THE ORIENTATION OF THE TRANSITION DIPOLE MOMENTS OF CHLOROPHYLL a and PHEOPHYTIN a IN THEIR MOLECULAR FRAME

Abstract

Abstract— In this paper we describe the determination of the orientation of the absorption and emission transition dipoles of chlorophyll a and pheophytin a in their molecular frame. For this purpose we have embedded the pigments in anhydrous nitrocellulose films with a concentration of 2 × 10‐7 mol/g. We have shown previously that under these conditions the pigments are in a purely monomeric state, are distributed uniformly both before and after stretching and that no intermolecular energy transfer among the molecules takes place. Using a combination of steady‐state anisotropy experiments on unstretched films and angle‐resolved fluorescence depolarization measurements on stretched films, we obtain the orientation of the transition dipole moments of both pigments in their molecular frame and the orientational distribution function of the molecules relative to the stretching direction of the film. The steady‐state anisotropy measurements indicate that chlorophyll a has two distinct emission dipole moments and that excitation in the Soret‐region results in simultaneous excitation of two or more absorption transition dipole moments. On the other hand, excitation in the QY‐band involves only a single dipole moment. The directions of the transition dipole moments in the molecular frame are obtained from the angle‐resolved measurements. Pheophytin a also exhibits two emission dipole moments, but the angle between them is much smaller than that between the corresponding dipoles for chlorophyll a. As a consequence the dipole moments contributing to the Soret‐region could not be resolved and only an effective absorption transition dipole moment in the Soret‐region is extracted.

Red Chlorophylls in the Exciton Model of Photosystem I

“…In the case of the Chl a molecules the dipole moment of the longest wavelength transition is oriented almost along the line connecting N II and N IV atoms in the chlorine plane (van Zandvoort et al 1995). By neglecting the specificity of the protein surrounding of Chls for W ij given in cm )1 and d in Debye 2 , we can assume C=5.04 and d=23 Debye 2 (van Amerongen et al 2000).…”

Water soluble chlorophyll binding protein of higher plants: A most suitable model system for basic analyses of pigment–pigment and pigment–protein interactions in chlorophyll protein complexes

“…The orientation of the transition dipole moments with respect to the x-and y-axes of Chls (and bacteriochorophylls) is a matter of long lasting discussions and markedly different values have been reported (Fragata et al, 1995;van Zandvoort et al, 1995;Linke et al, 2008;Shibata et al, 2010). When using the values for the rotational strength of CD spectra determined by Hughes et al (2006) for Chl a in class IIa WSCP we obtained a clockwise Q y deviation of 6-8 • from the y-axis with our best fit on the basis of the structure data of Horigome et al (2007).…”

Determining the Three‐Dimensional Electronic Transition Dipole Moment Orientation: Influence of an Isomeric Mixture

“…Consequently, angles 1058 and 758 are in effect equivalent, in these previous determinations. [2,3] In contrast, our approach can distinguish between these possibilities, and our solution indicates a slightly larger angle of 788 with the x-axis, as well as an angle of 48 out of the X-Y plane.…”

“…[2]). Figure 2 illustrates that the most probable normalized Q y electronic tdm vector is (x/y/z) = (0.19/0.98/ 0.07); this vector is also depicted in Figure 1 [2] a Q y angle of 708 with the x-axis, and for Chl a in nitrocellulose films [3] an angle of (105 AE 2)8 with the x-axis. Note that these determinations could not give information if the Q y tdm makes an angle with the X-Y plane, nor could they in principle distinguish between the angles 908 AE V with respect to the axis of orientation.…”