The thesis presents preparation and characteristics of two types of biologically active heterocyclic small molecules: 1) N-sulfonylpurines (potential antitumor agents and inhibitors of the purine nucleoside phosphorylase enzyme, and 2) peptide phenanthridine derivatives (potential antitumor agents and fluorescent probes). Series of C6, N9 substituted purines bearing sulfonamide pharmacophore has been prepared. Series of 6-chloropurine derivatives has been prepared by introduction of sulfonyl substituent at the N9 position of the purine base. Substitution of chlorine at the C6 position yielded 6- morpholino- and 6-bis(2-hydroxyethyl)amino-derivatives of N-9-sulfonylpurine. Biological activities 6-morpholino- and 6-bis(2-hydroxyethyl)aminopurine derivatives have been tested by MTT test on the leukemia and lymphoma (Raji, HuT 78, K-562), carcinoma (CaCo-2, HeLa) and normal (MDCK 1) cell line. Compound 17, bearing trans--styrenesulfonyl substituent at N9 position of the purine base produced the strongest cyctotoxic effect on all of the tested cell lines. 6-morpholino-derivatives with tosyl, naphtylsulfonyl and biphenylsulphonyl substituent at the N9 position of the purine (8, 9 and 10) exhibited moderate antiproliferative effect on the Raji and HuT 78 cell lines, while 9 and 10 exhibted moderate antiproliferative effect on the K-562 cell line too. The inhibition of the PNP enzyme isolated from two strains of Helicobacter pylori and PNP from Escherichia coli was also tested. Best results are obtained with 6-morpholino-derivatives 8 and 18. Peptide phenanthridine derivatives were prepared by classic peptide chemistry methods (synthesis in solution). Alanine derivative of phenanthridine 22 was prepared by Negishi reaction, and subsequently functionalized by peptide backbone constructed starting from the Nterminus of 22. Di-, tri- and tetrapeptide phenanthridine derivatives with guanidinopyrrole, guanidine (arginine), pyrene or fluorene moiety have been prepared as well. Finally, guanidinopyrrole and arginine derivatives have been converted into hydrochloride salts. ix Interactions of chosen compounds (guanidinopyrrole derivatives 28b and 29b) with polynucleotides have been studied via several spectrophotometric methods, namely UV/visible, circular dichroism and fluorescence spectroscopies. The compounds have shown different types of interactions, depending on their conformation. Compound 28b, whose chromophores are not interacting mutually in mildly acidic media, most likely binds to minor groove of DNA and major groove of RNA. Compound 29b, whose chromophores are involved in intramolecular - interaction, binds on the phosphate backbone of the polynucleotide. In neutral media both compounds agglomerate along the phosphate backbone.