Elektronen‐Donator‐Acceptor‐Komplexe bei Polypeptiden II. Problemstellung und orientierende Versuche mit Acceptoren des neuen Phtalimid‐ Typs

Abstract

Abstract Phthalimides, 4‐nitrophthalimides, and tetrachlorophthalimides (exemplified by methyl N α ‐phthalyl‐glycinate, 4‐nitrophthalimide, ethyl N α ‐(4‐nitrophthalyl)‐glycinate, and methyl N α ‐tetrachlorophthalyl‐glycinate) give colored molecular complexes with N α ‐benzyloxycarbonyl‐ ar ‐pentamethyl‐phenylalanine, N α ‐acetyl‐ p ‐dimethylamino‐phenylalanine, and N α ‐acetyl‐tryptophane, as well as with the simpler compounds hexamethylbenzene, N, N‐dimethylaniline, N, N‐dimethyl‐ p ‐toluidine, and indole, in various organic solvents. The association constants are small, and vary between 0.4 and 1.2 l/mol as measured spectroscopically. Enthalpies of complex formation lie between (−) 0.5 and (−) 2.5 kcal/mole. They are within the range assumed for V AN DER W AALS interactions between hydrophobic side‐chains of amino‐acids. This is essential if one would want to use such complexes as intra‐ or intermolecular conformational probes in synthetic polypeptides. The broad absorption maxima of the complexes lie between 350 and 490 nm, extinction coefficients are in the range reported for charge transfer complexes , 500 to 2000. Assignment of the long wavelength absorption to a charge transfer transition seems quite justified by the calculated values of energy coefficients (HMO‐method) for the new class of phthalimide acceptors and the donors, as well as by the applicability of the B RIEGLEB relationship between h ν max , ionisation energy, electron affinity, and distance between the planes of donor and acceptor in the complex between ethyl N α ‐(4‐nitrophthalyl)‐glycinate and hexamethylbenzene. On the basis of absorption maxima, the relative acceptor strengths are: 4‐ nitrophthalimide > tetrachlorophthalimide > phthalimide. Phthalyl, nitrophthalyl, and tetrachlorophthalyl derivatives of tryptophane, pentamethylphenylalanine, and p ‐dimethylamino‐phenylalanine are colored compounds and behave like intramolecular charge transfer complexes . The syntheses of all required compounds are described. N‐Ethoxycarbonyl derivatives of 4‐nitrophthalimide and of tetrachlorophthalimide are used to prepare 4‐nitrophthalyl and tetrachlorophthalyl derivatives of amino‐acids.