The visible absorption spectra of chlorophyll a monomer, dimer, and oligomers in solution have been analyzed with respect to peak positions, extinction coefficients, oscillator strengths, and dipole strengths. Exciton theory has been used to relate features in the dimer and oligomer spectra to features in the monomer spectrum. Conversion from penta-to hexacoordination at Mg strongly red shifts the Qx transition so that Qx(O,O) appears between Qy(O,O) and QJ(O,l). The relative bulk solvent environmental shifts of the Qy transition in carbon tetrachloride and n-octane have been computed. Because chlorophyll a oligomers have absorption spectra strikingly similar to antenna chlorophyll in green plants, oligomeric chlorophyll a is proposed as a model for antenna chlorophyll.

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A new model is proposed for the structure of the special pair of chlorophyll a molecules believed to correspond to the P700 species in green plants and algae. The proposed model, although admittedly speculative, is based upon exciton-theoretical considerations and on in vitro infrared and visible absorption spectra of a 700 nm absorbing ethanol adduct of chlorophyll a. In the new model, two chlorophyll a molecules are held together by (a) (Fig. 1 of ref. 13), the two Chl a molecules are held together by two water molecules, each of which coordinates to the Mg of one Chl a molecule and simultaneously hydrogen bonds to the carbomethoxy C=GO of the other Chl a molecule. The entire structure has C2 symmetry. We estimate from molecular models that the separation between the macrocycle planes in the Fong structure is approximately 5.6 A; thus, the 7r-systems of the two macrocycles in this model are also not in contact. In the Fong model the ring V keto C=O is not bound or used in special pair formation in any way.In the present paper we propose a new model for the structure of Chls based upon exciton-theoretical considerations and on the in vitro visible absorption and infrared spectra of a 700 nm absorbing Chl a-ethanol adduct. 700 nm absorbing chlorophyll a-ethanol adduct Brody and Brody (14) observed the formation of about 700 nm absorbing Chl a species in ethanol that fluoresces at about 720 nm. We have taken a new look at this species because of the close similarity between the position of its major visible absorption peak and the position of the major absorption peak of P700 in vivo. The visible absorption spectrum of a 0.094 M solution of Chi a in dry toluene with a 1.5 molar ratio of ethanol (about 0.14 M ethanol) at 298 K, 273 K, 247 K, 224 K, and 175 K is shown in Fig. 1. As the temperature is lowered there is a shift from a predominance of a species that absorbs near 670 nm to a species that absorbs near 700 nm. The C=O stretch region of the infrared spectrum of a 0.064 M Chl a solution in dry toluene with a 1.5 molar ratio of ethanol (about 0.096 M ethanol) at 298 K (spectrum A) and 183 K (spectrum B) is shown in Fig. 2. At the higher temperature, the 670 nm species predominates, and at the lower temperature, the 700 nm species predominates. As the temperature is lowered, there is very little change in the area of the ester C=O absorption, and the ester peak shifts only slightly from 1737 cm-1 to 1733 cm-1. On the other hand, the transformation of the 670 nm 700 nm species

The empirical potential using electrons and nuclei (EPEN) has been applied to a study of water clusters containing from two to five molecules. Cluster structures corresponding to potential energy minima have been characterized structurally and energetically. Among dimers and trimers, the only minimum-energy structures (on the sixand twelve-dimensional potential energy surfaces, respectively) that have been found are the trans near-linear dimer (TNLD) and the cyclic trimer. The cyclic dimer and open trimer are not potential-energy minima. In contrast, many local minimum-energy tetramers and pentamers exist. The most stable of these are cage-like structures which owe their low potential energy to the formation of a large number of highly strained hydrogen bonds. No open tetramer or pentamer structures have been found which are minima on their 18-and 24-dimensional respective potential energy surfaces. For the dimer and trimer, transition states for the interconversion between superimposable or enantiomeric minimum-energy structures are discussed along with their effects on the dynamics of these systems. An approximate statistical mechanical calculation of the thermodynamics of water cluster association equilibria in the gas phase is presented and analyzed. Selected water clusters obtained with other published empirical potentials are analyzed; also, the implications of the EPEN results for the structures of bulk liquid water, water surfaces, and droplet nuclei are discussed.