A model system for the hydrogen-bonded chain with large proton polarizability present in the L550 intermediate of bacteriorhodopsin : an FTIR study

Abstract

Mixtures of 1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2-a]pyrimidine (MTBD) with 4-tert-butylphenol have been studied as a function of the 4-tert-butylphenol: MTBD ratio in chloroform and acetonitrile by FTIR spectroscopy. In chloroform N⋯HO⋯HO⋯HO⋯HO N⁺H⋯OH⋯OH⋯OH⋯O^– hydrogen-bonded chains are formed which show large proton polarizability due to collective proton tunnelling. These chanis are particularly suitable for very fast proton conduction. In acetonitrile solutions even for 1:1 mixtures, homoconjugated OH⋯O^––O⋯HO bonds with large proton polarizability are formed in addition to the OH⋯N O^–⋯H⁺N bonds. A non-polar environment is necessary for the formation of the hydrogen-bonded chains. The relevance for the proton pathway present in the L₅₅₀ intermediate in bacteriorhodopsin is discussed.