Novel anoplin-based (lipo)-peptide models show potent antimicrobial activity

The subject of this study is the synthesis and biological evaluation of anoplin-based (Gly-Leu-Leu³ -Lys-Arg⁵ -Ile-Lys-Thr⁸ -Leu-Leu-NH₂ )-designed (lipo)-peptides, aiming at the development of new antibiotic substances. The design of synthetic compounds based on natural bioactive molecules is an optimistic strategy for the development of new pharmaceutics. Antimicrobial peptides (AMPs) and (lipo)-peptides are two classes of promising compounds, with characteristics that allow them to express their activity by differentiated mechanisms of action. On this basis, anoplin, a natural AMP, was used as a scaffold to design five peptides and seven lipopeptide analogs of them. Substitutions were made on residues Leu³ and Arg⁵ of the interphase and on Thr⁸ of the polar phase, as well as N-terminus conjunctions with octanoic and decanoic acid. The outcome of the biological evaluation revealed that some analogs might have substantial clinical potential. Specifically, Ano 1-F, Ano 3-F, Ano 4-C₁₀ , and Ano 5-F are strongly active against Gram-negative bacteria at minimum inhibitory concentration (MIC) values of 3 μg/ml, while Ano 4-F is active against Gram-positive bacteria at 1 μg/ml. Ano 2-C₁₀ , C₁₀ -Gly-Leu-Lys³ -Lys-Ile⁵ -Ile-Lys-Lys⁸ -Leu-Leu-NH₂ , is the most promising compound (MIC = 0.5 μg/ml) for the development of new pharmaceutics. The conformational features of the synthetic peptides were investigated by circular dichroism spectroscopy, and their physicochemical parameters were calculated. Our study shows that appropriate substitutions in the anoplin sequence in combination with N^α -acylation may lead to new effective AMPs.

HLA-DQ7 beta(1) and beta(2) derived peptides as immunomodulators

Modulation of protein-protein interactions involved in the immune system by using small molecular mimics of the contact interfaces may lead to the blockage of the autoimmune response and the development of drugs for immunotherapy. The nonpolymorphic beta-regions, exposed to the microenvironment, of the modeled HLA-DQ7, which is genetically linked to autoimmune diseases, were determined. Peptides 132-141 and 58-67, located at the beta(1) and beta(2) domains of HLA-DQ7, respectively, were tested for their involvement in the interactions with CD4(+) T lymphocytes. Linear, cyclic, and dimeric analogs that mimic the exposed surfaces of HLA-DQ7 were designed and synthesized. Their immunosuppressory activities, found in the secondary, humoral immune response to sheep erythrocytes (SRBC) in mice in vitro, ranged from 11% to 53%. The significance of the total charge of the peptides, the pattern of the hydrogen bonding, and the presence of secondary structure were investigated in relation to the immunomodulatory effect of the peptides. Two dimeric analogs of the HLA-DQ7 58-67 fragment, consisting of the two monomers covalently linked by a polyethylene glycol (PEG) spacer, able to mimic the superdimers, were also synthesized and studied. As the 58-67 segment is located at the beta(1) region of HLA-DQ7, close to the major histocompatibility complex (MHC) groove, one may assume that the 58-67 peptide could accommodate the association between T-cell receptor (TCR) and human leukocyte antigen (HLA) by activating a co-stimulatory molecule of the TCR/HLA interaction. This hypothesis is supported by the confocal laser image of the fluorescein-labeled 58-67 peptide and by the fact that it is an immunostimulator at low concentration.

A palmitoylated peptide, derived from the acidic carboxyl-terminal segment of the integrin alphaIIb cytoplasmic domain, inhibits platelet activation

Platelet integrin alpha(IIb)beta(3) contains an acidic membrane distal motif, 1000LEEDDEEGE1008, in the cytoplasmic domain of the alpha(IIb) subunit. We showed that a lipid-modified peptide corresponding to the above region, palmitoyl-K-LEEDDEEGE (pal-K-1000-1008), is platelet permeable and has inhibited platelet aggregation induced by 0.4 U/ml of thrombin (IC50 = 164 microM). Moreover the peptide inhibited both Fibrinogen and PAC-1, binding to activated platelets. The non palmitoylated analog was inactive. A modified, scrambled acidic peptide (palmitoyl-K-GDDEELEEE), showed significant lower inhibitory activity than pal-K-1000-1008. A palmitoylated peptide corresponding to the membrane proximal cytoplasmic domain of alpha(IIb), 989KGVFFKR995 (pal-989-995), is known to specifically induce platelet aggregation. Pal-K-1000-1008 was an inhibitor of human washed platelet aggregation induced by pal-K-989-995 (IC50 = 15 microM). Moreover, pal-K-1000-1008 inhibited phosphorylation of ERK and FAK, two protein kinases involved in platelet activation and aggregation. Our results favour the assumption that the interaction of the membrane proximal sequence 989KGVFFKR995 of the cytoplasmic domain of alpha(IIb) with the acidic terminal 1000LEEDDEEGE1008 motif may be an important structural factor in platelet signaling, leading to platelet activation and aggregation.

A complementary La/SSB epitope anchored to Sequential Oligopeptide Carrier regulates the anti-La/SSB response in immunized animals

Complementary peptide epitopes, derived from complementary RNA sequences, have been used for suppressing the autoimmune response in experimental autoimmune diseases as myasthenia gravis, allergic neuritis and allergic encephalomyelitis. Aiming at contributing to the development of a tool that could regulate the autoantibody production against La/SSB, which is the main target of autoantibodies in Sjogren's syndrome (SS) and systemic lupus erythematosus (SLE), the complementary epitope, cpep349-364, of the minor T/major B cell epitope of La/SSB, pep349-364, was utilized for the induction of neutralizing anti-cpep349-364 antibodies in rabbit immunizations. Complementary peptides were coupled to an artificial carrier, developed in our laboratory, in order to enhance the complementary potency of cpep349-364 and its counterpart. This carrier, named Sequential Oligopeptide Carrier, SOC(n), formed by the repeating tripeptide Lys-Aib-Gly, adopts helical conformation, which allows the anchored peptide epitopes to preserve their initial reactivity such as molecular recognition, antigenicity/immunogenicity. Our study provides proof of evidence of specific interactions between idiotypic (Id)/anti-idiotypic (anti-Id) antibodies generated in immunized animals by the sense epitope (conjugate I) of La/SSB and its complementary counterpart (conjugate II). It was also demonstrated that the Id/anti-Id association is specifically disrupted by adding either the sense epitope (conjugate I) or its complementary counterpart (conjugate II). A mutual neutralization of Id/anti-Id antibodies was observed in vivo, which implies that generation of anti-Id antibodies by immunization with the complementary La/SSB epitope could scavenge the anti-La/SSB response.

A palmitoyl-tailed sequential oligopeptide carrier for engineering immunogenic conjugates

The main guideline in designing effective immunogens as vaccine candidates capable of eliciting potent and specific immune responses is to combine B/T cell epitopes and adjuvants as immunostimulators on the same carrier that links the major histocompatibility complex with T cell receptors. Aiming at contributing to the development of carriers for human usage a helicoid type sequential oligopeptide carrier, SOC(n)-II, formed by the repeating tetrapeptide unit (Aib-Lys-Aib-Gly)(n), n=2-7, elongated from the amino-terminus by the palmitoyl group, known for its adjuvanticity, is now presented. The main B cell epitope, PPGMRPP, of the Sm autoantigen against which the majority of antibodies in patients with systemic lupus erythematosus is directed, was coupled to the Lys-N(epsilon)H(2) groups of the carrier in four copies and the resulting conjugate Palm-SOC(4)-II-Sm(4) was subjected to animal immunizations without utilizing any adjuvant. The induced immune response was comparable with that produced when Ac-SOC(4)-II-Sm(4) was administered in animals following the conventional immunization protocol of complete/incomplete Freund's adjuvant. High titers of anti-Palm-SOC(4)-II-Sm(4) antibodies were generated, which recognize the priming immunogenic conjugate, as well as reconstituted Sm mimics but not the carrier alone. It is concluded that Palm-SOC(n)-II carrier is a valuable tool for engineering immunogens eliciting enhanced and specific humoral immune responses.

Collagen models as a probe in the decay of works of art: synthesis, conformation and immunological studies

Proteinaceous substances such as collagen, casein and albumin have been widely used as binding media in a variety of works of art. Damages of these 'sensitive' materials, mainly caused of the influence of the environment, are responsible for the overall decay of works of art, and their identification is essential to understand ancient technologies, determine the extent of deterioration and help in future restoration and preservation processes. The most commonly used techniques for the identification of proteinaceous binding media are staining techniques, chromatography, spectrometry and immunological methods, although for the latter no systematic studies have been carried out until now. Aiming at contributing to the development of a reliable and reproducible immunoassay for the evaluation of the collagen-based decay of works of art, sequential polypeptides (Pro-X-Gly)n where X represents amino acid residues Val, Lys, Glu and (Hyp-Val-Gly)n were prepared as models of collagen fragments derived from artificially and naturally aged animal collagens. Conformational studies of the polypeptides by CD revealed the occurrence of polyproline II-like structures comparable with those of collagen. Polypeptides and collagen I were administered to animals, and the induced antibodies were used for the immunochemical detection and differentiation of collagen and collagen fragments. The combined application of (i) anti-collagen antibodies, which strongly interact with native collagen, but poorly recognized by artificially aged collagen and (ii) anti-polypeptide antibodies, which do not associate with native collagen, but are strongly recognized by collagen fragments in naturally or artificially aged collagen, is a valuable tool in determining the extent of decay in works of art.

Artificial carriers: a strategy for constructing antigenic/immunogenic conjugates

The rational design of artificial carriers for anchoring multiple copies of B and/or T cell epitopes, built-in vaccine adjuvants and "promiscuous" T cell epitopes for the construction of conjugates as antigenic substrates or potent immunogens has been the stimulus of intensive efforts nowadays. The unambiguous composition, the reliability and the versatility of the production of reconstituted antigens or immunogens has found a great number of biochemical applications in developing immunoassays of high sensitivity, specificity and reproducibility and in generating site-specific antibodies for usage as human vaccine candidates. In this review are summarized different types of artificial carriers currently used as dendrimers bearing branching segments, multimeric core matrices and templates with built-in folding devices. Emphasis is given to the construction and application of a helicoid-type Sequential Oligopeptide Carrier (SOCn) developed in our laboratory. The beneficial structural elements of SOCn induce a favorable arrangement of the conjugated peptides, which also retain their initial "active" conformation, so that potent antigens and immunogens are generated.

Phosphorylated and nonphosphorylated epitopes of the La/SSB autoantigen: comparison of their antigenic and conformational characteristics

La/SSB phosphoprotein is the target antigen of autoantibodies in sera of patients with Sjögren's syndrome (SS) and systemic lupus erythematosus (SLE). Among other structural and function motifs, four phosphorylation sites are encompassed in the primary sequence of La/SSB. Two of them (Thr-362 and Ser-366) are located within GSGKGKVQFQGKKTKFASDD (346-368) and one (Thr-302) within VTWEVLEGEVEKEALKKI (301-318), which are main B-cell epitopes of La/SSB. With the aim to investigate how phosphorylation, one of the most common posttranslational protein modifications, affects the antigenic and conformational characteristics of the La/SSB epitopes, we synthesized and studied the phosphorylated epitopes La/SSB(346-368)-P, La/SSB(359-368)-P, and La/SSB(301-318)-P with respect to their nonphosphorylated counterparts. Anti-La/SSB positive sera from SS and SLE patients are better recognized by the phosphorylated epitopes compared to their nonphosphorylated counterparts. Conformational analysis by (1)H nuclear magnetic resonance spectroscopy and molecular dynamics showed that the phosphorylated epitopes adopt different structural characteristics from those of the corresponding nonphosphorylated epitopes. It is concluded that phosphorylation can create neoepitopes with altered functions, compared to the nonphosphorylated epitopes, which might be seen from the immune system as "foreign."

A peptide carrier with a built-in vaccine adjuvant: construction of immunogenic conjugates

A multifunctional carrier combining B/T cell epitopes (i), a built-in vaccine adjuvant (ii), and a universal T cell epitope (iii) for the construction of potent and specific immunogenic conjugates is presented. The IL-1beta(163-171) fragment known to reproduce the immunostimulatory and adjuvant effects of the whole IL-1beta without possessing any of the pro-inflammatory properties of IL-1beta was covalently anchored to the N-terminus of the Sequential Oligopeptide Carrier, SOC(n), formed by the repeating tripeptide unit Lys-Aib-Gly. A promiscuous T cell epitope derived from the tetanus toxin, TT(593-599), was also positioned in the carboxy terminus of SOC(n) as a universal immunogen to provide broad immunogenicity. Selected B/T cell epitopes from the Sm and La/SSB autoantigens, against which is directed the humoral autoimmunity in patients with systemic lupus erythematosus and Sjögren's Syndrome, respectively, were coupled to the Lys-N(epsilon)H2 groups of the carrier, and the formulated constructs were administered in animals following the conventional immunization protocol of complete/incomplete Freund's adjuvant. The induced immune responses were compared with that produced when the Sm- and La/SSB-reconstituted immunogenic conjugates were injected alone. High titers of specific antibodies recognizing the priming construct, as well as the cognate autoantigen, were obtained when administered alone without the assistance of Freund's adjuvant. It is concluded that our approach provides the conceptual and experimental framework for the development of multifunctional immunogenic conjugates eliciting enhanced, specific, and prolonged humoral response for usage as human vaccine candidates.

Innovative, Multifunctional Sequential Oligopeptide Carriers Socn-I and SOCn-II: Functions-Technology-Perspectives

An innovative type of multifunctional helicoid artificial carriers, formed by the repetitive Lys-Aib-Gly (SOC(n)-I) or by the Aib-Lys-Aib-Gly (SOC(n)-II), with structural rigidity and regularity were successfully applied in our laboratory for anchoring antigenic/immunogenic peptides. The carriers, designed to display a predetermined 3D structure, adopt the 3(10) helical conformation, while the attached peptides preserve their original "active" conformation. The constructed conjugates were used as substrates in solid phase immunoassays, as well as for generating potent and specific immune responses.

Zinc Ion Dependent B-Cell Epitope, Associated with Primary Sjogren's Syndrome, Resides within the Putative Zinc Finger Domain of Ro60kD Autoantigen: Physical and Immunologic Properties

The Ro/La ribonucleoprotein (RNP) complex is composed of the proteins Ro60kD, Ro52kD, and La48kD that are in association with one small cytoplasmic RNA (YRNA). Specific protein-RNA and protein-protein interactions are thought to occur through the RNP and zinc-finger secondary structure elements of the Ro60kD protein. The aim of our study was to investigate the antigenic properties of the zinc finger domain of the Ro60KD autoantigen and its contribution to the formation of Ro/La RNP complex. It was found that the peptide VSLVCEKLCNEKLLKKARIHPFHILIA (Zif-1), which corresponds to the natural sequence of the zinc finger domain (301-327), and the peptide C(Acm)NEKLLKKARIC(Acm), analogous to the intermediate loop 310-319 (Zif-3) of the same domain of Ro60KD, are recognized by the majority of anti-Ro/SSA and anti-La/SSB positive sera (82.6% and 77.1%, respectively) in the absence of zinc ions. The same sera failed to react with Zif-1 peptide in the presence of Zn2+. In contrast, the addition of zinc ions was necessary for the binding of Zif-1 to recombinant Ro52KD as shown by direct binding experiments of the recombinant protein with synthetic peptides. Our data suggest the zinc finger domain of Ro60kD contains a B-cell epitope with high specificity for primary Sjogren's syndrome. Furthermore, depending on the presence of zinc ions, the zinc finger domain of the Ro60KD protein can exist in two different conformational states favoring either an interaction with the Ro52KD protein or binding with autoantibodies.

A three-residue cyclic scaffold of non-RGD containing peptide analogues as platelet aggregation inhibitors: design, synthesis, and structure--function relationships

Antagonists of fibrinogen at the GPIIb/IIIa receptor, which is the most abundant membrane protein on the platelet surface, are under active investigation as potential antithrombotics. The critical interaction between GPIIb/IIIa and fibrinogen can be inhibited by either linear or cyclic RGDS-containing peptides, which have been proved as lead compounds in the design of platelet aggregation inhibitors. In this study we present the design and construction of a new class of cyclic (S,S) non-RGD containing peptide sequences, using two Cys as a structural scaffold for the development of antiaggregatory agents. The (S,S)-CDC- sequence was incorporated as a conformational constraint, in molecules bearing at least one positive charge with the general formula (S,S)XCDCZ, where X = Ac-Arg, Pro-Arg, Pro-Ser-Lys, and Pro-Ser-Arg, and Z = -NH(2) and Arg-NH(2). Investigation of the structure-function relationships was performed on the basis of (a) the local conformation induced by the (S,S)-CDC motif, (b) the distance of the positively (R-C(zeta) or K-N(zeta)) and negatively (D-C(gamma)) charged centers, (c) the presence of a second positive or negative charge on the molecule, and (d) the orientation of the basic and acidic side chains defined by the pseudo dihedral angle (Pdo), which is formed by the R-C(zeta), R-C(alpha), D-C(alpha), and D-C(gamma) atoms in the case of (S,S)-RCDC and by the K-N(zeta), K-C(alpha), D-C(alpha), and D-C(gamma) atoms in the case of (S,S)-KCDC.

A diepitopic sequential oligopeptide carrier (SOCn) as mimic of the sm autoantigen: synthesis, conformation and biological assays

Anti-Sm (Sm: U1-U6 RNA-protein complex) antibodies are usually considered highly specific for systemic lupus erythematosus (SLE), while anti-U1RNP (U1RNP: U1RNA-protein complex) are thought of as diagnostic criteria for the mixed connective tissue disease (MCTD). However, both antibody specificities coexist in SLE and MCTD, in varying percentages. Although the anti-Sm/anti-U1RNP immunological cross-reactivity has been initially attributed to a common motif, PPXY(Z)PP (where X, Y, Z are various amino acids), found in the Sm, U1-A and U1-C autoantigens, it appears that the conformational features of the Sm epitopes also play an important role in the immunoreactivity. The PPGMRPP and PPGIRGP main epitopes of the Sm antigen were coupled in duplicate to the tetrameric Ac-(Lys-Aib-Gly)4-OH, SOC4, carrier to form the [(PPGMRPP)2, (PPGIRGP)2]-SOC4 construct as a mimic of the native Sm. It was found that: (i) the 3(10) helical structure of SOC4 allows the epitopes to adopt an exposed orientation, similar to their free forms, that facilitates their recognition from the anti-Sm antibodies, and (ii) the U1-RNP cross-reactivity is minimized.

B-Cell epitope mapping of DNA topoisomerase I defines epitopes strongly associated with pulmonary fibrosis in systemic sclerosis

We hypothesized that B-cell epitope mapping of DNA Topoisomerase I (type-I topoisomerase, or Topo I) may define epitopes strongly associated with pulmonary interstitial fibrosis (PIF) in systemic sclerosis (SSc). B-cell epitope mapping of Topo I was performed using 63 20-mer peptides overlapping by eight residues and spanning the entire length of the Topo I sequence. These peptides, coupled to polystyrene pins, were tested for antibody binding by enzyme-linked immunosorbent assays (ELISAs) using immunoglobulin G fractions from anti-Topo I, anticentromere, anti-U3RNP-positive, and normal sera. Four major epitopes were recognized by anti-Topo I sera, but not from the control sera: WWEEERYPEGIKWKFLEHKG (205-224, epitope I), RIANFKIEPPGLFRGRGNHP (349-368, epitope II), PGHKWKEVRHDNKVTWLVSW (397-416, epitope III), and ELDGQEYVVEFDFLGKDSIR (517-536, epitope IV). Peptide-epitopes were then synthesized in their soluble forms and ELISA systems were developed. Epitopes II to IV are localized at highly exposed sites of the Topo I tertiary structure, whereas epitope I is localized at a less accessible site. In a cohort of 81 patients with SSc with clinical data on the evolution of their disease, patients with antibodies in their sera recognizing at least three of the four epitopes had 3.1 times (P = 0.02) the hazard of developing PIF compared with patients whose sera recognized no epitopes or only one or two of the four epitopes. The discrimination was much stronger than that achieved by the simple determination of Topo I antibodies by counterimmunoelectrophoresis and immunoblot (hazard ratio 1.7, P = 0.30) in the same patients. B-cell epitope mapping of the anti-Topo I response has identified four major epitopes which cumulatively show a strong association with the development of PIF in SSc.

The clinical relevance of antibodies to ribosomal-P common epitope in two targeted systemic lupus erythematosus populations: a large cohort of consecutive patients and patients with active central nervous system disease

OBJECTIVES

To develop an enzyme linked immunosorbent assay (ELISA) using as substrate a synthetic 22-aminoacid peptide, corresponding to the ribosomal P0, P1 and P2 common epitope. To study the specificity and sensitivity of the method and evaluate the frequency and clinical associations of anti-P antibodies in two groups of systemic lupus erythematosus (SLE) patients: (a) unselected SLE patients and (b) SLE patients with central nervous system (CNS) involvement.

PATIENTS AND METHODS

The C-terminal 22 aminoacid peptide of the ribosomal P proteins (Lys-Lys-Glu-Glu-Lys-Lys-Glu-Glu-Lys-Ser-Glu-Glu-Glu-Asp-Glu-Asp-Met- Gly-Phe-Gly-Leu-Phe-Asp) was synthesised according to Merrifield's solid phase procedure. Purification of the peptide was performed by preparative high performance liquid chromatography and confirmed by amino acid analysis. Using this peptide, in a concentration 5 microg/ml, an ELISA was developed. The presence of anti-P antibodies was evaluated by western blot using purified ribosomal proteins from rat liver. Sera from 178 consecutive patients with SLE and 28 patients with SLE and CNS manifestations were tested. Sera from 58 patients with rheumatoid arthritis and 57 patients with primary Sjögren's syndrome were used as controls. The cut off point of the assay was defined using 124 normal sera.

RESULTS

The specificity of the assay was evaluated by homologous inhibition. Pretreatment of positive sera with soluble 22mer peptide of the ribosomal P proteins resulted in 88% inhibition. The concordance between the peptide assay and western blot was found to be 83%. Thirty three of 178 (18. 6%) of the unselected SLE patients had antibodies to P-protein common epitope. Their presence was associated with more active disease (European Consensus Lupus Activity Measurement, ECLAM scoring system) (p<0.001), higher levels of anti-ds DNA antibodies (p<0.05) and lower levels of the C4 component of complement (p<0.01). Eleven of 28 (39.3%) patients with SLE and active CNS involvement had antibodies to P-protein. The overall prevalence of anti-P antibodies in active CNS disease patients was statistically significantly higher, as compared with unselected SLE patients (chi(2)=6.04, p<0.05). These antibodies were found in a high proportion of patients without anticardiolipin antibodies (52.4%) and they were associated with diffuse CNS involvement (psychiatric disorders (71%) and epilepsy (75%)).

CONCLUSIONS

A synthetic analogue of the common epitope of ribosomal P-proteins can be use as an antigen for the detection of anti-P antibodies. These antibodies are associated with active SLE and CNS involvement particularly in patients without anticardiolipin antibodies.

Arg side-chain-backbone interactions evidenced in model peptides by 17O-NMR spectroscopy

The guanidinium group of arginine possesses a variety of biochemical functions, either by participating in direct interactions in recognition processes, or by stabilizing secondary structures. Three model compounds, selectively (17)O enriched, Ac-Arg-Ala-[(17)O]Pro-NH(2) (1), Piv-Arg-Pro-[(17)O]Gly-NH(2) (2) (C-terminal segment of the luteinizing hormone releasing hormone), and Piv-Nle-Pro-[(17)O]Gly-NH(2) (3), were prepared and studied by (17)O-nmr spectroscopy. A direct hydrogen-bonded interaction between the Arg side chain and the carbonyl main chain carboxy-terminus was found, thus confirming the tendency of Arg to participate in proton-acceptor functions.

Immunoreactivity and conformation of the P-P-G-M-R-P-P repetitive epitope of the Sm autoantigen

Anti-Sm antibodies are usually considered highly specific for systemic lupus erythematosus (SLE), while anti-U1 RNP antibodies are found in high titers in patients with mixed connective tissue disease (MCTD). The sequence P1-P-G-M-R-P-P7, present in three copies in the Sm (U1-U6 RNA-protein complex) autoantigen, is an important functional domain of the antigenic determinants. The immunoreactivity of this proline-rich repetitive epitope was investigated by testing sera with various autoantibody specificities for reactivity against this epitope, as well as its conformational properties by means of 1D and 2D 1HNMR spectroscopy. It was found that the P-P-G-M-R-P-P epitope is recognized mainly by anti-U1RNP and/or anti-Sm positive sera, but also by anti-Ro(SSA) (hY1RNA-protein complex) and anti-La(SSB) (hY1RNA-protein complex) positive sera, although these sera are negative for anti-U1RNP and anti-Sm. Conformational analysis of the proline-rich epitope in DMSO-d6 solution obtained from lyophilized aqueous solution at pH 5 showed the presence of at least three conformers. The main conformer A (62%) is stabilized by an ionic interaction between the guanidinium and the C-terminal carboxylate groups, and the Pro6-Pro7 peptide bond adopts the cis form. A type II beta-turn is also present in the N-terminal sequence (Pro1-Pro-Gly-Met4-) of this conformer. Conformer B (21%) is also stabilized by a similar ionic interaction, as in conformer A, while the NMR data indicate the absence of a folded structure in the N-terminal tetrapeptide of this conformer. Conformer C (17%) adopts a completely extended structure. The multiple conformers of the P-P-G-M-R-P-P may offer some explanation for the reactivity of sera with various autoantibody specificities against this epitope.

1H-NMR studies on arginine tripeptides: evidence for guanidinium-C-terminal carboxylate interactions

Guanidinium-C-terminal carboxylate interactions are involved in the establishment of the secondary structure of various biologically active peptide sequences. The conformational properties of a series of arginine-containing tripeptides, L-Arg-X-Gly (X = L-Ala, Val, Leu), in DMSO solutions at physiological pH, have been studied by means of 1D and 2D 1H-NMR spectroscopy. Measurements of the chemical shifts, NOE effects and temperature coefficients showed that the ArgN epsilon H and ArgN eta H2 groups form two hydrogen bonds with the C-terminal carboxylate moiety, whereas the ArgN alpha-terminal nitrogen is in the amino state. Our results point out the significant contribution of the C-terminal carboxylate group, at physiological pH, in the stabilization of the Arg side-chain structure in peptides simultaneously containing arginine residues and carboxy terminal sequences.

Changes in the c.d. spectra of calf thymus DNA induced by sequential polypeptides in aqueous solutions. Part I

Interactions of calf thymus DNA with sequential polypeptides were studied using c.d. spectroscopy in aqueous solutions. It was found that DNA structural alterations induced by sequential polypeptides (L-Arg-X-Gly)n (where X = L-Val, Leu, Ile, Nva, Nle) are modulated by the nature of the X residue. Thus, the polypeptide (L-Arg-L-Nva-Gly)n induced the 10.2B-DNA form, whereas the polypeptides (L-Arg-L-Ile-Gly)n having one methyl group less on the X residue side chain, did not provide any significant modification to the structure of DNA. The effect of ionic strength from 0.14 M NaCl (physiological value) to zero was also analysed on the basis of the observed c.d. changes and the degree of complexation in the DNA-polypeptides was estimated.

Changes in the c.d. spectra of calf thymus DNA induced by sequential polypeptides in trifluoroethanol solutions. Part II

Interactions between calf thymus DNA and (L-Arg-X-Gly)n sequential polypeptides (where X = L-Ala, Val, Leu, Ile, Nva, Nle) in trifluoroethanol: water (40:60) solutions in the salt range of 0.12-0.5 M NaCl, were studied using c.d. spectroscopy. It was found that DNA tertiary structure (psi form) is modulated by the nature of the polypeptides (variation of X residue). The effect of the secondary structure of polypeptides on the formation of psi-DNA was also analysed. Unordered polypeptides destabilized psi aggregates, while helical polypeptides favoured DNA tertiary structure. A loss of tertiary structure was observed in the presence of the (L-Arg-L-Val-Gly)n, which can be attributed to the ability of valine to suppress psi-type DNA.

Circular dichroism studies on chromatin models. Interactions between DNA and sequential polypeptides containing arginine

The sequential polypeptides (L-Arg-Xaa-Gly)n where Xaa represents amino acid residues Ala, Val and Leu, were employed as models of arginine-rich histones, in studying their interactions with nucleic acids. These polypeptide-DNA complexes were prepared using gradient dialysis and their conformational properties were investigated by circular dichroism spectroscopy. It was found that poly(L-Arg-L-Val-Gly) caused pronounced structural changes in the DNA molecule (conformational transition from B to the more compact and asymmetric C form) as a function of ionic strength and polypeptide: DNA ratio. In contrast the DNA interaction with poly (L-Arg-L-Ala-Gly) and poly (L-Arg-L-Leu-Gly) increased in the order of Ala----Leu, but with slight structural changes in the DNA secondary conformation. Thus, the importance of the composition, amino acid sequence and conformation of the polypeptides which bind to DNA was demonstrated. The significance of the hydrophobic forces besides the arginine-phosphate charge interaction, which modulate the nature of the polypeptide-DNA complexes and their condensation into higher-ordered tertiary structures, as found in chromatin, was also confirmed.