaction products (MRP) were prepared between endogenous opioid peptide leucine enkephalin (1) and D-glucose / D-glucuronic acid. The complexation ability of prepared MRP with metal ions (Ca2+, Zn2+, Al3+, Pb2+ and Cu2+) was investigated and compared to the complexation ability of parent peptide using ECD and FTIR spectroscopic measurements