Acid-base properties of 3-[2-(n-quinolinyl)benzoxazol-5-yl]alanine derivatives in the ground and excited state. Experimental and theoretical studies

Acid-base properties of 3-[2-(n-quinolinyl)benzoxazol-5-yl]alanine (n = 2,3,4,6,8 and 3-iso) were studied by means of absorption and fluorescence spectroscopy as well as theoretical calculations. It was found that basicity of heterocyclic derivatives in the ground and excited state depends on the position of the heteroatom in the substituent. Experimental as well as theoretical calculations revealed that quinolinyl nitrogen atom is the protonation center. The weakest base is 3-[2-(2-quinolinyl)benzoxazol-5-yl]alanine (pKa = 0.99) whereas the strongest one is 3-[2-(8-quinolinyl)benzoxazol-5-yl]alanine (pKa = 4.69).The prototropic equilibrium in the excited state is plausibly not established.