403 research outputs found
Staggered Ladder Spectra
We exactly solve a Fokker-Planck equation by determining its eigenvalues and
eigenfunctions: we construct nonlinear second-order differential operators
which act as raising and lowering operators, generating ladder spectra for the
odd and even parity states. These are staggered: the odd-even separation
differs from even-odd. The Fokker-Planck equation describes, in the limit of
weak damping, a generalised Ornstein-Uhlenbeck process where the random force
depends upon position as well as time. Our exact solution exhibits anomalous
diffusion at short times and a stationary non-Maxwellian momentum distribution.Comment: 4 pages, 2 figure
A New and Unusual Pathway for the Reaction of Neocarzinostatin Chromophore with Thiols. Revised Structure of the Protein-Directed Thiol Adduct
Neocarzinostatin (holo-NCS) is an antitumor antibiotic comprising a nonprotein chromophore component (1) and a 113-
amino acid carrier protein (apo-NCS). Goldberg and coworkers
first demonstrated that the reaction of the isolated
chromophore (1) with thiols in the presence of double-stranded DNA leads to DNA cleavage by a free-radical mechanism. The pathway shown in Scheme 1 was later proposed to account for this activity, a proposal that is now supported by a considerable body of evidence. In 1992, Saito and co-workers showed that the reaction of holo-NCS with small thiols, such as β-mercaptoethanol (BME), takes a different course, to form a product that is formally a 1:1:1 adduct of thiol, 1, and water. Structure 2 was proposed for this adduct, along with the mechanistic pathway shown in Scheme 2. Complicating the analysis was the fact that 2 was an inseparable mixture of two components, present in equal parts
Generalised Ornstein-Uhlenbeck processes
We solve a physically significant extension of a classic problem in the
theory of diffusion, namely the Ornstein-Uhlenbeck process [G. E. Ornstein and
L. S. Uhlenbeck, Phys. Rev. 36, 823, (1930)]. Our generalised
Ornstein-Uhlenbeck systems include a force which depends upon the position of
the particle, as well as upon time. They exhibit anomalous diffusion at short
times, and non-Maxwellian velocity distributions in equilibrium. Two approaches
are used. Some statistics are obtained from a closed-form expression for the
propagator of the Fokker-Planck equation for the case where the particle is
initially at rest. In the general case we use spectral decomposition of a
Fokker-Planck equation, employing nonlinear creation and annihilation operators
to generate the spectrum which consists of two staggered ladders.Comment: 24 pages, 2 figure
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Pediatric feeding and swallowing rehabilitation: An overview
Children with neurological disabilities frequently have problems with feeding and swallowing. Such problems have a significant impact on the health and well-being of these children and their families. The primary aims in the rehabilitation of pediatric feeding and swallowing disorders are focused on supporting growth, nutrition and hydration, the development of feeding activities, and ensuring safe swallowing with the aim of preventing choking and aspiration pneumonia. Pediatric feeding and swallowing disorders can be divided into four groups: transient, developmental, chronic or progressive. This article provides an overview of the available literature about the rehabilitation of feeding and swallowing disorders in infants and children. Principles of motor control, motor learning and neuroplasticity are discussed for the four groups of children with feeding and swallowing disorders
A New and Unusual Pathway for the Reaction of Neocarzinostatin Chromophore with Thiols. Revised Structure of the Protein-Directed Thiol Adduct
Neocarzinostatin (holo-NCS) is an antitumor antibiotic comprising a nonprotein chromophore component (1) and a 113-
amino acid carrier protein (apo-NCS). Goldberg and coworkers
first demonstrated that the reaction of the isolated
chromophore (1) with thiols in the presence of double-stranded DNA leads to DNA cleavage by a free-radical mechanism. The pathway shown in Scheme 1 was later proposed to account for this activity, a proposal that is now supported by a considerable body of evidence. In 1992, Saito and co-workers showed that the reaction of holo-NCS with small thiols, such as β-mercaptoethanol (BME), takes a different course, to form a product that is formally a 1:1:1 adduct of thiol, 1, and water. Structure 2 was proposed for this adduct, along with the mechanistic pathway shown in Scheme 2. Complicating the analysis was the fact that 2 was an inseparable mixture of two components, present in equal parts
Classical motion in force fields with short range correlations
We study the long time motion of fast particles moving through time-dependent
random force fields with correlations that decay rapidly in space, but not
necessarily in time. The time dependence of the averaged kinetic energy and
mean-squared displacement is shown to exhibit a large degree of universality;
it depends only on whether the force is, or is not, a gradient vector field.
When it is, p^{2}(t) ~ t^{2/5} independently of the details of the potential
and of the space dimension. Motion is then superballistic in one dimension,
with q^{2}(t) ~ t^{12/5}, and ballistic in higher dimensions, with q^{2}(t) ~
t^{2}. These predictions are supported by numerical results in one and two
dimensions. For force fields not obtained from a potential field, the power
laws are different: p^{2}(t) ~ t^{2/3} and q^{2}(t) ~ t^{8/3} in all dimensions
d\geq 1
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Comparing videofluoroscopy and endoscopy to assess swallowing in bottle-fed young infants in the neonatal intensive care unit
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