'X' ENCON,TRO NACIONAL DE FISICA DA MATERIA CONDENSADA

XX ENFMC - Resumos
constituica"o de amostras que facilitam a determinacao
da estrutura de membranas atraves de XSW.
INTERAcAO DE FARMACOS COM
SISTEMAS LIPiDEOS/AGUA: ESTUDO
ESTRUTURAL
LEIDE PASSOS CAVALCANTI, IRIS L. TORRIANI
Institute de Fi'sica 'Gleb Wataghin', UNICAMP,
Campinas, SP, Brasil
Lipideos e sistemas lipideo/agua apresentam urn variado
mesomorfismo. As mesofases lamelares de sistemas
simples, como lipideos naturais ou sinteticos, sejam
puros ou misturados, fornecem urn modelo racional,
que serve de base para estudar sistemas mais cornplexos
destinados a aplicacaes bioquimicas, medicas
ou industriais. A relacao entre estrutura molecular
e comportamento da mesofase em escudo e estabelecida,
geralmente, atraves de diagramas de fase que representam
a composicao relativa de fases coexistentes
para uma temperatura dada. E importante determinar
parametros composicionais que podem ser adotados na
formulacao de farmacos encapsulados ou corn propricdades
de liberacao controlada. Em experiencias anteriores
estudamos os sistemas dipalmitoilfosfatidilcolina
(DPPC)/cardiolipina (CL) corn incorporacEo de elipticina
(ELPT), urn alcaloide de mac, anti-cancerigena,
verificando que a presenca de lipideos anionicos, como
a CL, favorecem a incorporacao da ELPT nos sistemas
de lipideos mistos DPPC/CL. Neste trabalho utilizamos
a difracao de raios-X para identificar e caracterizar
estruturalmente as transigoes de fase encontradas
nos sistemas DPPC/CL e DPPC/CL/ELPT, corn
o objetivo de estabelecer relacOes estrutura-atividade
do farmaco ern quest5o. As medidas foram realizadas
em urn intervalo de temperatura de 25°C a 85°C corn
urn arranjo experimental que permute obter as diagramas
de difracao de raios-X "in-situ". Com esse arranjo
foi possivel monitorar as reflexoes lamelares correspondentes
a espacamentos da ordem de 60Ae o halo difuso
devido a distancia entre as cadeias acilicas que sao indicadores
da transican entre a fase gel e a fase liquidocristalina.
CNPq.
TEMPERATURE AND IONIC STRENGTH
DEPENDENT LIGHT SCATTERING OF
DMPG DISPERSIONS
KARIN A. RISKE, MARIA TERESA LAMY-FREUND
Institute of Physics, Universidade de Sao Paulo, SP
MARIO J. POLITI
Institute of Chemistry, Universidade de Scio Paulo, SP
WAYNE F. REED
Department of Physics, Tulane University, New Orleans,
LA, USA
The temperature dependence of the intensity of light
scattered by aqueous dispersions of the anionic lipid
DMPG (dimyristoyl phosphatidylglycerol) was studied
at different ionic strengths. The lipid main transition,
gel-liquid crystal, can be well monitored by a sharp decrease
in turbidity. As expected, the temperature of
the main transition (T m ) was found to increase with
the increase of the ionic strength. For low ionic strength,
a DMPG second temperature transition, named
post-transition, can be monitored by both an increase
in light scattering and a decrease in conductivity. Zimm
plot analysis indicates that bellow T n, the liposomes
tend to aggregate, yielding a negative second virial coefficient
A2, and particles of large molecular weight. At
the phase transition, parallel to the decrease in turbidity,
there is an increase in the sample conductivity,
A 2 becomes positive and the particle molecular weight
decreases, indicating a non-aggregated state. Moreover,
at the post-transition (T p ) A2 becomes very small,
and the molecular weight increases again. Opposite to
the main transition on the post-transition could not be
detected by spin labels placed either at the membrane
surface, or in the bilayer core. Considering an increase
in area per lipid headgroup upon the main transition,
and the Gouy-Chapman-Stern model, it was possible
to show that the main transition increases the vesicle
degree of dissociation, hence increasing the ion concentration
in solution. However, within the framework of
that simple model, it was necessary to consider different
Na+-PG — association constants, in the gel and liquid
phases, to explain an increase in vesicles repulsion
upon the main transition. The mechanisms governing
the post-transition are even less clear.
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