Limited information is available on inherent stabilities of four-chain coiled-coils. We have developed a model system to study this folding motif using synthetic peptides derived from sequences contained in the tetramerization domain of Lac repressor. These peptides are tetrameric as judged by both gel filtration and sedimentation equilibrium and the tetramers are fully helical as determined by CD. The four-chain coiled-coils are well folded as judged by the cooperativity of thermal unfolding and by the extent of dispersion in aliphatic chemical shifts seen in NMR spectra. In addition, we measured the chain length dependence of this four-chain coiled-coil. To this end, we developed a general procedure for nonlinear curve fitting of denaturation data in oligomeric systems. The dissociation constants for bundles that contain a-helical chains 21, 28, and 35 amino acids in length are 3.1 x 6.7 X and 1.0 x M3, respectively. This corresponds to tetramer stabilities (in terms of the peptide monomer concentration) of 180 pM, 51 nM, and 280 f M , respectively. Finally, we discuss the rules governing coiledcoil formation in light of the work presented here.

1047highest probability via direct collisional activation at high translational energies (So > 0.01 for EN > 65 kJ/mol) rather than via trapping or precursor mediated-processes at low incident translational energies (ET < 40 kJ/mol).Optimal control theory is introduced for the control of quantum molecular rotational excitations induced by electric fields. Particular emphasis is given to the case where the electric field pulse is sufficiently short (-1 ps) so that the sudden approximation can be made. Co:isequently, the time evolution of the rotational wave functions can be obtained analytically for general molecular rotations. A hyperbolic curve is shown to explicitly describe the relationship between the rotational energy and the action integral, x ( T ) = JrdSc(t) dt where d is the molecular permanent dipole moment, S the direction cosine matrix, and c ( r ) the applied electric field over time [0, r ] . For the case that the control cost functional has the form Q = F(x(T)) + /3.frc2(t) dt, it is found that the optimal fields are constant in time. The controllability of both rotational energy and transition probability is investigated. A detailed discussion on properties of the optimal fields, as well as the initial and final rotational states, is presented. Numerical calculations are performed for the molecule CsF.

This paper considers vibronic population inversion in the presence of molecular rotation. The objective is to invert the population of a vibronic level with populated rotational levels to a specific vibrational level in the excited electronic state regardless of the detailed population distribution in the final rotational levels. The control of the multilevel population inversion is achieved by design of a pump pulse through optimal control theory. The total energy fluence and the pulse peak intensity are imposed as physical constraints in the design cost functional. A model diatomic molecule is used as an example to investigate molecular rotational effects on the vibronic population inversion and the control properties at different target times. The numerical results indicate that the shape-optimized pulses can achieve nearly complete population inversion and largely overcome the difficulty that a rectangular pulse faces due to energy mismatches and multiple dipole transition moments.

In this paper, the Schrijdinger equation is linearized with regard to a low-intensity controlling electric field.For such a linearized quantum dynamical system, the present work answers the issue of controllability and explicitly provides the control field. Starting in a particular eigenstate, the resultant necessary and sufficient conditions for controllability require that the system satisfy the following two criteria: (1) the N eigenstates of the field-free Hamiltonian superimposed to form the coherent final state must be nondegenerate and (2) the electric dipole transition moments from the initial state to each of the above eigenstates must be nonzero. The control field is obtained analytically in terms of N monochromatic electric fields, each of which has a frequency corresponding to the transitions of the field-free Hamiltonian. We show that the physical properties of the control field are not affected by the overall phase of the coherent wave function. Using Liz as an example, we investigate the control properties of creating specified coherent wave functions on the excited potential energy surface A'Z: by excitation from an initial state on the X'Z; surface. The numerical results suggest that the required control field is reasonable for laboratory realization.

No abstract