On the significance of the fourth-rank orientational order parameter of fluorophores in membranes

Abstract Using information theory, the orientational distribution function of cylindrically symmetric probe molecules in uniaxial systems is constructed from orientational order parameters. If only the second-rank order parameter 〈 P 4 > is known, a distribution of the gaussian type results. If also the fourth-rank order parameter 〈 P 4 > is known, it is possible to distinguish between several hypothetical models. A reanalysis of time-resolved fluorescence anisotropy data indicates that the so-called 〈 P 4 >-distribution (introduced by Zannoni and described below) is a good model for diphenylhexatriene in non-oriented membranes. The behaviour of the distribution function is related to the fluctuation in P 2 . This fluctuation is proportional to 〈 P 4 >, at fixed 〈 P 2 >.