Ultrasonic absorption in aqueous solutions of amino acids at neutral pH

TitleUltrasonic absorption in aqueous solutions of amino acids at neutral pH
Publication TypeJournal Article
Year of Publication2003
AuthorsNishikawa, S, Ohno T, Huang H, Yoshizuka K, Jordan F
JournalJournal of the Acoustical Society of America
Volume113
Pagination2884-2888
Date PublishedMay
Type of ArticleArticle
ISBN Number0001-4966
Accession Numberhttp://apps.isiknowledge.com/InboundService.do?Func=Frame&product=WOS&action=retrieve&SrcApp=EndNote&Init=Yes&SrcAuth=ResearchSoft&mode=FullRecord&UT=000182752500049
KeywordsKINETICS, PROTON-TRANSFER REACTION, RELAXATION
Abstract

Ultrasonic absorption coefficients in aqueous solutions of glycine, L-alanine, imidazole, L-phenylalanine, L-histidine and L-tryptophan at neutral pH were measured in the range from 0.8 to 220 MHz at 25degreesC. A characteristic ultrasonic relaxation phenomenon was observed only in the solution of L-histidine with a relaxation frequency at around 2 MHz at neutral pH. It was proposed from the concentration independent relaxation frequency and the linear concentration dependence of the maximum absorption per wavelength that the relaxation mechanism was associated with a perturbation of the rotational isomeric equilibrium of the L-histidine molecule. The existence of two rotational isomeric forms of L-histidine in water was examined by semiempirical quantum chemical methods, in order to determine the free energy difference between the two states. The forward and backward rate constants were determined from the relaxation frequency and the energy change. Also, the standard volume change of the reaction was estimated from the concentration dependence of the maximum absorption per wavelength. It was speculated that L-histidine fulfills a specific function among amino acids because of the rotational motion in the molecule, in addition to its well-established acid-base properties. (C) 2003 Acoustical Society of America.

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Alternate JournalJ. Acoust. Soc. Am.