Search PubMed⌕ Search

Biomedical subjects

D Kleinfeld

Publications and source records attributed to D Kleinfeld.

42 records · Page 3Linked to original sources
PubMed1987

"Unlearning" increases the storage capacity of content addressable memories.

The storage and retrieval of information in networks of biological neurons can be modeled by certain types of content addressable memories (CAMs). We demonstrate numerically that the amount of information that can be stored in such CAMs is substantially increased by an unlearning algorithm. Mechanisms for the increase in capacity are identified and illustrated in terms of an energy function that describes the convergence properties of the network.

Animals↗
PubMed1986

Sequential state generation by model neural networks.

Sequential patterns of neural output activity form the basis of many biological processes, such as the cyclic pattern of outputs that control locomotion. I show how such sequences can be generated by a class of model neural networks that make defined sets of transitions between selected memory states. Sequence-generating networks depend upon the interplay between two sets of synaptic connections. One set acts to stabilize the network in its current memory state, while the second set, whose action is delayed in time, causes the network to make specified transitions between the memories. The dynamic properties of these networks are described in terms of motion along an energy surface. The performance of the networks, both with intact connections and with noisy or missing connections, is illustrated by numerical examples. In addition, I present a scheme for the recognition of externally generated sequences by these networks.

Artificial Intelligence↗
PubMed1985

Charge recombination kinetics as a probe of protonation of the primary acceptor in photosynthetic reaction centers.

The kinetics of the charge recombination D+QA-----DQA was used to probe the protonation of the primary acceptor in reaction centers from Rhodopseudomonas sphaeroides, in which the native ubiquinone was replaced by anthraquinone. We found that QA- is stabilized by the rapid (t less than 10(-2) s) binding of a proton, with a pK of 9.8. The distance between QA- and the proton binding site was estimated to be larger than approximately 5 A.

Bacterial Proteins↗
PubMed1984

Electron-transfer kinetics in photosynthetic reaction centers cooled to cryogenic temperatures in the charge-separated state: evidence for light-induced structural changes.

We have compared the electron-transfer kinetics in reaction centers (RCs) cooled in the dark with those cooled under illumination (i.e., in the charge-separated state). Large differences between the two cases were observed. We interpreted these findings in terms of light-induced structural changes. The kinetics of charge recombination D+QA-----DQA in RCs containing one quinone were modeled in terms of a distribution of donor-acceptor electron-transfer distances. For RCs cooled under illumination the distribution broadened and shifted to larger distances compared to the distribution for RCs cooled in the dark. The model accounts for the nonexponential decay observed at low temperatures [McElroy, J. D., Mauzerall, D. C., & Feher, G. (1974) Biochim. Biophys. Acta 333, 261-277; Morrison, L.E., & Loach, P.A. (1978) Photochem. Photobiol. 27, 751-757]. A possible physiological role of the structural changes is an enhanced charge stabilization. For RCs with two quinones, the recombination kinetics D+QAQB-----DQAQB were found to be strongly temperature dependent. This was interpreted in terms of temperature-dependent transitions between structural states [Agmon, N., & Hopfield, J.J. (1983) J. Chem. Phys. 78, 6947-6959]. This interpretation requires that these transitions occur at cryogenic temperatures on a time scale t greater than or approximately 10(3) s. The electron transfer from QA- to QB was found to not take place in RCs cooled in the dark (tau ABdark greater than 10(-1) s). In RCs cooled under illumination, we found tau ABlight less than 10(-3) s. We suggest the possibility that the drastic decrease in tau AB observed in RCs cooled under illumination is due to the trapping of a proton near QB-.

Bacterial Proteins↗
PubMed1984

Electron transfer in reaction centers of Rhodopseudomonas sphaeroides. I. Determination of the charge recombination pathway of D+QAQ(-)B and free energy and kinetic relations between Q(-)AQB and QAQ(-)B.

The electron-transfer reactions and thermodynamic equilibria involving the quinone acceptor complex in bacterial reaction centers from R. sphaeroides were investigated. The reactions are described by the scheme: (Formula: see text). We found that the charge recombination pathway of D+QAQ(-)B proceeds via the intermediate state D+Q(-)AQB, the direct pathway contributing less than approx. 5% to the observed recombination rate. The method used to obtain this result was based on a comparison of the kinetics predicted for the indirect pathway (given by the product kAD-times the fraction of reaction centers in the Q-AQB state) with the observed recombination rate, kobsD+----D. The kinetic measurements were used to obtain the pH dependence (6.1 smaller than or equal to pH smaller than or equal to 11.7) of the free energy difference between the states Q(-)AQB and QAQ(-)B. At low pH (less than 9) QAQ(-)B is stabilized relative to Q(-)AQB by 67 meV, whereas at high pH Q(-)AQB is energetically favored. Both Q(-)A and Q(-)B associate with a proton, with pK values of 9.8 and 11.3, respectively. The stronger interaction of the proton with Q(-)B provides the driving force for the forward electron transfer.

Animals↗