Use of autoionization to measure microwave-driven transitions between high-n strontium Rydberg states.

The use of autoionization as a tool to quantitatively analyze transitions between high-n strontium Rydberg states with low-to-intermediate values of L is described and is demonstrated through observations of Rabi oscillations when driving 5snf 1F3 → 5s(n + 1)g 1G4, 5snf 1F3 → 5s(n + 2)h 1H5, and 5snp 1P1 → 5s(n + 1)d 1D2 transitions using microwave fields. The technique is shown to offer advantages when compared to selective field ionization and can be used with other alkaline-earth and alkaline-earth-like metals.

Hyperhydroxylation: a new strategy for neuronal targeting by venomous marine molluscs.

Venomous marine molluscs belonging to the genus Conus (cone snails) utilize a unique neurochemical strategy to capture their prey. Their venom is composed of a complex mixture of highly modified peptides (conopeptides) that interact with a wide range of neuronal targets. In this chapter, we describe a set of modifications based upon the hydroxylation of polypeptidic chains that are defining within the neurochemical strategy used by cone snails to capture their prey. In particular, we present a differential hydroxylation strategy that affects the neuronal targeting of a new set of a-conotoxins, mini-M conotoxins, conophans, and y-hydroxyconophans. Differential hydroxylation, preferential hydroxylation and hyperhydroxylation have been observed in these conopeptide families as a means of augmenting the venom arsenal used by cone snails for neuronal targeting and prey capture.

Improved synthesis of 5-hydroxylysine (Hyl) derivatives.

The synthesis of 5-hydroxylysine (Hyl) derivatives for incorporation by solid-phase methodologies presents numerous challenges. Hyl readily undergoes intramolecular lactone formation, and protected intermediates often have poor solubilities. The goals of this work were twofold: first, develop a convenient method for the synthesis of O-protected Fmoc-Hyl; secondly, evaluate the efficiency of methods for the synthesis of O-glycosylated Fmoc-Hyl. The 5-O-tert-butyldimethylsilyl (TBDMS) fluoren-9-ylmethoxycarbonyl-Hyl (Fmoc-Hyl) derivative was conveniently prepared by the addition of tert-butyldimethylsilyl trifluoromethanesulfonate to copper-complexed Hyl[epsilon-tert-butyloxycarbonyl (Boc)]. The complex was decomposed with Na+ Chelex resin and the Fmoc group added to the alpha-amino group. Fmoc-Hyl(epsilon-Boc, O-TBDMS) was obtained in 67% overall yield and successfully used for the solid-phase syntheses of 3 Hyl-containing peptides. The preparation of Fmoc-Hyl[epsilon-Boc, O-(2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)] was compared for the thioglycoside, trichloroacetimidate and Koenigs-Knorr methods. The most efficient approach was found to be Koenigs-Knorr under inverse conditions, where Fmoc-Hyl(epsilon-Boc)-OBzl and peracetylated galactosyl bromide were added to silver trifluoromethanesulfonate in 1,2-dichloroethane, resulting in a 45% isolated yield. Side-reactions that occurred during previously described preparations of glycosylated Hyl derivatives, such as lactone formation, loss of side-chain protecting groups, orthoester formation, or production of anomeric mixtures, were avoided here. Research on the enzymology of Lys hydroxylation and subsequent glycosylation, as well as the role of glycosylated Hyl in receptor recognition, will be greatly aided by the convenient and efficient synthetic methods developed here.

Synthetic matrix metalloproteinase-sensitive hydrogels for the conduction of tissue regeneration: engineering cell-invasion characteristics.

Synthetic hydrogels have been molecularly engineered to mimic the invasive characteristics of native provisional extracellular matrices: a combination of integrin-binding sites and substrates for matrix metalloproteinases (MMP) was required to render the networks degradable and invasive by cells via cell-secreted MMPs. Degradation of gels was engineered starting from a characterization of the degradation kinetics (k(cat) and K(m)) of synthetic MMP substrates in the soluble form and after crosslinking into a 3D hydrogel network. Primary human fibroblasts were demonstrated to proteolytically invade these networks, a process that depended on MMP substrate activity, adhesion ligand concentration, and network crosslinking density. Gels used to deliver recombinant human bone morphogenetic protein-2 to the site of critical defects in rat cranium were completely infiltrated by cells and remodeled into bony tissue within 4 wk at a dose of 5 microg per defect. Bone regeneration was also shown to depend on the proteolytic sensitivity of the matrices. These hydrogels may be useful in tissue engineering and cell biology as alternatives for naturally occurring extracellular matrix-derived materials such as fibrin or collagen.

Evaluation of radiolabeled type IV collagen fragments as potential tumor imaging agents.

The objective of this study was to examine radiopharmaceuticals that target the alpha3beta1 integrin to determine if these agents target tumors for diagnostic imaging and/or targeted radiotherapy of cancer. Prior studies had shown that residues 531-542 from the alpha1 chain of type IV collagen bind a variety of tumor cell alpha3beta1 integrins. A peptide mimic of this sequence containing all D-amino acids (designated D-Hep-III) was synthesized by solid-phase methods. The tetraazamacrocyclic chelator, TETA, was conjugated to the peptide while it was resin-bound. TETA-D-Hep-III and D-Hep-III were radiolabeled with 64Cu and 125I, respectively, in high specific activity and radiochemical purity. Heterologous competitive binding assays between D-Hep-III and either 125I-D-Hep-III or 64Cu-TETA-D-Hep-III indicated low micromolar affinity of D-Hep-III. The biodistribution of each radiolabeled analogue of D-Hep-III was carried out in rats and tumor-bearing mice. Both analogues were rapidly cleared from the blood in normal rats, with the kidneys receiving the highest accumulation of each. SKOV3 human ovarian tumor cells, known to strongly express alpha3beta1, were xenografted in SCID mice. Localization of 125I-D-Hep III and 64Cu-TETA-D-Hep III in the xenografts were low (<2% ID/g), and in the case of 125I-D-Hep III, not inhibited by a competitive dose of D-Hep III. The low tumor accumulation is likely not due to receptor down-regulation, but rather due to the weak affinity of the radioligands for the alpha3beta1 integrin.

Kinetic analysis of matrix metalloproteinase activity using fluorogenic triple-helical substrates.

Matrix metalloproteinase (MMP) family members are involved in the physiological remodeling of tissues and embryonic development as well as pathological destruction of extracellular matrix components. To study the mechanisms of MMP action on collagenous substrates, we have constructed homotrimeric, fluorogenic triple-helical peptide (THP) models of the MMP-1 cleavage site in type II collagen. The substrates were designed to incorporate the fluorophore/quencher pair of (7-methoxycoumarin-4-yl)acetyl (Mca) and N-2,4-dinitrophenyl (Dnp) in the P(5) and P(5)' positions, respectively. In addition, Arg was incorporated in the P(2)' and P(8)' positions to enhance enzyme activity and improve substrate solubility. The desired sequences were Gly-Pro-Lys(Mca)-Gly-Pro-Gln-Gly approximately Leu-Arg-Gly-Gln-Lys(Dnp)-Gly-Ile/Val-Arg. Two fluorogenic substrates were prepared, one using a covalent branching protocol (fTHP-1) and one using a peptide self-assembly approach (fTHP-3). An analogous single-stranded substrate (fSSP-3) was also synthesized. Both THPs were hydrolyzed by MMP-1 at the Gly approximately Leu bond, analogous to the bond cleaved in the native collagen. The individual kinetic parameters for MMP-1 hydrolysis of fTHP-3 were k(cat) = 0.080 s(-1) and K(M) = 61.2 microM. Subsequent investigations showed fTHP-3 hydrolysis by MMP-2, MMP-3, MMP-13, a C-terminal domain-deleted MMP-1 [MMP-1(Delta(243-450))], and a C-terminal domain-deleted MMP-3 [MMP-3(Delta(248-460))]. The order of k(cat)/K(M) values was MMP-13 > MMP-1 approximately MMP-1(Delta(243-450)) approximately MMP-2 >> MMP-3 approximately MMP-3(Delta(248-460)). Studies on the effect of temperature on fTHP-3 and fSSP-3 hydrolysis by MMP-1 showed that the activation energies between these two substrates differed by 3.4-fold, similar to the difference in activation energies for MMP-1 hydrolysis of type I collagen and gelatin. This indicates that fluorogenic triple-helical substrates mimic the behavior of the native collagen substrate and may be useful for the investigation of collagenase triple-helical activity.

Adhesion of alpha5beta1 receptors to biomimetic substrates constructed from peptide amphiphiles.

Biomimetic membrane surfaces functionalized with fragments of the extracellular matrix protein, fibronectin, are constructed from mixtures of peptide and polyethylene glycol (PEG) amphiphiles. Peptides from the primary binding loop, GRGDSP, were used in conjunction with the synergy site peptide, PHSRN, in the III(9-10) sites of human fibronectin. These peptides were attached to dialkyl lipid tails to form peptide amphiphiles. PEG amphiphiles were mixed in the layer to minimize non-specific adhesion in the background. GRGDSP and PEG amphiphiles or GRGDSP, PHSRN, and PEG amphiphiles were mixed in various ratios and deposited on solid substrates from the air-water interface using Langmuir-Blodgett techniques. In this method, peptide composition, density, and presentation could be controlled accurately. The effectiveness of these substrates to mimic native fibronectin is evaluated by their ability to generate adhesive forces when they are in contact with purified activated alpha5beta1 integrin receptors that are immobilized on an opposing surface. Adhesion is measured using a contact mechanical approach (JKR experiment). The effects of membrane composition, density, temperature, and peptide conformation on adhesion to activated integrins in this simulated cell adhesion setup were determined. Addition of the synergy site, PHSRN, was found to increase adhesion of alpha5beta1, to biomimetic substrates markedly. Increased peptide mobility (due to increased experimental temperature) increased integrin adhesion markedly at low peptide concentrations. A balance between peptide density and steric accessibility of the receptor binding face to alpha5beta1 integrin was required for highest adhesion.

Difficulties encountered during glycopeptide syntheses.

Methods for the efficient solid-phase synthesis of glycopeptides have developed rapidly over the past two decades. Incorporation of both O- and N-linked glycosides, as well as branched carbohydrates, into peptides is readily achieved. Synthetic glycoproteins of modest size have also been constructed. As glycopeptide synthesis protocols have progressed, so has the recognition of distinct categories of synthetic difficulties. Such categories include (a) unstable glycosidic linkages, (b) multifunctional amino acids not easily glycosylated and incorporated into peptides, and (c) glycosylated peptide sequences that are subject to side reactions. In the present overview,we describe specific examples for each category of problematic glycopeptide syntheses, as well as solutions to these problems.

Use of Edman degradation sequence analysis and matrix-assisted laser desorption/ionization mass spectrometry in designing substrates for matrix metalloproteinases.

The matrix metalloproteinase (MMP) family has been implicated in the process of a variety of diseases such as arthritis, atherosclerosis, and tumor cell metastasis. We have been designing single-stranded peptides (SSPs) and triple-helical peptides (THPs) as potential discriminatory MMP substrates. Edman degradation sequence and matrix-assisted laser desorption/ionization mass spectrometric (MALDI-MS) analyses of proteolytic activity have been utilized to aid in further substrate design. THP models of the alpha1(I)772-786 sequence from type I collagen were synthesized to examine the triple-helical substrate specificity of MMP family members. Sequence and MALDI-MS analyses were used in conjunction with a fluorometric assay to determine the exact point of cleavage by each MMP. MMP-1 (interstitial collagenase) cleaved the substrates at a single Gly-Ile bond, analogous to the cleavage site in type I collagen. MMP-2 (Mr 72 000 type IV collagenase; gelatinase A) was found to cleave the substrates at two sites, a Gly-Ile bond and a Gly-Gln bond. MMP-3 (stromelysin 1) was found to cleave only one of the substrates after reaction for 48 h. Ultimately, sequence and MALDI-MS analyses allowed us to detect an additional cleavage site for MMP-2 in comparison to MMP-1, while MMP-3 was found to cleave a substrate after an extended time period. The second cleavage site would cause the kinetic parameters for MMP-2 to be overestimated by the fluorometric assay. Further design variations for these substrates need to consider the presence of more stable triple-helical conformation (to eliminate MMP-3 binding) and the removal of Gly-Gln bonds that may be susceptible to MMP-2.

Induction of protein-like molecular architecture by monoalkyl hydrocarbon chains.

Numerous approaches have been described for creating relatively small folded biomolecular structures. "Peptide-amphiphiles," whereby monoalkyl or dialkyl hydrocarbon chains are covalently linked to peptide sequences, have been shown previously to form specific molecular architecture of enhanced stability. The present study has examined the use of monoalkyl hydrocarbon chains as a more general method for inducing protein-like structures. Peptide and peptide-amphiphiles have been characterized by CD and one- and two-dimensional nmr spectroscopic techniques. We have examined two structural elements: alpha-helices and collagen-like triple helices. The alpha-helical propensity of a 16-residue peptide either unmodified or acylated with a C(6) or C(16) monoalkyl hydrocarbon chain has been examined initially. The 16-residue peptide alone does not form a distinct structure in solution, whereas the 16-residue peptide adopts predominantly an alpha-helical structure in solution when a C(6) or C(16) monoalkyl hydrocarbon chain is N-terminally acylated. The thermal stability of the alpha-helix is greater upon addition of the C(16) compared with the C(6) chain, which correlates to the extent of aggregation induced by the respective hydrocarbon chains. Very similar results are seen using a 39-residue triple-helical model peptide, in that structural thermal stability (a) is increasingly enhanced as alkyl chain length is increased and (b) correlates to the extent of peptide-amphiphile aggregation. Overall, structures as diverse as alpha-helices, triple helices, and turns/loops have been shown to be induced and/or stabilized by alkyl chains. Increasing alkyl chain length enhances stability of the structural element and induces aggregates of defined sizes. Hydrocarbon chains may be useful as general tools for protein-like structure initiation and stabilization as well as biomaterial modification.

Identification of the (183)RWTNNFREY(191) region as a critical segment of matrix metalloproteinase 1 for the expression of collagenolytic activity.

Matrix metalloproteinase 1 (MMP-1) cleaves types I, II, and III collagen triple helices into (3/4) and (1/4) fragments. To understand the structural elements responsible for this activity, various lengths of MMP-1 segments have been introduced into MMP-3 (stromelysin 1) starting from the C-terminal end. MMP-3/MMP-1 chimeras and variants were overexpressed in Escherichia coli, folded from inclusion bodies, and isolated as zymogens. After activation, recombinant chimeras were tested for their ability to digest triple helical type I collagen at 25 degrees C. The results indicate that the nine residues (183)RWTNNFREY(191) located between the fifth beta-strand and the second alpha-helix in the catalytic domain of MMP-1 are critical for the expression of collagenolytic activity. Mutation of Tyr(191) of MMP-1 to Thr, the corresponding residue in MMP-3, reduced collagenolytic activity about 5-fold. Replacement of the nine residues with those of the MMP-3 sequence further decreased the activity 2-fold. Those variants exhibited significant changes in substrate specificity and activity against gelatin and synthetic substrates, further supporting the notion that this region plays a critical role in the expression of collagenolytic activity. However, introduction of this sequence into MMP-3 or a chimera consisting of the catalytic domain of MMP-3 with the hinge region and the C-terminal hemopexin domain of MMP-1 did not express any collagenolytic activity. It is therefore concluded that RWTNNFREY, together with the C-terminal hemopexin domain, is essential for collagenolytic activity but that additional structural elements in the catalytic domain are also required. These elements probably act in a concerted manner to cleave the collagen triple helix.

Hydrolysis of triple-helical collagen peptide models by matrix metalloproteinases.

The matrix metalloproteinase (MMP) family has been implicated in the process of a variety of diseases such as arthritis, atherosclerosis, and tumor cell metastasis. To study the mechanisms of MMP action on collagenous substrates, we have constructed homotrimeric triple-helical peptide (THP) models of the collagenase cleavage sites in types I and II collagen. The THPs incorporate either the alpha1(I)772-786 or the alpha1(II)772-783 sequence. The alpha1(I)772-786 and alpha1(II)772-783 THPs were hydrolyzed by MMP-1 at the Gly-Ile and Gly-Leu bonds, respectively, analogous to the bonds cleaved in corresponding native collagens. Thus, the THPs contained all necessary information to direct MMP-1 binding and proteolysis. Subsequent investigations using the alpha1(I)772-786 THP showed hydrolysis by MMP-2, MMP-13, and a COOH-terminal domain-deleted MMP-1 (MMP-1(Delta(243-450))) but not by MMP-3 or a COOH-terminal domain-deleted MMP-3 (MMP-3(Delta(248-460))). Kinetic analyses showed a k(cat)/K(m) value of 1,808 s(-1) m(-1) for MMP-1 hydrolysis of alpha1(I)772-786 THP, approximately 10-fold lower than for type I collagen. The effect is caused primarily by relative K(m) values. MMP-2 and MMP-13 cleaved the THP more rapidly than MMP-1, but MMP-2 cleavage occurred at distinct multiple sites. Comparison of MMP-1 and MMP-1(Delta(243-450)) hydrolysis of alpha1(I)772-786 THP showed that both can cleave a triple-helical substrate with a slightly higher K(m) value for MMP-1(Delta(243-450)). We propose that the COOH-terminal domain of MMPs is necessary for orienting whole, native collagen molecules but may not be necessary for binding to and cleaving a THP. This proposal is consistent with the large distance between the MMP-1 catalytic and COOH-terminal domains observed by three-dimensional structural analysis and supports previous suggestions that the features of the catalytic domain contribute significantly toward enzyme specificity.

Chemical synthesis of proteins.

The manipulation of protein structure enables a better understanding of the principles of protein folding, as well as the development of novel therapeutics and drug-delivery vehicles. Chemical synthesis is the most powerful approach for constructing proteins of novel design and structure, allowing for variation of covalent structure without limitations. Here we describe the various chemical methods that are currently used for creating proteins of unique architecture and function.

Ligand accessibility as means to control cell response to bioactive bilayer membranes.

We report a new method to create a biofunctional surface in which the accessibility of a ligand is used as a means to influence the cell behavior. Supported bioactive bilayer membranes were created by Langmuir-Blodgett (LB) deposition of either a pure poly(ethylene glycol) (PEG) lipid, having PEG head groups of various lengths, or 50 mol % binary mixtures of a PEG lipid and a novel collagen-like peptide amphiphile on a hydrophobic surface. The peptide amphiphile contains a peptide synthetically lipidated by covalent linkage to hydrophobic dialkyl tails. The amphiphile head group lengths were determined using neutron reflectivity. Cell adhesion and spreading assays showed that the cell response to the membranes depends on the length difference between head groups of the membrane components. Cells adhere and spread on mixtures of the peptide amphiphile with the PEG lipids having PEG chains of 120 and 750 molecular weight (MW). In contrast, cells adhered but did not spread on the mixture containing the 2000 MW PEG. Cells did not adhere to any of the pure PEG lipid membranes or to the mixture containing the 5000 MW PEG. Selective masking of a ligand on a surface is one method of controlling the surface bioactivity.

Cellular recognition of synthetic peptide amphiphiles in self-assembled monolayer films.

The incorporation of lipidated cell adhesion peptides into self-assembled structures such as films provides the opportunity to develop unique biomimetic materials with well-organized interfaces. Synthetic dialkyl tails have been linked to the amino-terminus, carboxyl-terminus, and both termini of the cell recognition sequence Arg-Gly-Asp (RGD) to produce amino-coupled, carboxyl-coupled, and looped RGD peptide amphiphiles. All three amphiphilic RGD versions self-assembled into fairly stable mixed monolayers that deposited well as Langmuir-Blodgett films on surfaces, except for films containing amino-coupled RGD amphiphiles at high peptide concentrations. FT-IR studies showed that amino-coupled RGD head groups formed the strongest lateral hydrogen bonds. Melanoma cells spread on looped RGD amphiphiles in a concentration-dependent manner, spread indiscriminately on carboxyl-coupled RGD amphiphiles, and did not spread on amino-coupled RGD amphiphiles. Looped RGD amphiphiles promoted the adhesion, spreading, and cytoskeletal reorganization of melanoma and endothelial cells while control looped Arg-Gly-Glu (RGE) amphiphiles inhibited them. Antibody inhibition of the integrin receptor alpha3beta1 blocked melanoma cell adhesion to looped RGD amphiphiles. These results confirm that novel biomolecular materials containing synthetic peptide amphiphiles have the potential to control cellular behavior in a specific manner.

Induction of protein-like molecular architecture by self-assembly processes.

One of the most intriguing self-assembly processes is the folding of peptide chains into native protein structures. We have developed a method for building protein-like structural motifs that incorporate sequences of biological interest. A lipophilic moiety is attached onto an N(alpha)-amino group of a peptide chain, resulting in a 'peptide-amphiphile'. The alignment of amphiphilic compounds at the lipid solvent interface is used to facilitate peptide alignment and structure initiation and propagation. Peptide-amphiphiles containing potentially triple-helical structural motifs have been synthesized. The resultant head group structures have been characterized by circular dichroism and NMR spectroscopies. Evidence for a self-assembly process of peptide-amphiphiles has been obtained from: (a) circular dichroism spectra and melting curves characteristic of triple-helices, (b) one- and two-dimensional NMR spectra indicative of stable triple-helical structure at low temperatures and melted triple-helices at high temperatures, and (c) pulsed-field gradient NMR experiments demonstrating different self-diffusion coefficients between proposed triple-helical and non-triple-helical species. The peptide-amphiphiles described here provide a simple approach for building stable protein structural motifs using peptide head groups.

Structure and dynamics of peptide-amphiphiles incorporating triple-helical proteinlike molecular architecture.

Organized polymeric assemblies that incorporate bioactive sequences and structures are finding important applications for the study of protein structure-function relationships. We have recently described a heteropolymeric peptide-amphiphile system that forms organized structures in solution and on surfaces. While the overall three-dimensional features of peptide-amphiphiles have been studied previously, the precise environment of specific residues, particularly those within biologically active regions, have not been examined in detail. In the present study, we have used heteronuclear single quantum coherence (HSQC) and inverse-detected 1H-15N NMR spectroscopy to examine the structure and dynamics of a peptide and peptide-amphiphile that incorporate the alpha1(IV)1263-1277 ([IV-H1]) amino acid sequence from type IV collagen. Three variants of the sequence (Gly-Pro-Hyp)4-[IV-H1]-(Gly-Pro-Hyp)4 were constructed with a single 15N-labeled Gly placed in the middle of the N-terminal (Gly-Pro-Hyp)4 region (residue Gly7), in the middle of the [IV-H1] sequence (residue Gly19), or in the middle of the C-terminal (Gly-Pro-Hyp)4 region (residue Gly34). These peptides were also N-terminally acylated with hexanoic acid to create an analogous series of 15N-labeled peptide-amphiphiles. HSQC spectra indicated that both the peptide and the peptide-amphiphile were in triple-helical conformation at low temperature, supporting prior circular dichroism (CD) spectroscopic results. The intensities of the triple-helical cross-peaks were stronger for the peptide-amphiphile, consistent with an enhanced triple-helical thermal stability within the peptide-amphiphile construct compared to that of the peptide alone. Relative relaxation values for the peptide-amphiphile monomeric and trimeric species were consistent with those reported previously for other triple-helical peptides. Relaxation measurements indicated that the triple-helical [IV-H1] region did not appear to be dramatically more flexible than the Gly-Pro-Hyp regions. The angle between Gly N-H bonds and the helix dyad axis, determined from the relaxation data, was within the range expected for triple helices. Overall, the peptide headgroup of the C6-(Gly-Pro-Hyp)4-[IV-H1]-(Gly-Pro-Hyp)4 peptide-amphiphile appears to form a continuous triple helix that behaves similarly, in a dynamic sense, to a triple-helical peptide. The enhanced thermal stability of the peptide-amphiphile compared to the analogous triple-helical peptide, along with the multitude of organized structures formed by lipidlike compounds, suggest that peptide-amphiphiles could be utilized as targeted liposomes, sensors, receptors, or enzymes.