Solid-state geometry and conformation of linear, diastereoisomeric oligoprolines

Pseudodiproline (Pro-Cyp) Oligomers Fold into Helical Polyproline Type secondary structures

The synthesis and conformational properties of oligo-proline mimetics composed of dimeric and tetrameric Pro-Cyp constructs linked by a hydroxymethylene unit are reported. Oligomers were studied both in the solid state and in solution, unveiling right-handed helical conformation depending on the configuration of the vicinally substituted trans-cyclopentane carboxylic acid unit (Cyp). Unlike polyproline oligomers, the alternating synthetic Pro-Cyp counterparts are not stabilized by n-π* interactions but rely instead on the steric demands of the extended backbone conformation within the hydroxymethylene-linked Pro-Cyp repeating units.

Helical screw-sense preferences of peptides based on chiral, Cα-tetrasubstituted α-amino acids

The preferred helical screw senses of chiral α-amino acids with a C(α)-tetrasubstituted α-carbon atom, as determined in the crystal state by X-ray diffraction analyses on derivatives and peptides, are reviewed. This survey covers C(α)-methylated and C(α)-ethylated α-amino acids, as well as α-amino acids cyclized on the α-carbon, including those characterized by the combination of lack of chirality at the α-carbon with either side-chain or axial chirality. Although, in general, chiral C(α)-tetrasubstituted α-amino acids show a less pronounced bias toward a single helical screw sense than their proteinogenic (C(α)-trisubstituted) counterparts, our analysis highlights significant differences in terms of magnitude and direction of such a bias among the various sub-families of residues, and between individual amino acids within each sub-family as well. The experimental findings can be rationalized, at least in part, on the basis of steric considerations.

Multiple, consecutive, fully-extended 2.0₅-helix peptide conformation

The peptide 2.0(5)-helix does exist. It has been experimentally authenticated both in the crystalline state (by X-ray diffraction) and in solution (by several spectroscopic techniques). It is the most common conformation for C(α)-tetrasubstituted α-amino acids with at least two atoms in each side chain, provided that cyclization on the C(α)-atom is absent. X-Ray diffraction has allowed a detailed description of its geometrical and three-dimensional (3D)-structural features. The infrared absorption and the nuclear magnetic resonance parameters characteristics of this multiple, consecutive, fully-extended structure have been described. Conformational energy calculations are in agreement with the experimental findings. As the contribution per amino acid residue to the length of this helix is the longest possible, its exploitation as a molecular spacer is quite promising. However, it is a rather fragile 3D-structure and particularly sensitive to solvent polarity. Interestingly, in such a case, it may reversibly convert to the much shorter 3(10)-helix, thus generating an attractive molecular spring.

Solution synthesis, conformational analysis, and antimicrobial activity of three alamethicin F50/5 analogs bearing a trifluoroacetyl label

We prepared, by solution-phase methods, and fully characterized three analogs of the membrane-active peptaibiotic alamethicin F50/5, bearing a single trifluoroacetyl (Tfa) label at the N-terminus, at position 9 (central region) or at position 19 (C-terminus), and with the three Gln at positions 7, 18, and 19 replaced by Glu(OMe) residues. To add the Tfa label at position 9 or 19, a γ-trifluoroacetylated α,γ-diaminobutyric acid (Dab) residue was incorporated as a replacement for the original Val(9) or Glu(OMe)(19) amino acid. We performed a detailed conformational analysis of the three analogs (using FT-IR absorption, CD, 2D-NMR, and X-ray diffraction), which clearly showed that Tfa labeling does not introduce any dramatic backbone modification in the predominantly α-helical structure of the parent peptaibiotic. The results of an initial solid-state (19)F-NMR study on one of the analogs favor the conclusion that the Tfa group is a very promising reporter for the analysis of peptaibioticmembrane interactions. Finally, we found that the antimicrobial activities of the three newly synthesized analogs depend on the position of the Tfa label in the peptide sequence.

Single and multiple peptide γ-turns: literature survey and recent progress

Published data on peptide isolated and repetitive γ-turns are reviewed. Advancements in our laboratories on these 3D-structures are also presented.

A crystal structure of an oligoproline PPII-helix, at last

The first crystal structure of an oligoproline adopting an all-trans polyproline II (PPII) helix is presented. The high-resolution structure provides detailed insight into the dimensions and conformational properties of oligoprolines that are important for, e.g., their use as "molecular rulers" and "molecular scaffolds". The structure also showed that the amides interact with each other within a PPII helix and that water is not necessary for PPII helicity.

Building a bridge between peptide chemistry and organic chemistry: intramolecular macrocyclization reactions and supramolecular chemistry with helical peptide substrates

In our ongoing efforts to build a bridge between peptide chemistry and organic chemistry, we are currently investigating: (1) two types of intramolecular macrocyclization reactions in 3(10)-helical peptides, and (2) a peptido[2]rotaxane molecular machine as a supramolecular tool using a 3(10)-helical peptide as the axle. More specifically, we studied the following two reactions: (a) the intramolecular ring-closing olefin metathesis between two amino acid residues with side chains bearing an allyl group, and (b) the intramolecular Paternò-Yang photoreaction, using a benzophenone-based amino acid as a photoaffinity reagent for a Met residue. Both reactions involve formation of a new C--C bond. As for the supramolecular system examined, we were able to identify the two stations of a new peptido[2]rotaxane characterized by an -(Aib)(6)- axle and to reversibly switch the aromatic tetramide macrocyclic wheel from one station to the next. This article summarizes the information available in the literature from other groups and the published/unpublished data originated from our laboratory on these research areas.

rac-2-Methyl-3,4,5,6-tetra-hydro-2H-2,6-methano-1,3-benzoxazocin-4-one

The title compound, C(12)H(13)NO(2), represents a conformationally restricted 2-pyridone analogue of 1,4-dihydro-pyridine-type calcium antagonists and was selected for a crystal structure determination in order to explore some aspects of drug-receptor inter-action. In the mol-ecule, two stereogenic centres are of opposite chirality, whereas a racemate occurs in the crystal. It was found that the formally aminic N atom of the heterocycle is essentially sp(2)-hybridized with the lone-pair electrons partially delocalized through conjugation with the adjacent carbonyl bond. As a result, the central pyridone ring assumes an unsymmetrical half-chair conformation. The critical 4-phenyl ring is fixed in a pseudo-axial and perpendicular orientation [dihedral angle 85.8 (1)°] with respect to the pyridone ring via an oxygen bridge. In the crystal a pair of centrosymmetric N-H⋯O hydrogen bonds connect mol-ecules of opposite chirality into a dimer. The dimers are packed by hydrophobic van der Waals inter-actions.

Total synthesis, characterization, and conformational analysis of the naturally occurring hexadecapeptide integramide A and a diastereomer

Integramide A is a 16-amino acid peptide inhibitor of the enzyme HIV-1 integrase. We have recently reported that the absolute stereochemistries of the dipeptide sequence near the C terminus are L-Iva(14)-D-Iva(15). Herein, we describe the syntheses of the natural compound and its D-Iva(14)-L-Iva(15) diastereomer, and the results of their chromatographic/mass spectrometric analyses. We present the conformational analysis of the two compounds and some of their synthetic intermediates of different main-chain length in the crystal state (by X-ray diffraction) and in solvents of different polarities (using circular dichroism, FTIR absorption, and 2D NMR techniques). These data shed light on the mechanism of inhibition of HIV-1 integrase, which is an important target for anti-HIV therapy.

Is the backbone conformation of C(alpha)-methyl proline restricted to a single region?

C(alpha)-methyl-L-proline, or L-(alphaMe)Pro, is probably the most conformationally constrained alpha-amino acid. In particular, its omega and phi torsion angles are restricted to about 180 and -60 degrees, respectively, and only three ranges of values are theoretically available for psi in mono- or longer peptides, namely, about -30 degrees (cis', 3(10)/alpha-helical structure), 60 degrees (inverse gamma turn), or 140 degrees (trans', poly(L-Pro)(n) II structure). In this work, we examined the tendency of a number of N(alpha)-acyl dipeptide N'-alkylamides of the type RCO-(alphaMe)Pro-Xxx-NHR' or RCO-Xxx-(alphaMe)Pro-NHR', in which Xxx is L (or D)-Ala, Aib (alpha-aminoisoburyric acid), or L (or D)-(alphaMe)Pro, long enough to fold into intramolecularly hydrogen-bonded gamma or beta turns. The results are compared with those obtained for the corresponding dipeptides based on Pro, a well-known turn-forming residue. For the crystal-state 3D-structural analysis we used X-ray diffraction, whereas our solution conformational analysis was heavily based on the FTIR absorption and (1)H and (13)C NMR spectroscopy techniques. We conclude that (alphaMe)Pro is able to explore both trans' and cis' psi areas of the conformational space, but in (alphaMe)Pro the latter is overwhelmingly more populated, in marked contrast to the Pro preference. This finding is a clear indication that in (alphaMe)Pro the major 3D-structural determinant is the C(alpha)-methyl group. The circular dichroism (CD) signature of a peptide type III' beta-turn conformation is also proposed.

Can N-methylated amino acids serve as substitutes for prolines in conformational design of cyclic pentapeptides?

The incorporation of proline into cyclic peptides seems to be the most promising way to induce beta-turn structures. Recently, however, it was shown that N-methylated amino acids might be even better suited than proline for introducing turn structures. Another property of proline, the ability to effect cis-peptide bonds, has also been reported for N-methylated amino acids. These findings raise the question if it might be possible to replace a proline by an N-methylated amino acid without altering the desired conformational features. The most important benefit of replacing proline by an N-methylated residue is that one recovers the side-chain functionalities, which could be used for enhancing binding selectivity, or to tune a cyclic peptide concerning its pharmacological properties.Here, we compare cyclic peptides containing one or two prolines or N-methylated alanines and a combination of both with respect to preferred conformations and cis-peptide bonds. In addition, the positions have been investigated where an N-alkylated amino acid has to be incorporated to mimic structural aspects usually introduced by proline residues.

Cα-Methyl proline: A unique example of split personality

Methylation at the C(alpha)-position of a Pro residue was expected to lock the preceding tertiary amide (omega) torsion angle of the resulting (alphaMe)Pro to the trans disposition and to restrict the phi,psi surface to the single region where the 3(10)/alpha-helices are found (in this five-membered ring residue phi is severely constrained to about +/-65 degrees by its cyclic nature). The results of the present X-ray diffraction work on a selected set of four N(alpha)-blocked, (alphaMe)Pro-containing, dipeptide N'-alkylamides clearly show that, although the region of the conformational map largely preferred by (alphaMe)Pro would indeed be that typical of 3(10)/alpha-helices, the semi-extended [type-II poly(Pro)(n) helix] region can also be explored by this extremely sterically demanding C(alpha)-tetrasubstituted alpha-amino acid. In addition, the known high propensity for beta-turn formation of the Pro residue is further enhanced in peptides based on its C(alpha)-methylated derivative.

Peptide helices based on alpha-amino acids

Relevant parameters and stereochemical consequences of helices [alpha-helix, 3(10)-helix, beta-bend ribbon spiral, gamma-helix, 2.0(5)-helix, poly(Pro)(n) type-I and -II helices, and collagen triple helix] of peptides based on alpha-amino acids for use as templates in various branches of chemistry are briefly discussed.

Laminated morphology of nontwisting beta-sheet fibrils constructed via peptide self-assembly

A synthetic peptide has been de novo designed that self-assembles into beta-sheet fibrils exhibiting a nontwisted, stacked morphology. The stacked morphology is constituted by 2.5 nm wide filaments that laterally associate to form flat fibril laminates exceeding 50 nm in width and micrometers in length. The height of each fibril is limited to the length of exactly one peptide monomer in an extended beta-strand conformation, approximately 7 nm. Once assembled, these highly ordered, 2-D structures are stable over a wide range of pH and temperature and exhibit characteristics similar to those of amyloid fibrils. Furthermore, the rate of assembly and degree of fibril lamination can be controlled with kinetic parameters of pH and temperature. Finally, the presence of a diproline peptide between two beta-sheet-forming strands in the peptide sequence is demonstrated to be an important factor in promoting the nontwisting, laminated fibril morphology.

Molecular spacers for physicochemical investigations based on novel helical and extended peptide structures

A proper understanding of the detailed nature and mechanism of physicochemical interactions depends heavily upon our ability to design and synthesize conformationally constrained 3D structures whose intercomponent geometry (either rigorously rigid or able to undergo destructuration, if required, but in all cases precisely tunable) would be well defined. To this end we have recently reported a few initial studies and we are currently working on the exploitation of stable, short, helical peptide spacers based on achiral and/or chiral Calpha-tetrasubstituted alpha-amino acids. These building blocks are known to force the peptides either to predominantly fold into a 3(10)-helical structure or to adopt a fully extended, planar 2.0(5)-helix. The systems under investigation involve a donor and an acceptor moiety linked to the N- and C-termini of the oligopeptide spacer main chain. By increasing the number of intervening residues the donor.acceptor separation can be easily modulated. This review highlights details of these two novel peptide secondary structures and their use as molecular spacers in physicochemical investigations.

N-benzhydryl-glycolamide: the first protecting group in peptide synthesis with a strong conformational bias

We have synthesized and examined the preferred conformation of a set of N-benzhydryl-glycolamide esters from N(alpha)-protected (or N(alpha)-blocked) alpha-amino acids. Experiments were performed in CDCl(3) solution by Fourier transform infrared absorption and (1)H-NMR techniques, and in the crystalline state by x-ray diffraction. The results of our analysis strongly support the view that this type of N(alpha)-acylated alpha-aminoacyl esters has a marked tendency to fold into a beta-turn conformation, the nature of which is dictated by the structural propensity of the amino acid constituent at the i+1 position.

Non-hydrogen-bonded secondary structure in beta-peptides: evidence from circular dichroism of (S)-pyrrolidine-3-carboxylic acid oligomers and (S)-nipecotic acid oligomers

[formula: see text] Homooligomers of beta-amino acids (S)-3-pyrrolidine-3-carboxylic acid (PCA) and (S)-nipecotic acid (Nip) were studied by circular dichroism (CD) in methanol. In each series, a profound change in the far-UV CD spectrum was observed from monomer to tetramer, but little change was observed from tetramer to hexamer. A comparable pattern is observed in the CD spectra of short proline oligomers. We conclude that both PCA and Nip oligomers with > or = four residues adopt a characteristic secondary structure.

Conformational study of linear alternating and mixed D- and L-proline oligomers using electronic and vibrational CD and Fourier transform IR

Vibrational CD (VCD) spectra of a series of blocked linear, alternating D- and L-proline containing oligopeptides, dissolved in D2O and in CDCl3, are reported. For the Boc-LDL-Pro3 to Boc-DLDLDLDL-Pro8 oligomers, the VCD spectra in the amide I band is a positive couplet, opposite in sense to that obtained for (L-Pro)n oligomers. While this admits the possibility of their favoring a right-handed helical chain conformation, the amide I ir spectra for these DL oligomers in D2O indicate a mixed, apparently alternate, cis-trans conformation that prevents a simple conclusion. Their VCD in D2O evidence no narrowing and has a progressive loss in intensity (measured as delta A/A) with an increase in chain length. In CDCl3 a similar pattern of positive VCD couplets decreasing in intensity with length was seen, but the ir spectra are narrower. Their electronic CD (ECD), in the uv, also indicates a loss in intensity with increasing length. Oligomers with odd or even numbers of Pro residues have different ECD patterns, indicating that those spectra are strongly influenced by local contributions arising in the N-terminal groups. The VCD arises from dipolar and vibrational coupling of the amides in the helical structure. All the spectra are consistent with the chiral end groups leading to formation of an excess of one helical handedness. With an increase in length, the influence of this selectiveness is less and the overall CD measured decreases.

First characterization at atomic resolution of the C-activating groups in a peptide synthesis acid chloride, acid azide and carboxylic-carboxylic mixed anhydride

The X-ray diffraction structures of three N alpha-protected, C alpha-activated MeAib derivatives are reported. They are Tos-MeAib-Cl, Tos-MeAib-N3 and Tos-MeAib-O-Piv. The geometry and conformation of these classical carboxyl activating groups, which have been characterized at atomic resolution for the first time, are discussed.

Three-dimensional molecular modeling of bovine caseins: a refined, energy-minimized kappa-casein structure

A refined three-dimensional molecular model of kappa-casein has been produced using energy minimization techniques and a Kollman force field on a previously reported predicted three-dimensional structure. This initial model was constructed via molecular modeling techniques from sequence-based secondary structural prediction algorithms. Both the initial and refined structures agreed with global secondary structure analysis from vibration spectroscopy. The refined structure contained many of the features of the initial model, including two sets of antiparallel beta-sheet structures containing predominantly hydrophobic side chains, which could form interaction sites with alpha s1-casein. Two types of energy-minimized dimer and tetramer models are presented: 1) using Cys as potential intermolecular disulfide binding sites and 2) using the two sheets as possible hydrophobic self-association sites, without Cys interactions. All structures yielded good stabilization energies and are in agreement with chemical, biochemical, and physical chemical results obtained for kappa-casein.

Three-dimensional molecular modeling of bovine caseins: an energy-minimized beta-casein structure

To obtain a molecular basis for the similarities and dissimilarities in the functional, chemical, and biochemical properties between beta-casein and the other caseins, a predicted three-dimensional model is presented. The predicted structure was assembled using molecular modeling techniques, as well as secondary structural prediction algorithms, in conjunction with global secondary structural information from Raman spectroscopy. To add validity to this model, the structure was refined using energy minimization techniques to diminish the likelihood of structural overlaps and energetically unfavorable van der Waals contacts arising from the large number of proline residues present in the beta-casein sequence. The refined model overall showed a loosely packed, asymmetrical structure with an axial ratio of 2:1. Hydrophobic side chains were uniformly dispersed over one end (C terminal) and the center surface of the structure; the other end (N terminal) was hydrophilic. The hydrophobic section also possessed a large loop through which water could easily travel. Such a suprasurfactant structure could account for the micellar type of hydrophobically driven self-association exhibited by beta-casein. Other chemical and biochemical data are in good agreement with the refined structure.

Alpha-beta-dehydro-amino acid residues in the design of peptide structures. Molecular and crystal structures of two folded dehydro peptides

The molecular and crystal structures of two N alpha-protected tripeptide amides, containing in the central position the alpha-beta-dehydro-amino acid residue delta Phe (Z-configurational isomer), were determined by X-ray diffraction. While Z-Gly-delta Phez-L-Pro-NH2 is characterized in the crystal state by the presence of a type I beta-bend conformation (at the delta Phez-L-Pro sequence), Z-D-Ala-delta Phez-Gly-NH2 is folded into two consecutive beta-bends (type II' followed by type I), at the D-Ala-delta Phez and delta Phez-Gly sequences, respectively. In both cases the achiral delta Phez residue adopts a set of phi, psi angles typical of the right-handed helical conformation. The delta Phe residue may be exploited to design aromatic peptides with preferred secondary structures.

Stereochemistry of peptides containing 1-aminocycloheptane-1-carboxylic acid (Ac7c)

The crystal structures of four peptides incorporating 1-aminocycloheptane-1-carboxylic acid (Ac7c) are described. Boc-Aib-Ac7c-NHMe and Boc-Pro-Ac7c-Ala-OMe adopt beta-turn conformations stabilized by an intramolecular 4----1 hydrogen bond, the former folding into a type-I/III beta-turn and the latter into a type-II beta-turn. In the dipeptide esters, Boc-Aib-Ac7c-OMe and Boc-Pro-Ac7c-OMe, the Ac7c and Aib residues adopt helical conformations, while the Pro residue remains semi-extended in both the molecules of Boc-Pro-Ac7c-OMe found in the asymmetric unit. The cycloheptane ring of Ac7c residues adopts a twist-chair conformation in all the peptides studied. 1H-NMR studies in CDCl3 and (CD3)2SO and IR studies in CDCl3 suggest that Boc-Aib-Ac7c-NHMe and Boc-Pro-Ac7c-Ala-OMe maintain the beta-turn conformations in solution.

Three-dimensional molecular modeling of bovine caseins: kappa-casein

Three-dimensional structures derived from X-ray crystallography are extremely important in elucidating relationships between structure and function for many proteins. However, not all proteins can be crystallized. The caseins of bovine milk are one class of noncrystallizable proteins. The complete primary and partial secondary structures of these proteins are known, but homologous proteins with known crystallographic structure are not available. In this report, sequence-based predictions of secondary structure were made and adjusted to conform with global secondary structures derived from Fourier transform infrared spectroscopy. With this information, a three-dimensional structure for kappa-casein was constructed using molecular modeling computer programs. The constructed model contains two unstranded beta-sheets; both are predominantly hydrophobic and capable of forming quaternary structural interaction sites with alpha s1-casein. This unrefined structure is in good agreement with much of the biochemical information available for kappa-casein.

Three-dimensional molecular modeling of bovine caseins: alpha s1-casein

Structures derived from X-ray crystallography are extremely important in elucidating functional relationships for many proteins. However, the caseins of bovine milk are one class of noncrystallizable proteins. The complete primary and partial secondary structures of these proteins are known, but homologous proteins of known crystallographic structure cannot be found. Therefore, sequence-based predictions of secondary structure were made and adjusted to conform with global secondary structures determined by Raman spectroscopy. With this information, a three-dimensional structure for alpha s1-casein was constructed using molecular modeling programs. The predicted structure of alpha s1-casein contains a hydrophobic and a hydrophilic domain, which are connected by a segment of alpha-helix. This unrefined structure shows good agreement with global biochemical and chemical information concerning alpha s1-caseins A, B, and C.

Conformationally restricted peptides through short-range cyclizations

The various types of conformationally restricted peptides obtained by short-range cyclizations, from residue i to residue i + 1, are presented. Relevant examples of N in equilibrium C alpha, C' in equilibrium C alpha, N in equilibrium C', C alpha in equilibrium C alpha, C' in equilibrium C', and N in equilibrium N cyclizations are reported and the pertinent literature listed. In the discussion emphasis is place on the conformational consequences for peptides from the incorporation of such ring structures.

Structural versatility of peptides containing C alpha, alpha-dialkylated glycines. An X-ray diffraction study of six 1-aminocyclopropane-1-carboxylic acid rich peptides

The molecular and crystal structures of six fully blocked, Ac3c-rich peptides to the tetramer level were determined by X-ray diffraction. The peptides are Fmoc-(Ac3c)2-OMe-CH3OH, Ac-(Ac3c)2-OMe, t-Boc-Ac3c-L-Phe-OMe, pBrBz-(Ac3c)3-OMe.H2O, Z-Gly-Ac3c-Gly-OTmb.(CH3)2CO, and t-Boc-(Ac3c)4-OMe.2H2O. Type-I (I') beta-bends and distorted 3(10)-helices were found to be typical of the tri- and tetrapeptides, respectively. In the dipeptides, too short to form beta-bend conformations, other less common structural features may be observed. The average geometry of the cyclopropyl moiety of the Ac3c residue is asymmetric and the N-C alpha-C' bond angle is significantly expanded from the regular tetrahedral value. A comparison with the structural preferences of other extensively investigated C alpha, alpha-dialylated alpha-amino acids is made and the implications for the use of the Ac3c residue in conformational design are examined.

Crystal-state structural analysis of two gamma-lactam-restricted analogs of Pro-Leu-Gly-NH2

The crystal structures of two analogs of Pro-Leu-Gly-NH2 (1), containing a gamma-lactam conformational constraint in place of the -Leu-Gly- sequences, are described. The highly biologically active (S,R)-diastereomer 2a is semi-extended at the C-terminus, with the N-terminal Pro residue in the unusual "C5" conformation [psi 1 = -0.8(15) degrees] stabilized by a (peptide)N-H...N(amino) intramolecular H-bond [the N(3)...N(4) separation is 2.687(11)A]. Conversely, the N,N'-isopropylidene aminal trihydrate of the (S,S)-diastereomer 2b, compound 3, adopts a beta-bend conformation at the C-terminus, as already reported for 1. However, the backbone torsion angles [phi 2 = 57.4(4), psi 2 = -129.9(3) degrees; psi 3 = -92.3(4), phi 3 = 6.4(5) degrees] lie close to the values expected for the corner residues of an ideal type-II' beta-bend. A weak intramolecular 4----1 H-bond is seen between the Gly carboxyamide anti-NH and Pro C = O groups. In the newly formed 2,2,3,4-tetraalkyl-5-oxo-imidazolidin-1-yl moiety the psi 1 torsion angle is 12.9(4) degrees and the intramolecular N(3)...N(4) separation is 2.321(4)A.

Structure of conformationally constrained peptides: from model compounds to bioactive peptides

The use of backbone conformational constraints has acquired increasing importance in the design and synthesis of structurally restricted agonists and antagonists of bioactive peptides. Here I discuss the preferred conformations of four among the most popular types of such peptide surrogates: (a) Peptides from C alpha, alpha-dialkylated residues, (b) tetrazolyl peptides, (c) (gamma- and delta-) lactam-containing peptides, and (d) thiated peptides. Emphasis is given to conformational energy computations and x-ray diffraction analyses of selected model compounds and analogues of small bioactive peptides such as the formylmethionyl tripeptide chemoattractant and MIF.

Isolation of tryptic peptides of myelin basic protein by reversed-phase high-performance liquid chromatography

A reversed-phase high-performance liquid chromatography (HPLC) system was developed to obtain individual tryptic peptides of myelin basic protein (BP). Because of the similar charge and hydrophobicity of some of the tryptic peptides of the whole protein, several of these were not clearly separated by a single HPLC system. Therefore, the BP was first cleaved specifically between residues 97 and 98 with thrombin, and the two resulting fragments were separated by ion-exchange chromatography. When the thrombic fragments were digested with trypsin separately and subjected to HPLC, all of the peptides were satisfactorily separated. Elution times of all of the tryptic peptides of human BP were established. Differences among homologous peptides, derived from different mammalian BPs, were readily detected from their elution patterns inasmuch as a change in a single amino acid residue was usually sufficient to cause a shift in the retention time of the peptide. An amino acid difference detected by a peak shift could be confirmed by amino acid analysis. The technique has been used to isolate short peptides of rabbit, monkey, porcine, bovine, and human BP for sequence analysis.