No abstract

The relationship between calcium current and transmitter release was studied in squid giant synapse. It was found that the voltage-dependent calcium current triggers the release of synaptic transmitter in direct proportion to its magnitude and duration. Transmitter release occurs with a delay of approximately 200 mus after the influx of calcium. A model is presented which describes these relations formally.

No abstract

SynopsisThe kinetics of the fluorescence decay of the energy donor in a homologous series of oligopeptides each containing a t its ends a donor and an acceptor of electronic excitation energy was investigated in solvent mixtures of different viscosities. The repeating unit in the peptides was N5-(2-hydroxyethyl)-~-glutamine and the chromophores used as donor and acceptor were naphthalene and dansyl, respectively. The number of units in the peptides studied varied from four to nine. The solvents used were mixtures of glycerol and trifluoroethanol in various proportions. The decay rate of the donor fluorescence increases when the solvent viscosity decreases. This behavior is due to the disturbance of the equilibrium end-to-end distribution of distances of the excited molecules by the energy transfer process, which is more favorable for short than for long distances. The subsequent rearrangement towards the equilibrium distribution by diffusion of the molecular ends relative to one another enhances the efficiency of the energy transfer. Assuming a modified Fick equation to describe this diffusion motion, the fluorescence decay data were analyzed in terms of a diffusion coefficient describing the Brownian motion of the molecular ends. The diffusion coefficients thus evaluated increase systematically upon decreasing the solvent viscosity. For example, for the oligopeptides studied it changes from unmeasurahly small values in glycerol solution to values varying between cmz/sec a t room temperature in a glycerol trifluoroethanol solvent mixture of viscosity of 8 centipoise. The values obtained for the diffusion coefficient are smaller by about an order of magnitude than the values expected for the diffusion coefficients of the free chromophores in solvents of comparable viscosity. It is thus concluded that the backbone of the polymeric chains possesses appreciable internal friction which exerts resistance to the Brownian motion of the polymer chains. The diffusion coefficient of the end-to-end motion is systematically smaller for the shorter than for the longer chains. For example, a t room temperature in a solvent mixture of 8 centipoise it is 3 X 5 X 7.6 X and 8.5 X lop8 cm2/sec for oligomers containing four, five, eight, and nine N5-(2-hydroxylethy1)-L-glutamine repeating units, respectively. The internal friction thus impedes the motion of the molecular ends more effectively in the shorter chains than in the longer ones. Analysis of the energy-transfer experiments in solvents spanning a wide range of viscosities shows that the orientational factor appearing in Forster's equation for energy transfer does not interfere perceptibly with the evaluation of the e n d -b e n d distances from the fluorescence decay data when naphthalene and dansyl serve as donor and acceptor, respectively. This is due to the fact that both the donor emission and acceptor absorption in the range of spectral overlap are characterized by more than one transition dipole moment, as is evident from the corresponding polarization data.

A homologous series of oligopeptides each containing at its ends a donor and an acceptor of electronic excitation energy was synthesized by the solidphase method. N5-(2-Hydroxyethyl)-L-glutamine was the repeating unit, and peptides containing 4,5,6,7,8, and 9 of these amino-acid residues were prepared. The chromophores naphthalene and dansyl, which were used as donor and acceptor, respectively, fulfil the conditions necessary for energy transfer according to the Forster mechanism. A distance corresponding to 50% efficiency of energy transfer, Ro = 22 i 1 A, was calculated.The kinetics of fluorescence decay of an oligomer containing the naphthalene chromophore only could be described precisely by a monoexponential function. In contrast, the kinetics of the decay curves of the fluorescence of the donor of all of the oligomers containing both donor and acceptor, as measured in viscous solution, deviated markedly from monoexponential behavior. The deviation was interpreted in terms of the great number of different conformations that the various molecules of each of the oligomers attain in solution, leading to characteristic end-to-end distribution functions between the donor and acceptor. Numerical adjustment of the parameters of some of the previously proposed expressions to describe the end-to-end distribution enabled the reconstruction of the kinetics of the fluorescence decay of the donor with great precision. The end-to-end distribution functions for the various oligopeptides were thus evaluated.The common methods used for the study of the dimensions of polymeric molecules, such as viscosity, light scattering, or ultracentrifugal sedimentation, are not applicable to oligomers of relatively small molecular weight. The conformations of such molecules are, however, of considerable interest, since they are more amenable to computational analysis by fast computers. Comparison between computed and measured data, if available, may yield valuable information concerning the basic steric properties of the repeating units within the oligomers under consideration. Since many low molecular weight oligomers, such as some of the native and synthetic peptides, show biological activity, information as to their conformation in solution seems of particular interest.Stryer and Haugland (1) have suggested that distances of the order of 15-50 X between definite sites on macromolecules can be derived from the efficiency of transfer of electronic excitation energy by the F6rster mechanism (2), between suitably chosen chromophoric groups that have been attached to these sites. This is made possible by the relationship given in Eq. 1in which E denotes the efficiency of energy transfer, and r is the distance between the participating chromophores. E = Ro6/(r6 + Ro6) Ro is a quantity of dimension length given by Eq. 2.R 06 -9000 In 10 X270o A f(F)(v dp 128 ir5Nn4 Jo[2]in Eq. 2, f(P) and e(P) denote the fluorescence intensity of the donor and the molar extinction coefficient of the acceptor, respectively, at wave number P; 77o is t...