Localization of carotenoids in plasma low-density lipoproteins studied by surface-enhanced resonance Raman spectroscopy

Surface-enhanced resonance Raman scattering (SERRS) spectra were measured f or the beta-carotene and lycopene carotenoids present in low-density lipopr oteins (LDLs), which were isolated from human plasma and adsorbed on roughe ned silver surfaces. The silver surface was modified by formation of a self -assembled monolayer (SAM) of carboxylate-terminated linear alkanethiols in order to simulate the LDL binding region of the cellular LDL receptor. Thi ols of different chain length were used to produce SAMs of varying thicknes ses. It was shown that carotenoids are not released from the LDL particle u pon adsorption onto the bare and thiol modified silver surfaces. The SERRS studies indicated that beta-carotene and lycopene were present in the shell of the LDL particle. The dependence off SERRS on the distance from the sil ver surface was different for beta-carotene and lycopene in LDL. This obser vation suggests that the two carotenoids are located in different places of the LDL particle. (C) 2000 John Wiley & Sons, Inc.