XI Simpósio Brasileiro de Química Teórica
Caxambu, 18 a 21 Novembro de 2001
 
 
 
 

P302

A MOLECULAR AND QUANTUM MECHANICAL STUDY OF INDOLE-3-
ACETIC ACID

Rudolf Kiralj (PQ) and Márcia M. C. Ferreira (PQ), Instituto de Química, Universidade
Estadual de Campinas, Campinas, SP, 13083-970, Brazil. marcia@iqm.unicamp.br
 

Indole-3-acetic acid is the most important auxin,   a plant hormone  with  simple molecular
structure. A ll the attempts to quantify its structure-biological activity relationship were not
much  successsful  up  to  date,   mostly  due  to  missing knowledge about some intrinsic
molecular properties responsible for intermolecular interactions.  Low temperature crystal
structure  of  indole-3-acetic  acid¹  is  a  good  reference  point for s ystematic theoretical
studies.   In  this  work,   the   experimental  geometry   was   optimized   using   molecular
mechanics (MMF, Sybil), semi-empirical methods (MINDO/3, MNDOm AM1, PM3) and ab
initio  (about fifteen DFT and RHF basis sets).   Bond length matrix  mxn  (m = number of
bonds,  n = number  of  calculations)   and   its  tranpose  (nxm)   were  analyzed  utilizing
Principal   Component  Analysis   (PCA)   and   Hierarchical  Cluster  Analysis   (HCA).   A
carboxylic  acid cyclic  H-bond  dimer  from  crystal structure was optimized using various
methods.   In general,   ab initio  bond lengths are closer to experimental ones than those
from semi-empirical and molecular  mechanics.   All the methods do not reproduce well C-
OH bond which is  the most  one  affected  by  dimer's  H-bonds.  Detailed  PCA  on  both
matrices shows that bond lengths are described   at   most  by  six  Principal Components.
The data clustering is similar in PCA and in HCA.   Characteristic molecular fragments are
recognized among the scores,   while similar basis sets of  ab initio  and also methods  of
the  same  level  of  accuracy  (e.g.  molecular mechanics,  semi-empirical, ab initio)  are
distinguished in loadings.   Bond lengths of optimized cyclic  H-bond  dimer  are  close  to
experimental values than those of  isolated i ndole-3-acetic  acid,   especially considering
the  carboxylic  chain (by  more  than  0.01 Å).   The optimized dimer structures show that
the carboxyl group geometry is closely related to H-bond geometry.
1 B. Nigovic, S. Antolic, B. Kojic-Prodic, R. Kiralj, V. Magnus, B. Salopek-Sondi, Acta
Cryst. B56 (2000) 94-111.

(FAPESP