Changes in the D- and L-content of aspartic acid, glutamic acid, and alanine in a scleractinian coral over the last 300 years

The D- and L-contents of aspartic acid (Asp), glutamic acid (Glu), and alan ine (Ala) together with absolute and relative concentrations of 17 amino ac ids (AAs) were determined in samples of aragonite skeletons of a scleractin ian coral from the Caribbean through the last 308 years. Regular patterns o f increasing D/L ratios with increasing age are seen for the last 300 years . High and linear racemization rates occur through the first 250 years in A sp, the first 150 years in Glu, and the first 250 years in Ala. An evaluati on of the utility of the D/L ratios of these amino acids as a chronological tool through the last 250 years yields standard errors for individual age estimates ranging between 1.7 and 6.4 years. Thereafter the racemizations s low down and in the 308-years old terminal sample the D/L values are discor dant. A rapid decrease in the total amino acid concentration of 33% over ap proximately the last hundred years is identified. The rapid rate of decreas e in Asp and Glu may be responsible for this decrease in the total AA conce ntration. The relative concentrations of Asp, Glu, and Ala (g AA/100 g prot ein) show a general pattern of decrease, although not significant, through the time interval analyzed. A regularity of the decrease in L-contents and an irregularity of the decrease in D-contents are observed. This may imply that, in addition to possible diagenetic alterations and/or leaching of ami no acids, contamination of D-amino acids explains the discordant D/L values . The D-amino acids could originate from peptidoglycans in bacterial cell w alls. (C) 2001 Published by Elsevier Science Ltd.