Characterization of cooperative conformational transitions by Fourier transform infrared spectroscopy: application to phospholipid binary mixtures

Abstract

Temperature-induced cooperative conformational transitions in biopolymers may be examined in aqueous solution by the use of Fourier transform infrared spectroscopy. The transition is described in terms of a two-state model with a fractional population parameter (which describes the degree of transition at various values intermediate between the two limiting cases) that can be generated by a least-squares technique. This procedure allows for calculation of a conformational index without assuming a linear dependence of the infrared parameter on the extent of transition. Calculations based on the Zimm-Bragg theory of cooperative conformational transitions allow thermodynamic quantities to be derived using the conformational index in conjunction with calorimetric measurements.The method is illustrated with data from the gel/liquid-crystalline phase transitions of binary phospholipid vesicles where the use of isotopic substitution allows the conformation of each component of the binary mixture to be independently monitored. The data suggest the coexistence of well-defined structural domains of phospholipid as well as the preferential clustering of one of the lipid components at the interface between the two phases. On peut Ctudier les transitions conformationnelles coopCratives induites par la tempirature des biopolymkres en solution aqueuse en faisant appel B la spectroscopie infrarouge par transformation de Fourier. On peut dCcrire la transition en fonction d'un modkle a deux Ctats avec un paramktre de population fractionnaire (qui dtcrit le degrC de transition des diverses valeurs intermkdiaires entre les deux cas limites) que I'on peut gCnCrC par la technique des moindres c a d s . Ce procCdC permet de calculer un indice conformationnel sans faire I'hypothkse qu'il existe une relation linkaire entre le paramktre infrarouge et le degri de transition. Des calculs basks sur la thtorie de Zimm-Bragg des transitions conformationnelles coopCratives permettent dlCvaluer des quantitks thermodynamiques a l'aide de l'indice conformationnel couplC a des mesures calorimCtriques.On illustre la mCthode avec des donnees provenant de transitions de phases gel/liquide cristallin de vCsicules de phospholipides binaires oij I'utilisation de la substitution isotopique permet de determiner d'une manikre indkpendante la conformation de chacun des composants du mClange binaire. Les donnCes suggkrent qu'il y a une coexistence de domaines structuraux de phospholipides bien dCfinis ainsi qu'une aggregation prCfCrentielle de I'un des lipides composants a l'interface entre les deux phases.[Traduit par le journal]

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