To assess the ability of the effective fragment potential (EFP) method to describe the hydration of simple cations, calculations have been carried out on alkali metal (Li + , Na + , and K + )/alkaline earth (Mg 2+ and Ca 2+ ) cation water clusters containing up to six water molecules. The restricted Hartree-Fock (RHF) and secondorder Møller-Plesset (MP2) perturbation methods have also been employed in the study. It is shown that the EFP method is capable of accurately reproducing RHF differential standard enthalpies of hydration for the alkali metal and calcium cation water systems. The EFP method also reproduces experimental differential and total standard enthalpies of hydration for the alkali metal cation water complexes. Good agreement is also found among the EFP and ab initio levels of theory for the enthalpies of the calcium cation water clusters. Possible reasons for discrepancies between the EFP results and those obtained at the ab initio levels for the structures of the Na + (H 2 O) 1-6 and Mg 2+ (H 2 O) 1-6 clusters and the energetics of the Mg 2+ (H 2 O) 1-6 clusters are discussed. A model chemistry is suggested that is based upon EFP/6-31+G* optimizations and Hessians and single-point energies at the MP2/aug-cc-pVDZ level of theory (i.e., MP2/aug-cc-pVDZ//EFP/6-31+G*).

Light-induced fast transient absorbance changes were detected by time-resolved spectroscopy in 38 of 51 haloalkaliphilic isolates from alkaline salt lakes in Kenya and the Wadi Natrun in Egypt. They indicate the presence of two retinal pigments, Pf and Ps, which undergo cyclic photoreactions with half-times of 2 ms and 500 ms respectively. Pf absorbs maximally near 580 nm and Ps near 500 nm. The pigments differ in their sensitivity to hydroxylamine and detergent bleaching and the photoreactions of Pf are strongly dependent on chloride concentration. Of the 38 pigment-containing strains, 29 possess both Pf and Ps, 9 possess only Ps. Inhibition of retinal synthesis with nicotine blocks pigment formation and addition of retinal restores it. Hydroxylamine-bleached pigments can be reconstituted with retinal or retinal analogues. Their similarity to the retinal pigments of Halobacterium halobium strongly suggests that they are also rhodopsin-like retinyledene proteins. Pf in all properties tested is almost identical to halorhodopsin, the light-driven chloride pump of H. halobium, and may serve the same function in the haloalkaliphiles. Ps has photocycle kinetics similar to sensory rhodopsin and a far-blue-shifted long-lived photocycle intermediate, but its ground state absorption maximum is near 500 nm instead of 587 nm. We have not found a bacteriorhodopsin-like pigment in the haloalkaliphiles.

It appears that small movements (d d hitherto only by fluorescence resonance energy trer measurements and crosulinking studies) in a region of the myosin S-1 particle may mediate chemomechanical energy transduction In the contractile system. Here we find under conditions of hih precision at 10"C and 20°C that ATP binding to S-1 causes small (0.4%) changes In CD signal, Ae2m, as do temperature changes In the regime below 16°C. ATP binding perturbs

A weil-known indication that a nucleotide has bound to myosin Is the enhancement of the fluorescence of a speciflc tryptophan in the "subfragment 1" segment of the protein. Empirically the effect has been enormously useful in myosin enzymology. But beyond an early suggestion that it aries from a purine-tryptophan charge-transfer complex, the mechanism of the effect has not been considered. Here we consider the alternative that it arises from an Ionizable group (either another residue or the phosphate of the nucleotide) whose proimit to the tryptophan is altered by substrate bnding. We study this possibility by studying the interaction of an ionizable residue and tryptophan when both are incorporated in a diketopiperazine structure. The geometry of the ituation is inferred from molecular mehaics simulations. Unexpectedly, the best explanation seems to be that the field of the imposed charge, acting across space, affects events in the excited state of the indole.The perturbation of the absorbance (1) and emission (2) (8), but alternative explanations have not been considered. We undertook the present study, hoping to get clues about the aforementioned effect in myosin and, more generally, information about environmental effects on tryptophan. Our specific goal was to study the effect of a single charge positioned in a known relation to the indole ring, using a setup in which the sign and quantity of the charge (and to a limited extent the geometry of the arrangement) could be varied. We sought to accomplish this goal by using diketopiperazines constructed from. tryptophan and the L or D forms of amino acids such as histidine or aspartic acid, using appropriate molecular mechanical analysis to characterize the charge-indole relation. MATERIALS AND METHODSPreparation of the Diketopiperazines. The strategy followed was to prepare t-butoxycarbonyl (Boc)-protected amino acids, to couple these into linear dipeptide esters, and then to cyclize them and remove the protective groups.Boc-Trp and Boc-Asp(13-OBz) (where Bz = benzoyl) were prepared according to Morodes et al. (9). Glu(-OBz) OBz-The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. 6791TosOH and His-OMe*2HCl were prepared according to Zervas et al. (10) and Humphlett and Wilson (11), respectively. Linear protected dipeptide esters were prepared by the N,N'-dicyclohexylcarbodiimide/N-hydroxysuccinimide method (12). Dimethylformamide was the solvent for coupling. The products were purified from ethyl acetate petroleum ether, except Boc-L-Trp-L-His-OMe, which was purified from warm ethanol. In TLC, each of the products gave a single spot (ninhydrin negative) using the KI/toluidine reaction for detection (13). To prepare the diketopiperazines, a Boc group was removed by dissolving 10 mmol of Bocprotected dipeptide ester in 120 ml (240 ml for the histidinecontaining dipeptide) of 0.1 M HCl/96% formic acid...

The cross-helix separation of Tm molecules in acto-tropomyosin has been determined using neutron scattering. Deuterated Dictyostelium discoideum actin was density matched in a 93% D2O buffer so that effectively only the protonated tropomyosin was "visible" to neutrons. Analysis of the solution scattering pattern in the region of the first oscillation yielded a value for the cross-helix separation of 7.9 +/- 0.3 nm. The implications of this value for the mechanism of the regulation of muscle contraction are discussed in light of recent results by others.