A general ellipsoid cannot always serve as a model for the rotational diffusion properties of arbitrarily shaped rigid molecules.

An ellipsoidally shaped body, or more commonly, an ellipsoid of revolution, is generally assumed to serve as a convenient model for evaluating the rotational diffusion properties of macromolecules. If Perrin's equations for the rotational diffusion coefficients of general ellipsoids can be shown to generate all possible rotational diffusion coefficients, then there would exist at least one equivalent ellipsoidal shape for every arbitrarily shaped rigid body. We investigated the problem by first generating a space, r-space, representing all possible ellipsoidal shapes. We then generated another space, D-space, representing all possible combinations of rotational diffusion coefficients. We then mapped r-space into D-space by using Perrin's equations. Ellipsoidal shapes map into diffusion space in a well-defined manner. The mapping is either 1:1, 2:1, or 3:1; several distinctly different regions of r-space map onto the same regions of D-space. Thus, for some combinations of rotational diffusion coefficients, more than one ellipsoid can be used as a model. Not all of D-space is covered by the mapping of r-space. Therefore, there are combinations of rotational diffusion coefficients that cannot be generated from ellipsoidally shaped bodies. Several examples of rigid body shapes with nonellipsoidal diffusion properties are described.

Subnanosecond single photon counting fluorescence spectroscopy using synchronously pumped tunable dye laser excitation.

A synchronously pumped tunable dye laser has been constructed and interfaced with a modified Ortec 9200 photon counting system for the purpose of measuring subnanosecond relaxation phenomena. The dye laser excitation pulse, which has an intrinsic 35-ps FWHM for Rhodamine 6G, is 350 ps when measured by time-correlated single photon counting. This value appears to be characteristic of the transit time jitter in the RCA 8850 photomultiplier tube. Subnanosecond fluorescence lifetimes of Rhodamine B with KI as a quencher have been determined by deconvolution of photons counted versus elapsed time using the method of moments; the shortest lifetime measured was 68 ps. Various technical aspects of the system are discussed with emphasis on applications to biophysical problems.

In vitro preparation and characterization of a 700 nm absorbing chlorophyll-water adduct according to the proposed primary molecular unit in photosynthesis.

1. This study characterized chlorophyll a-H2O adducts in vitro in order to establish their generic relationship to the recently proposed [15, 18-20, 31] primary molecular adduct in photosynthesis. The effects of water titration and temperature on the absorption, fluorescence, excitation, and redox properties of the various in vitro chlorophyll a aggregate species are investigated. 2. From fluorescence measurements, we conclude that the driest chlorophyll a sample contains an equimolar amount of water. This conclusion is consistent with earlier experimental work [2, 3, 14, 17, 31], and clarifies the origin of the controversial [15] Katz model [14] of chlorophyll a-H2O interactions. 3. With increasing water concentration or as the temperature is lowered below room temperature, the A-663 monohydrate chlorophyll a-H2O (species absorbing at 663 nm) is favored at the expense of the A-678 anhydrous aggregate according to the equilibrium 2H2O+chlorophyll a2in equilibrium2 chlorophyll a-H2O. Under excess water conditions, A-663 is converted to A-743 (chlorophyll a-2H2O)n. 4. On slow sample cooling to T less than or approximately 200 degrees K, we observe the growth of A-700 at the expense of A-663. There is a direct correspondence between the increasing (decreasing) absorption by A-700 (A-633) and increasing (decreasing) fluorescence at 720 nm (664 nm). 5. It is concluded that A-700 is most probably the dimer participating in the equilibrium 2 chlorophyll a-H2O in equilibrium (chlorophyll a-H2O)2. The A-700 band consists of two exciton components (separated by approximately 280 cm1) that are interpretable in terms of the dimeric origin of A-700. 6. The deconvoluted A-700 absorption spectrum and the excitation spectrum of the 720 nm fluorescence are compared with the light-minus-dark spectra of P-700. 7. It is found that A-700 is reversibly bleached by I2 (E0 equals 0.54 V). The significance of this observation is discussed in terms of the redox properties of monomeric chlorophyll a and P-700.