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New thionation experimental conditions and new reagents for the synthesis of thioamide analogues of protected amino acids and peptides are presented. The interaction of thiocarbonyl analogues of model substrates of (alpha)-chymotrypsin and leucine aminopeptidase were also studied. Optically active dithioester derivatives of protected amino acids were prepared and used as thioacylating agents. / The synthesis of four thioamide-containing analogues of the chemotactic tripeptide f-Met-Leu-Phe was accomplished. The conformational properties of these novel analogues were studied by ('1)H and ('13)C NMR spectroscopy. Their biological activity was also evaluated in vitro and the results interpreted in terms of their molecular properties. / The regioselectivity of the new thionation methodology allowed for the rapid and efficient synthesis of the four possible monothioamide positional isomers of {Leu('5)}-enkephalin. Their biological activity was studied both in vitro and in vivo. / Amidoxime and amidrazide analogues of the peptidic bond were also obtained using thioamides as intermediates.

<p> Calculations have been done for H₂O, F₂CO, Cℓ₂CO, H₂CS, F₂CS and Cℓ₂CS, to determine the transition energy and oscillator strength of the ¹A₂ ← ¹A₁(n → π*), ¹A₁ ← ¹A₁(π → π*), ¹B₂ ← ¹A₁(n → σ*) and ¹B₁ ← ¹A₁(σ → π*) transitions. Eigenvalues were obtained from CNDO programs and used to calculate the oscillator strength at three levels of approximation. </p> <p> The ¹A₁ ← ¹A₁(π → π*) transition of Cℓ₂CS has been analysed. Four ground state and three excited state frequencies were found. Coon's method was used to establish the assignment and predict an excited state bent out of plane by 26.6°. </p> <p> A previously unassigned transition in Cℓ₂CS has been labelled as A₂ ← ¹A₁(n → π*), on the basis of CNDO calculations, and five ground state frequencies have been found in the spectrum. </p> / Thesis / Master of Science (MSc)