To be announced

[CM ICTP - Trieste]

CONDENSED MATTER SECTION

	
SEMINAR on  Disorder and strong electron correlations




Thursday, 6 March   -    11:00 a.m.


Seminar Room,  Main Bldg.- first floor





F. PIAZZA   ( Ecole Polytechnique Fédérale de Lausanne )


"Energy localization in complex networks of nonlinear oscillators: 
Interplay between disorder and nonlinearity in protein dynamics "



	Abstract

Discrete breathers (DB) are spatially-localized, time-periodic 
vibrations that arise under general conditions in discrete, extended 
nonlinear systems.  While much is known concerning their existence and 
stability in spatially periodic media, much less is known about the 
interplay of nonlinearity and spatial heterogeneity in disordered 
systems, where localization of vibrational energy is also fostered as a 
result of breaking of translational invariance.

We introduce a coarse-grained, topology-based nonlinear network model 
of protein dynamics with the aim of investigating the interplay of 
spatial disorder and nonlinearity in biological molecules.  DB 
solutions, characterized both numerically with the surface cooling 
technique and analytically, show that localization of energy occurs 
generically also in the presence of disorder, but is a site-dependent 
and, on a larger scale, a fold-dependent process.  In particular, we 
find that, as a sheer consequence of disorder,  a non-zero energy gap 
for exciting a DB at a given site either exists or not.  Interestingly, 
in the former case, the gaps arise as a result of the  impossibility of 
exciting small-amplitude modes in the first place.  In the latter case, 
a small subset  of linear localized modes act as accumulation points, 
whereby DBs can be continued to arbitrary small energies, while 
unavoidably approaching one of such normal  modes.

Remarkably, our cooling simulations show that localized modes of 
nonlinear origin form spontaneously in the stiffest parts of the 
structure.  Analytic calculations further reveal that such regions are 
also the sectors where DBs are characterized by the smallest energy 
gaps. Besides being interesting per se, such results  provide a 
straightforward way for interpreting the recently discovered link 
between local stiffness of proteins and enzymatic activity.  They 
strongly suggest that nonlinear vibrational modes may play an important 
role in enzyme function, allowing for a ready energy storage channel 
during the catalytic process.