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An empirical potential energy function based on the interactions of the electrons and nuclei in molecules has been developed and tested. The potential energy of interaction is approximated by the sum of the coulombic interactions between all point charge centers (electrons and nuclei), an exponential repulsion to represent electron-electron overlap repulsion, and an R(-6) (R = distance) attraction to simulate dispersion and other attractive energies between the heavy atom fragments of the molecules. The parameters of the potential energy function have been determined from experimental gas-phase and crystal data.The results indicate that both intramolecular and intermolecular interaction energies can be treated with the same set of parameters. In comparison to other empirical interaction potentials now in use, there are fewer independent parameters, there is no need for intrinsic torsional potentials to obtain the correct rotational barriers, and there is no need for special hydrogen bonding functions to account for the directionality and energetics of hydrogen bonding.

An empirical potential energy function based on the interactions of electrons and nuclei (EPEN) has been tested on molecules other than those used for its parameterization. The results indicate that this energy function is able to predict reliably the lowest energy conformations, the potential energy differences between conformations, rotational barrier heights, and dipole moments for a series of alkanes, amines, alcohols, and carbohydrates. Crystal packing studies on n-hexane, n-octane, methylamine, methanol, and alpha-d-glucose, using this same potential, indicate that it is also reliable for calculating intermolecular interaction energies and low-energy orientations.

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) two strong ring V keto C[unk]O[unk]H[unk]O (or keto C[unk]O[unk]H[unk]N, or keto C[unk]O[unk]H[unk]S) hydrogen bonds and by (b) pi-pi van der Waals stacking interactions between the two chlorophyll a macrocycles. Macrocycle stacking provides the intermolecular pi-pi overlap necessary to promote the (experimentally observed) delocalization of the unpaired electron of the in vivo radical over two chlorophyll pi systems. The new model provides an explicit role for the participation of protein in the formation of the chlorophyll special pair. The in vitro system yields an electron spin resonance signal indistinguishable from that of oxidized P700 in Chlorella vulgaris.

The chlorophyll a (Chl a) special-pair model of the primary donor of photosystem I (P700) does not account in a completely adequate fashion for the magnetic resonance properties observed for P700(+). Moreover, P700 is at least 420 mV easier to oxidize than is Chl a in vitro. Neither Chl a dimer formation nor selective ligation of Chl a can account for this potential difference. Enolization of the Chl a ring V beta-keto ester results in a very different pi electronic structure. The Chl a enol can be trapped as a silyl enol ether. In addition, the enol analog 9-desoxo-9,10-dehydro-Chl a can be prepared. Both the trapped enol and its 9-H analog are approximately 350 mV easier to oxidize than Chl a. The ESR spectrum of the cation radical consists of a single 6.1-G gaussian line that is line narrowed relative to that of Chl a(+) in a manner similar to P700(+). Electron-nuclear double resonance (ENDOR) spectroscopy resolves only a 3.5-MHz hyperfine splitting for the 3-methyl-group. The remaining splittings are all less than 3.5 MHz. The second moment of the ESR line of fully (13)C-enriched 9-desoxo-9,10-dehydro-Chl a(+) agrees with that of [(13)C]P700(+) to within 10%. Application of the special-pair model to the [(13)C]P700(+) second-moment data yields a 100% error. Ab initio molecular orbital calculations on ethyl chlorophyllide a enol cation bear out the ESR and ENDOR data. We conclude that a monomeric Chl a enol model provides a better description of the magnetic resonance parameters and oxidation potential of P700 than a Chl a special-pair model.

Evidence from electron paramagnetic resonance (e.p.r.) studies suggests that the unpaired spin in oxidized P700+. or P865+. is shared by two special chlorophyll a (Chl a) or bacteriochlorophyll a (Bchl a) molecules respectively. Three classes of models have been proposed for special pair reaction centre chlorophyll: asymmetric, in which one Chl a (or Bchl a) acts as electron donor to a second acting as acceptor; models with translational symmetry only; and models with C2 symmetry. Models with C2 symmetry have been synthesized in vitro with two chlorophyll macrocycles tied together by a covalent link. The singlet and triplet states of the in vitro models have been characterized by e.p.r., nuclear magnetic resonance, and optical studies involving absorption, emission, and lasting behaviour. The fact that lasting occurs only from the folded configuration of the linked dimers suggests the availability of a highly effective nonradiative decay path from the S1 state of the excited open dimer. A radical-pair mechanism that accounts for the unusual spin polarization of the special pair triplet is proposed for the primary photochemistry in the reaction centre.

The relative donor properties of the three carbonyl groups of chlorophyll a have been studied theoretically by a series of ab initio molecular fragment, floating spherical Gaussian orbital, self-consistent field calculations on ethyl chlorophyllide a and experimentally through a 13C magnetic resonance study on chlorophyll a. The approximate ground state electronic wavefunction of ethyl chlorophyllide a was perturbed by monopole and dipole point charges whose signs, magnitudes, and positions were chosen to mimic the coulombic interactions associated with carbonyl coordination to Mg. Because the polarizability of the ring V keto carbonyl binding site is substantially greater than that for the ester carbonyl binding sites, the ring V keto binding site binds with smallest binding energy for weak perturbations and with largest binding energy for strong perturbations. A comparison of 13C magnetic resonance chemical shifts in chlorophyll a monomer and dimer provides new experimental evidence that the donor-acceptor interactions that bind the chlorophyll dimer together involve a substantial participation by the ring V keto carbonyl and minimal participation by the two ester carbonyl groups, and thus are in agreement with conclusions derived from the ab initio calculations.

Ab initio calculations on di-, tri-, tetra-, and pentapeptides of glycine in various conformations are reported. Hydrogen bonding is observed as an important stabilizing force in alpha-helical conformations. These studies on polypeptides of glycine of reasonable size show for the first time that the forces acting to stabilize polypeptide conformations can be extracted directly from theoretical studies, without prior postulation of their existence or need for concern that neglect or approximation of various integrals may have biased the results.