L'objectif principal de cette thèse est l’étude spectroscopique de molécules ou de complexes portant plusieurs centres chiraux en phase gazeuse, pour comprendre les effets de la stéréochimie sur leurs propriétés structurales. Des alcaloïdes dérivés de la Cinchonine ont été introduits intacts en phase gazeuse par ablation laser. Ils ont été étudiés en combinant un jet supersonique avec de la spectroscopie laser. Les deux pseudo-énantiomères Quinine et Quinidine ont montré des spectres électroniques et vibrationnels similaires, en accord avec leur structure similaire. Leurs propriétés en solution diffèrent davantage, comme le montrent les expériences de dichroïsme circulaire vibrationnel (VCD). Cette différence est encore plus marquée dans l’Hydroquinine et l’Hydroquinidine. Enfin la reconnaissance chirale a été étudiée dans des complexes ioniques dans un piège à ions. La stabilité des complexes formés entre S-camphre et les R et S-Alanine protonées indique une préférence homochirale. Cependant, l'énergie d'interaction calculée ainsi que les spectres IRMPD dans la région des empreintes digitales sont identiques. Le rôle des conformères plus hauts en énergie dans la reconnaissance chirale a été discuté. / The main objective of this thesis is a spectroscopic study of molecules or complexes bearing multiple chiral centres in the gas phase, to understand the effects of stereochemistry on their structural properties. Neutral cinchona alkaloids have been introduced intact in gas phase by laser-ablation. They have been studied by combining supersonic expansion with laser spectroscopy. The two pseudo-enantiomers Quinine and Quinidine show similar electronic and vibrational spectra, in line with similar structure. Their properties in solution differ more, as shown by Vibrational Circular Dichroism (VCD) experiments. This difference is further enhanced in Hydroquinine and Hydroquinidine. Lastly chiral recognition has been studied in ionic complexes in an ion trap. A homochiral preference has been shown in the stability of the complexes formed between S-Camphor and R and S protonated Alanine. However, the calculated interaction energy as well as the IRMPD spectrum in the fingerprint region are identical. The role of higher energy conformers in chiral recognition has been discussed.

O nadolol é um agente bloqueador de receptores β-adrenérgicos empregado principalmente, na \"angina pectoris\", hipertensão, certas arritmias cardíacas e no tratamento do glaucoma (SING, 2006). A ivermectina e a abamectina são fármacos que apresentam ação antiparasitária (SHOOP, 1995). Na presente pesquisa, a cromatografia em fase líquida de alta eficiência foi uma das técnicas estudadas para a quantificação dos enantiômeros do nadolol e dos homólogos presentes na abamectina e ivermectina. A versatilidade desta técnica reside no grande número de fases estacionárias existentes, as quais possibilitam análises, separações e determinações quantitativas de uma ampla gama de compostos com alta eficiência (Aquino Neto e Nunes, 2003). Para identificação dos enantiômeros do nadolol foi utilizado o dicroísmo circular que permite a determinação da configuração absoluta de enantiômeros (LIMA, 1997). Para os enantiômeros do nadolol e dos homólogos presentes na abamectina e na ivermectina também foram realizados testes para desenvolvimento de uma metodologia de quantificação por meio de uma técnica relativamente recente chamada de eletroforese capilar (EC), a qual tem alcançado desde sua introdução um rápido desenvolvimento e ampla aplicação na análise de fármacos em medicamentos (SANTORO, 2000). Para a comprovação da qualidade e segurança dos sistemas computadorizados dos equipamentos de cromatografia em fase líquida de alta eficiência (CLAE) e de eletroforese capilar (EC) foram efetuadas, neste trabalho, as respectivas validações. Após esta validação, pode-se confirmar o correto funcionamento de um software, e suas interações com o hardware, onde devem ser levados em consideração, dentre outros, os aspectos relacionados à infra-estrutura, segurança e manutenção de dados (AGÊNCIA NACIONAL DE VIGILÂNIA SANITÁRIA, 2010). As metodologias analíticas desenvolvidas a para quantificação do nadolol, abamectina e ivermectina por cromatografia em fase líquida de alta eficiência foram validadas. A validação analítica deve garantir, por meio de estudos experimentais, que o método atenda às exigências das aplicações analíticas, assegurando a confiabilidade dos resultados. Para tanto, o método deve apresentar especificidade, linearidade, intervalo, precisão, sensibilidade, limite de quantificação e detecção, exatidão, adequados à análise (AGÊNCIA NACIONAL DE VIGILÂNIA SANITÁRIA, 2003). Portanto, o objetivo proposto nesta pesquisa é primeiramente a validação dos sistemas computadorizados dos equipamentos de cromatografia em fase líquida de alta eficiência (CLAE) e de eletroforese capilar (EC). Para isto, serão desenvolvidos e validados os métodos analíticos de separação, identificação e quantificação dos enantiômeros do nadolol e dos homólogos presentes na abamectina e na ivermectina, em medicamentos, empregando as técnicas analíticas selecionadas. / Nadolol is a blocking agent with activity in the β -adrenergic receptors. It is mainly used in angina, hypertension, certain heart arrhythmias and in the treatment of glaucoma (SING, 2006). Ivermectin and abamectin are drugs with antiparasitic activity (SHOOP, 1995). In the present research, high performance liquid chromatography is one of the techniques used in the quantification of the enantiomers of nadolol and homologues present in abamectin and ivermectin. The versatility of this technique and the large number of existing stationary phases, enables the separation and quantitative determination of a wide range of compounds with high efficiency (Aquino Neto e Nunes, 2003). For identification of the nadolol enantiomers, circular dichroism was used which allows the determination of the absolute configuration of the enantiomers (LIMA, 1997). Nadolol enantiomers and the homologues present in abamectin and ivermectin will be also quantified by capillary zone electrophoresis (CE), a separation technique relatively recent, which has achieved, since its introduction, a wide application in the analysis of drugs in pharmaceutical preparations (SANTORO, 2000). In order to assure the quality of the analytical results, the computer systems of the liquid chromatograph and capillary electrophoresis equipments, must be validated prior to the analytical methods validation. Computer systems validation is used to verify and confirm the proper operation of softwares, and their interactions with the hardwares, besides the infrastructure, safety and storage of data (AGÊNCIA NACIONAL DE VIGILÂNIA SANITÁRIA, 2010). The analytical methodologies developed for quantification of nadolol, abamectin, ivermectin by using high efficiency liquid chromatography and capillary electrophoresis were validated. The analytical methods validation should ensure, through experimental studies, that the method meets the requirements for analytical applications, ensuring the reliability of the results. Parameters like, specificity, linearity, range, accuracy, sensitivity, limits of detection and quantification and accuracy, must be determined (AGÊNCIA NACIONAL DE VIGILÂNIA SANITÁRIA, 2003). The objective of this study is to validate the computer systems of the high performance liquid chromatograph and capillary electrophoresis equipments and then to develop and validate analytical methods for separation, identification and quantification of nadolol enantiomers and the homologues of abamectin and ivermectin.

S100A12 porcina é um membro da família das proteínas S100, um grupo de pequenas proteínas ligantes de cálcio caracterizado pela presença de dois motivos “EF-hand". Estas proteínas estão envolvidas em diversos eventos celulares, como a regulação da fosforilação protéica, atividade enzimática, tamponamento de Ca+2, processos inflamatórios e a polimerização de filamentos intermediários. Adicionalmente, algumas dessas proteínas podem ligar Zn+2, o qual pode afetar a ligação do íon Ca+2, particularmente para as proteínas S100. Neste trabalho, a seqüência gênica que codifica a proteína S100A12 porcina foi obtida por meio da construção de um gene sintético usando códons preferenciais para E.coli, permitindo a produção recombinante de grandes quantidades da proteína. Um estudo termodinâmico da estabilidade estrutural foi realizado, assim como a interação da proteína recombinante com íons divalentes usando técnicas de dicroísmo circular (CD) e fluorescência extrínseca. A desnaturação e renaturação induzidas por uréia ou temperatura indicam que se trata de um processo reversível e que a ligação dos íons Zn+2 e ou Ca+2 à rS100A12 aumenta sua estabilidade. A interação da sonda ANS com a proteína na presença de seus ligantes expõe superfícies hidrofóbicas podendo assim facilitar sua interação com macromoléculas alvo. Analisados em conjunto, os resultados obtidos indicam que S100A12 porcina é capaz de assumir diferentes conformações as quais podem estar correlacionadas com sua função fisiológica. / Porcine S100A12 is a member of S100 family, a small acidic calcium-binding proteins group characterized by the presence of two EF-hand motifs. These proteins are involved in many cellular events as the regulation of protein phosphorylation, enzymatic activity, Ca+2 homeostasis, inflammatory processes and intermediate filament polymerization. In addition, some of these proteins can bind Zn+2, which can affect the binding of Ca+2 particularly to S100 proteins. In this study, the gene sequence encoding S100A12 was obtained by the synthetic gene approach using E. coli codon bias allowing the recombinant production of large amounts of the protein. We report here a thermodynamic study on the structural stability of this recombinant protein and its interaction with divalent ions using circular dichroism and extrinsic fluorescence. The folding/unfolding induced by urea or temperature indicated a reversible process and the binding of Zn+2 or Zn+2 and Ca+2 to S100A12 increasing its stability. The interaction of the ANS probe with the protein in the ligant presence can lead to exposition of hydrofobic regions allowing its interaction with target macromolecules. Taken together, the results indicated that porcine S100A12 may assume different conformations that could be correlated to its physiological function.

Les huiles essentielles sont connues pour leur richesse en molécules chirales. L'identification et la caractérisation de ces différentes molécules en termes de la configuration absolue des énantiomères majoritaires représente un important pas dans la compréhension des actions thérapeutiques des huiles essentielles. Dans le cadre de cette thèse, une étude est réalisée sur la composition chimique des huiles essentielles de quelques plantes aromatiques du Sahara algérien (l'Artemisia herba-alba, le Bubonium graveolens et l'Artemisia arborescens) ayant un usage fréquent dans la pharmacopée traditionnelle. Différentes techniques analytiques chromatographiques telles que la CG-SM classique et chirale, la CLHP chirale et spectroscopiques comme l'IR et le VCD seront mises en œuvre afin d’étudier les constituants chiraux de ces HE pour la connaissance de leurs signatures chiroptiques qui peuvent être des paramètres essentiels pour leurs caractérisations. En associant la performance du traitement chimiométrique, la fiabilité des techniques spectroscopiques et le potentiel discriminant de la signature chirale, nous avons développé des outils de caractérisation, de contrôle qualité et de traçabilité des HE. Les configurations absolues de la (-)-α-thujone, la (+)-β-thujone, l'acetate de (-)-cis-chrysanthenyl, le (+)-oxocyclonerolidol et l'acetate de (-)-cis-acetoxychrysanthenyl ont été obtenues par la comparaison des spectres VCD expérimentaux et calculés et on a montré qu'on peut utiliser le VCD pour l'étude et la modélisation des matrices complexes. / Essential oils are known for their richness in Chiral molecules. Identification and characterization of these different molecules in terms of absolute configuration of the majors enantiomers represents an important step in the understanding of the therapeutic actions of essential oils. In this thesis, a study was done to investigate the chemical composition of the essential oils of some aromatic plants of the Algerian Sahara (Artemisia herba-alba, Bubonium graveolens and Artemisia arborescens) frequently used in the traditional pharmacopoeia. Different chromatographic analytical techniques such as classic and chiral GC-MS, chiral HPLC and spectroscopic techniques as IR and VCD will be implemented to study the chiral constituents in these EO for the knowledge of their chiroptical signatures which can be essential parameters for their characterizations. Combining chemometrics processing performance, reliability of spectroscopic techniques and potential discriminating chiral signature, we have developed tools for the characterization, quality control and traceability of EO. Absolute configuration of (-)-α-thujone, (+)-β-thujone, (-)-cis-chrysanthenyl acetate, (+)-oxocyclonerolidol and (-)-cis-acetoxychrysanthenyl acetate were obtained by comparison of calculated and experimental VCD spectra and we demonstrated that VCD can be used for the study and modeling of complex matrices.

SUR2A-mediated regulation of KATP channels is affected by residues belonging to the C terminus of the first nucleotide binding domain (NBD1). We studied the C-terminal region of NBD1 by comparing experiments using NBD1 S615-D914 and NBD1 S615-K972 constructs to studies of NBD1 S615-L933 also performed in our laboratory. Our NMR data suggests that the C-terminal region of NBD1 from residues Q915 to L933 is disordered and transiently contacts the NBD1 core, which may affect NBD1 phosphorylation. Tryptophan quenching fluorescence experiments corroborate that the Q915-L933 C-terminal tail contacts the NBD1 core. Fluorescence thermal denaturation experiments suggest that NBD1 S615-D914 has a higher affinity for MgATP compared with NBD1 S615-L933, implying that the C-terminal tail varies MgATP binding. Additional experiments were performed to identify soluble constructs of hSUR1 NBD1 and rSUR2A NBD2 that would allow detailed biophysical studies of these domains. Some of the constructs studied showed improved solubility and stability.

Sulfonated phthalocyanines were characterized using capillary electrophoresis and mass spectrometry. Derivatives investigated included the copper, cobalt, zinc and metal-free sulfonated phthalocyanines. The electropherograms of commercially available copper phthalocyanine-3,4',4'',4'''-tetrasulfonic acid and 4,4',4'',4'''-tetrasulfonic acid were very different, consistent with the latter compound having a structure that is not fully sulfonated. Matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) were used to characterize the sulfonated phthalocyanines. Mass spectral evidence was obtained for a pentasulfonated species of both the metal-free phthalocyanine and zinc phthalocyanine when these species were made by sulfonation of the metal-free phthalocyanine (followed by zinc insertion in the latter case). Many pathogenic bacteria require heme and obtain it from their environment. Heme transverses the cytoplasmic membrane via an ATP binding cassette (ABC) pathway. Although a number of heme ABC transport systems have been described in pathogenic bacteria, there is as yet little biophysical characterization of the proteins in these systems. The sia (hts) gene cluster encodes a heme ABC transporter in the Gram positive Streptococcus pyogenes. The heme binding protein (HBP) of this transporter is SiaA (HtsA). Several biophysical techniques were used to determine the coordination state, and spin state of both the ferric and ferrous forms of this protein. Identifiers from these techniques suggested that the heme is six-coordinate and low spin in both oxidation states of the protein, with methionine and histidine as axial ligands. The pKa of SiaA was determined, as were the reductive and oxidative midpoint potentials. Guanidinium titration studies of wild-type SiaA showed that the ferric state is less stable than the ferrous state. Free energy of unfolding values [ÄG(H2O)] for the oxidized and reduced proteins were 7.3 ± 0.8 and 16.0 ± 3.6 kcal mol−1, respectively. Denaturation of the histidine mutant H229A was not able to be followed via absorbance spectrometry, possibly due to the large amount of apoprotein present or to non-specific binding of the heme in the binding pocket. The biophysical characterization described herein will significantly advance our understanding of structure-function relationships in HBP.

No description available.

Tyrosine Hydroxylase (TyrH) is a pterin-dependent mononuclear non-heme iron oxygenase. TyrH catalyzes the hydroxylation reaction of tyrosine to dihydroxyphenylalanine (DOPA). This reaction is the first and the rate-limiting step in the biosynthesis of the catecholamine neurotransmitters. The active site iron in TyrH is coordinated by the common facial triad motif, 2-His-1-Glu. A combination of kinetic and spectroscopic techniques was applied in order to obtain insight into the catalytic mechanism of this physiologically important enzyme. Analysis of the TyrH reaction by rapid freeze-quench Mossbauer spectroscopy allowed the first direct characterization of an Fe(IV) intermediate in a mononuclear nonheme enzyme catalyzing aromatic hydroxylation. Further rapid kinetic studies established the kinetic competency of this intermediate to be the long-postulated hydroxylating species, Fe(IV)O. Spectroscopic investigations of wild-type (WT) and mutant TyrH complexes using magnetic circular dichroism (MCD) and X-ray absorption spectroscopy (XAS) showed that the active site iron is 6-coordinate in the resting form of the enzyme and that binding of either tyrosine or 6MPH4 alone does not change the coordination. However, when both tyrosine and 6MPH4 are bound, the active site becomes 5-coordinate, creating an open site for reaction with O2. Investigation of the kinetics of oxygen reactivity of TyrH complexes in the absence and presence of tyrosine and/or 6MPH4 indicated that there is a significant enhancement in reactivity in the 5-coordinate complex in comparison to the 6-coordinate form. Similar investigations with E332A TyrH showed that Glu332 residue plays a role in directing the protonation of the bridged complex that forms prior to the formation of Fe(IV)O. Rapid chemical quench analyses of DOPA formation showed a burst of product formation, suggesting a slow product release step. Steady-state viscosity experiments established a diffusional step as being significantly rate-limiting. Further studies with stopped-flow spectroscopy indicated that the rate of TyrH reaction is determined by a combination of a number of physical and chemical steps. Investigation of the NO complexes of TyrH by means of optical absorption, electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) techniques revealed the relative positions of the substrate and cofactor with respect to NO, an O2 mimic, and provided further insight into how the active site is tuned for catalytic reactivity upon substrate and cofactor binding.

Synthetic construction of protein molecules has been widely pursued over the last two decades. A primary goal behind de novo protein design has been to build minimal systems by capturing the essential features of protein structures. Such minimal models can be used to understand underlying principles governing folding, structure, and function of proteins molecules. Several approaches envisioning successful construction of synthetic proteins have been described over the years, some of them being admirably successful (DeGrado et al, 1999; Richardson et al> 1992; Baltzer, 1998). Specific patterning of polar and apolar residues in synthetic sequences has been widely used to achieve designed polypeptide structures like helix bundles (DeGrado et ah, 1999) and (3-sheets (Smith and Regan, 1997; Lacroix et a/., 1998), with reliance on hydrophobic driving forces for folding. Our laboratory has been pursuing a distinctly alternative approach, that employs stereochemically constrained amino acids to generate specific secondary structures which can then be assembled into composite structures by appropriately chosen linking segments. This approach, which involves linking prefabricated modules of secondary structures can be termed as a "Meccano set" approach to protein design (Balaram, 1992). The studies embodied in the present thesis describe attempts at construction of synthetic polypeptide motifs using the stereochemically directing influence of conformationally constrained amino acid residues, such as DPro or Aib (α-aminoisobutyric acid). This thesis is subdivided into 8 chapters, with Chapter 1 providing a perspective of the field of protein design. Subsequent chapters (2-8) describe studies directed towards the specific goal of construction of polypeptide motifs. Chapter 2 describes synthesis and conformational characterization of two octapeptides Boc-Leu-Val-Val-DPro-LAla-Leu-Val-Val-OMe (1) and Boc-Leu-Val-Val-DPro-DAla-Leu-Val-Val-OMe (2), designed to investigate the effect of specific β-turn stereochemistry on β-hairpin structures. 500 MHz NMR studies establish that both peptides 1 and 2 adopt predominantly β-hairpin conformations in chloroform and methanol solutions, with interstrand registry established by observation of long-range nuclear Overhauser effects (NOEs). Specific NOEs provide evidence for a type II' β-turn conformation for the DPro-LAla segment in 1, while the NMR data suggest that a type I' DPro-DAla β-turn conformation predominates in the peptide 2. The crystal structure of 1 reveals two independent molecules in the crystallographic asymmetric unit, both of which adopt β-hairpin conformations nucleated by a type II’ β-turn across DPro-LAla and stabilized by 3 cross strand hydrogen bonds. These designed β-hairpins with defined tight turns produce characteristic vibrational circular dichroism (VCD) patterns, demonstrating the utility of VCD as a probe for conformational analysis of β-hairpins. In Chapter 3, we present conformational analysis on designed β-hairpin sequences incorporating a 'Phe-Phe' residue pair at a non-hydrogen bonding position. Two octapeptides Boc-Leu-Phe-Val-DPro-Gly-Leu-Phe-Val-OMe and Boc-Leu-Phe-Val-DPro-Ala-Leu-Phe-Val-OMe were synthesized and conformationally characterized by 500 MHz NMR spectroscopy. Specific NOEs observed in solution provide conclusive evidence favoring specific orientation effects pertaining to the 'Phe-Phe' pair. The peptides exhibited anomalous electronic CD, which has been explained in terms of aromatic contributions by the side chain chromophores. Interestingly, the VCD patterns obtained for these peptides were almost identical to those obtained for other β-hairpins, described in Chapter 2. Chapter 4 describes the synthesis and conformational analysis of designed decapeptide sequences with centrally located DPro-Xxx β-trun segments. Two sequences Boc-Met-Leu»Phe-Val'DPro-Ala-Leu-Val-Val-Phe-OMe (1) and Boc-Met-Leu-Val-Val-^ro-Gly-Leu-Val-Val-Phe-OMe (2) were designed to study the effect of chain length elongation, of β-strands, on designed β-hairpin structures. 500 MHz NMR studies establish β-hairpin folds in both these sequences, with strand segments aligned even at the termini of the structures. Multi-stranded, antiparallel β-sheet structures can be generated by successive placement of β-hairpin sequences in a single polypeptide chain. The successful construction of three stranded β-sheet structures is described in Chapter 5 of this dissertation. A 14-residue peptide Boc-Leu-Phe-Val-DPro-Gly-Leu-Val-Leu-Ala-DPro-Gly-Phe-Val-Leu-OMe (LFV14) was designed such that it is composed of three strand segments linked by two DPro-Gly turn segments. The peptide showed excellent solubility in apolar media, permitting detailed conformational analysis by 500 MHz NMR spectroscopy in organic solvents. Observation of long-range, interstrand NOEs, diagnostic of multiple hairpin structures, provides conclusive evidence for a predominantly populated three stranded β-sheet structure in solution. Extension of this strategy has been described in which an 18-residue peptide, Arg-Gly-Thr-Ile-Lys-DPro-Gly-Val-Thr-Phe-Ala-DPro-Ala-Thr-Lys-Tyr-Gly-Arg, was designed with enhanced solutility in water to probe (β-sheet structure formation in aqueous and mixed aqueous-methanol systems. NMR data provided conclusive evidence in favor of the desired structure being significantly populated in methanol and methanol-water mixtures (50 %, v/v). In water, spectroscopic evidence suggests that the long-range order expected of a three-stranded structure is lost, possibly due to water invading the interstrand hydrogen bonds. Successful construction of a four-stranded antiparallel β-sheet structure has been demonstrated in Chapter 6. A 26-residue peptide Arg-Gly-Thr-Ile-Lys»DPro-Gly-Ile-Thr- Phe-Ala-DPro-Ala-Thr-Val-Leu-Phe-Ala-Val-DPro-Gly-Lys-Thr-Leu-Tyr-Arg was designed to have four strand segments linked by three DPro-Xxx turn segments. The peptide exhibited excellent NMR properties permitting structure determination by analysis of NOE data, which revealed that a four stranded β-sheet structure is indeed populated in methanol. Structural studies on this peptide in mixed methanol-water established that the four stranded β-sheet is appreciably populated at a composition of 50 % (v/v) methanol-water mixture, with the β-sheet structure still detectable even at a composition of 70 % water-30 % methanol. In a completely aqueous environment, the β-sheet structures is significantly disrupted, presumably due to solvent invasion. The nucleating β-turns, however, appear to have retained their structural integrity even in this competitive environment. Chapter 7 describes the insertion of L-Lactic acid (Lac), a hydroxy acid, into polypeptide helices stabilized by a-aminoisobutyricacid (Aib). This study was undertaken to investigate the effect of hydrogen bond deletion on peptide helices. Crystal structure determination of three oligopeptides containing Lac residues has been performed. Peptide 1, Boc-Val-Ala-Leu-Aib-Val-Lac-Leu-Aib-Val-Ala-Leu-OMe, and peptide 2, Boc-Val-Ala-Leu-Aib-Val-Lac-Leu-Aib-Val-Leu-OMe adopt completely helical conformations in the crystalline state, with the Lac(6) residue comfortably accommodated in the center of a helix. NMR studies of peptide 1 and its all amide analog 4, Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-Aib-Val-Ala-Leu-OMe, provide firm evidence for a continuous helical segment in both the cases. In a 14-residue peptide 3, Boc-Val-Ala-Leu-Aib- Val- Ala-Leu- Val- Ala-Leu- Aib-Val-Lac-Leu-OMe, residues Val( 1 )-Leu( 10) adopt a helical conformation, which is terminated by formation of a Schellman motif, with Aib(ll) as the site of chiral reversal. The loss of the hydrogen bond at the C-terminus appears to facilitate the chiral reversal at Aib(l 1). In the final section of this thesis, Chapter 8, successful construction of a synthetic motif containing two distinct elements of secondary structure, a (β-hairpin and a helix, has been described. The design of a 17-residue peptide Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-Gly-Gly-Leu-Phe-Val-DPro-Gly-Leu-Phe-Val-OMe, BH17, is based on a modular approach, in which previously characterized β-hairpin (Leu-Phe-Val-DPro-Gly-Leu-Phe-Val) and helix (Val-Ala-Leu-Aib-Val-Ala-Leu) modules are linked by a Gly-Gly linker. The positioning of the achiral Gly residue at position 8 facilitates termination of the potential helical segment (residues 1-7) by formation of a Schellman motif. Gly(9) is anticipated to be the sole conformationally flexible residue. NMR studies on BH17 indicated the presence of both the helix (residues 1-7) and the β-hairpin (residues 10-17) structures in the sequence, with four major conformational possibilities at the linking segment. Crystal structure determination of BH17 revealed that the two elements of structure are approximately arranged in an orthogonal fashion. The crystal structure validates the original premise that a modular assembly strategy may be viable for the construction of larger synthetic structures. Chapter 9 summarises the major results of this thesis. (For formulae, please refer "pdf" format)

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal